AC Motors 2

Introduction to motors cont.:
An object in motion travels a distance in a given time. Speed is the ratio of the distance traveled and the time it takes to travel the distance.

The linear speed of an object is a measure of how long it takes the object to get from point A to point B. Linear speed is usually given in a form such as meters per second (m/s).

The angular speed of a rotating object is a measurement of how long it takes a given point on the object to make one complete revolution from its starting point. Angular speed is generally given in revolutions per minute (RPM).

An object can change speed. An increase in speed is called acceleration. Acceleration occurs only when there is a change in the force acting upon the object. An object can also change from a higher to a lower speed. This is known as deceleration (negative acceleration).

Mechanical systems are subject to the law of inertia. The law of inertia states that an object will tend to remain in its current state of rest or motion unless acted upon by an external force. This property of resistance to acceleration/deceleration is referred to as the moment of inertia. The English system of measurement is pound-feet squared (lb-ft2).



If we look at a continuous roll of paper, for example, we know that when the roll is stopped it would take a certain amount of force to overcome the inertia of the roll to get it rolling. The force required to overcome this inertia can come from a source of energy such as a motor. Once rolling, the paper will continue unwinding until another force acts on it to bring it to a stop.

AC Motors 1

AC motors are used worldwide in many residential, commercial, industrial, and utility applications. Motors transform electrical energy into mechanical energy. An AC motor may be part of a pump or fan, or connected to some other form of mechanical equipment such as a winder, conveyor, or mixer. AC motors are found on a variety of applications from those that require a single motor to applications requiring several motors.

Before discussing AC motors it is necessary to understand some of the basic terminology associated with motor operation. Many of these terms are familiar to us in some other context. Later in the course we will see how these terms apply to AC motors.

In simple terms, a force is a push or a pull. Force may be caused by electromagnetism, gravity, or a combination of physical means.

Net force is the vector sum of all forces that act on an object,  including friction and gravity. When forces are applied in the same direction they are added. For example, if two 10 pound forces were applied in the same direction the net force would be 20 pounds.

If 10 pounds of force were applied in one direction and 20 pounds of force applied in the opposite direction, the net force would be 10 pounds and the object would move in the direction of the greater force.
If 10 pounds of force were applied equally in both directions, the net force would be zero and the object would not move.

Torque is a twisting or turning force that causes an object to rotate. For example, a force applied to the end of a lever causes a turning effect or torque at the pivot point. Torque (τ) is the product of force and radius (lever distance).
τ = Force x Radius
In the English system torque is measured in pound-feet (lb-ft) or pound-inches (lb-in).If 10 lbs of force were applied to a lever 1 foot long, for example, there would be 10 lb-ft of torque.

An increase in force or radius would result in a corresponding increase in torque. Increasing the radius to two feet, for example, results in 20 lb-ft of torque.

Circuit Breakers Types

Instantaneous magnetic-trip-only circuit breakers do not provide overload protection and are used on motor circuits where overload protection is provided by a motor starter. The current level at which an instantaneous trip circuit breaker trips is adjustable. The name comes from the electromagnet used to sense short circuit current. The purpose of overload protection is to prevent the motor from operating beyond its full-load capability. In the schematic illustrated below, a motor is supplied through a 3-pole circuit breaker, motor starter contacts and separately supplied overload contacts. Heat generated from excessive current will cause the overload contacts to open, removing power from the motor.

Thermal-magnetic circuit breakers have both overload and instantaneous trip features. When an overload condition exists, the excess current will generate heat, which is detected in the circuit breaker. After a short period of time, dependent on the rating of the breaker and amount of overload, the breaker will trip, disconnecting the load from the voltage source. If a short circuit occurs, the breaker responds instantaneously to the fault current and disconnects the circuit.

The user does not have access to the trip unit on some circuit breakers. This means the trip unit cannot be changed with another. Interchangeable trip is actually a design feature that is available on some thermal-magnetic and some solid state breakers. The advantage of a breaker with an interchangeable trip unit is the user can change the continuous current rating of the breaker without replacing the breaker. This is done by replacing the trip unit with one of a different rating.

molded case circuit breakers are available as a molded case switch. Molded case switches employ the same operating mechanism as the thermal magnetic and magnetic only units.  A preset instantaneous function is factory installed to allow the switch to trip and protect itself at a high fault current, but the switch provides no thermal overload protection.

Many electrical distribution systems can deliver large short circuit currents to electrical equipment. This high current can cause extensive damage. Current limiting circuit breakers will reduce the current flowing in the faulted circuit to substantially less magnitude. This helps protect expensive equipment. One way to accomplish current limiting is with an additional set of contacts that feature two moveable arms. These are referred to as dual-pivot contacts, which separate even more quickly than the single-pivot contacts. The dual-pivot contacts are connected in series with the single-pivot contacts. As with the single-pivot design, current flows in opposite directions through the contact arms, creating a magnetic repulsion. As current increases, the magnetic repulsion force increases.

In an overload condition where current may only be one to six times normal current, the contacts remain closed until the breaker trips. In a short circuit condition fault current is extremely high, both sets of contact arms may open simultaneously, generating high impedance arcs. The contact gap of the dual-pivot contacts increases more rapidly, therefore generating arc impedance more rapidly. Once the arcs are extinguished, the dual-pivot contacts close on their own due to spring tension. The single-pivot contacts are held open by the breaker mechanism, which will have tripped during the fault and must be manually reset.

Solid state circuit breakers function similarly to thermal-magnetic breakers. The basic breaker mechanism is still mechanical. The tripping unit is solid state.  As with the thermal-magnetic tripping unit, the Sensitrip circuit breaker tripping unit performs the following three functions:
 Senses magnitude of current flow,Determines when current becomes excessive,Determines when to send a trip signal to the breaker.


 Mechanism of these circuit breakers use a microprocessor to execute numerous functions programmed in the unit. These units have a greater degree of accuracy and repeatability. Adjustments on the trip unit allow the user to select numerical values the microprocessor will use in performing protective functions. Current sensors mounted in the trip unit monitor the value of load current. The value of current is reduced to a low level and converted to a digital voltage, which is used by the microprocessor. The microprocessor continuously compares the line current with the value set by the user. When current exceeds a preset value for the selected time, the trip unit sends a signal to a magnetic latch. The magnetic latch opens the breaker’s contacts, disconnecting the protected circuit from the power source.

Circuit Breaker Design

Circuit breakers are constructed in five major components:
Frame (Molded Case) Contacts, Arc Chute Assembly,Operating Mechanism and Trip Unit.

The frame provides an insulated housing to mount the circuit breaker components. The construction material is usually a thermal set plastic such as glass-polymer. The construction material can be a factor in determining the interruption rating of the circuit breaker. Frame ratings indicate several pieces of important information such as; maximum voltage, ampere rating, interrupting rating, and physical size.

Circuit breakers use contacts to break the circuit and stop the flow of energy. Some conventional circuit breakers use a straight-through contact arrangement. The electrical path through the contacts is a straight line.a magnetic field is developed around a current carrying conductor. The magnetic fields developed around the contact arms of a straight-through contact arrangement have little or no effect on the contacts arms. During a fault, the contacts are only opened by the mechanical operation of the circuit breaker spring.

But current causes heat, which is destructive to electrical equipment. A rise in current causes a corresponding rise in heat. In reality, the thermal energy the circuit will see is proportional to the square of the current multiplied by the time the current flows (I2T). This means that the higher the level of current, the shorter the time it takes for heat to damage equipment. In the following illustration, IP represents the peak level the fault current rises before the breaker contacts open.

The blow-apart contacts are two contact arms are positioned parallel to each other as shown in the following illustration. As current flows through the contact arms, magnetic fields are set up around each arm. Because the current flow in one arm is opposite in direction to the current flow in the other arm, the two magnetic fields oppose each other. The strength of the magnetic field is directly proportional to the amount of current. During normal current conditions, the magnetic field is not strong enough to force the contacts apart.

When a fault develops, current increases which increases the strength of the magnetic field. The increased strength of the opposing magnetic fields actually helps to open the contacts faster by forcing them apart.

As the contacts open a live circuit, current continues to flow for a short time by jumping the air space between the contacts in the form of an arc. When the contacts open far enough the arc is extinguished and the current flow stops.

The arc can cause burning on the contacts. In addition, ionized gases form inside the molded case. If the arc isn’t extinguished quickly the pressure from the ionized gases could cause the molded case to rupture. An arc chute assembly is used to quench the arc. This assembly is made up of several “U” shaped steel plates that surround the contacts. As the arc is developed it is drawn into the arc chute where it is divided into smaller arcs, which are extinguished faster.

An operating handle is provided to manually open and close the contacts. Molded case circuit breakers (MCCBs) are trip free, meaning that they can’t be prevented from tripping by holding or blocking the operating handle in the “ON” position. There are three positions of the operating handle: “ON” (contacts closed), “OFF” (contacts open), and “TRIPPED” (mechanism in tripped position). The circuit breaker is reset after a trip by moving the handle to the “OFF” position and then to the “ON” position.

The operating handle is connected to the moveable contact arm through an operating mechanism. Molded case circuit breakers use an over-center toggle mechanism that is a quickmake and quick-break design. In the following illustration, the operating handle is moved from the “OFF” to the “ON” position. In this process a spring begins to apply tension to the mechanism. When the handle is directly over the center the tension in the spring is strong enough to snap the contacts closed. This means that the speed of the contact closing and opening is independent of how fast the handle is operated.To open the contacts, the operating handle is moved from the “ON” to the “OFF” position. In this process a spring begins to apply tension to the mechanism. When the handle is directly over the center the tension in the spring is strong enough to snap the contacts open. As in closing the circuit breaker contacts, contact opening speed is independent of how fast the handle is operated.

The trip unit is the “brain” of the circuit breaker. It consists of components that will automatically trip the circuit breaker when it senses an overload or short circuit. The tripper bar is moved by a manual “PUSH TO TRIP” button, a thermal overcurrent sensing element or an electromagnet.

A trip mechanism is held in place by the tripper bar. As long as the tripper bar holds the trip mechanism, the mechanism remains firmly locked in place.The operating mechanism is held in “ON” position by the trip mechanism. When a trip is activated, the trip mechanism releases the operating mechanism, which opens the contacts.

MCCBs, heavy duty and above, can be manually tripped by depressing the red “PUSH TO TRIP” button on the face of the circuit breaker. When the button is pressed the tripper bar rotates up and to the right. This allows the trip mechanism to “unlock” releasing the operating mechanism. The operating mechanism opens the contacts. The “PUSH TO TRIP” button also serves as a safety device by not allowing access to the circuit breaker interior in the “ON” position. If an attempt is made to remove the circuit breaker cover while the contacts are in the closed (“ON”) position, a spring located under the pushbutton will cause the button to lift up. This action will also trip the breaker.

Power Supply Systems

Homes built prior to 1936, especially in rural areas, used a two wire supply system. This system provided 120 volts between a hot conductor and a grounded conductor. A two-wire system is usually inadequate for today’s residential electrical demands.

The most common supply system used in residential applications today is a three-wire supply system. There are 120 volts between any phase and neutral and 240 volts between phases.

Load centers can also be used in commercial applications. Electric power is brought into a building at one location and then is distributed through the building by means of separate circuits. Two distribution systems used in commercial applications that are suitable for load centers are three-wire, three-phase, 240 volts; and four-wire, three-phase, 208Y/120 volts.

Surge Protection

Today’s homes have many semiconductor-based devices such as televisions, stereos, computers and microwave ovens.These devices are highly susceptible to voltage spikes. Devices used in the home which generate voltage spikes include vacuum cleaners and other motor driven devices, and spark igniters on gas ranges, furnaces and water heaters. The most damaging voltage spikes are caused by lighting strikes. A lighting strike on a power line several miles away still has the potential to cause extensive electrical damage in a home.Lightning strikes on high voltage lines are generally dissipated by utility transmission and arresters. The average home, however, will experience eight to ten voltage surges of 1,000 to 10,000 volts annually. Damage to expensive electrical equipment can be instantaneous or cumulative.

A typical lightning strike consists of 25,000 amps at 30 million volts. The following map shows the approximate mean annual number of days with thunderstorms in the United States.

An electrical surge, whether it is caused by electrical equipment or lightning, always seeks ground. Any component between the source of the surge and ground can be damaged.

Installation is as simple as mounting a conventional circuit breaker. After power is switched off and the trim removed, the circuit breaker/surge arrestor plugs into place. A lead wire is provided to connect the ground side of the module to the load center’s neutral bus. It is best to position the circuit breaker/surge arrestor in the first position of the load center and connect the lead wire in the first neutral position.

One device provides protection for the electrical system. Two red LEDs indicate that the device is working. The device does not require a dedicated space and can be added on to existing load centers. The circuit breaker portion of the surge arrester can be used on noncritical lighting circuits to provide additional visual indication that the device is working. If the device trips due to a high voltage surge, it is reset like any other circuit breaker in the panel.


Clamping voltage is the amount of voltage allowed across a surge suppression device when it is conducting a specific current created by a surge.

Peak current rating specifies the maximum energy that can be dissipated from a single surge without causing the protecting device to sacrifice itself.

Arc Fault Protection

GFCI devices are designed to protect a person from getting a shock when touching an ungrounded appliance. Arc Fault Circuit Interrupters (AFCI), in comparison, protect against a fire being started from an unintended arc. An arc fault occurs when a current-carrying conductor has an arching condition to ground or another conductor. Damaged insulation, for example, can lead to an arc fault, which may not generate enough fault current to trip a circuit breaker. In the following example a staple has been driven through the insulation of a wire during installation.

An AFCI device is intended to provide protection from the effects of arc faults by recognizing the characteristics unique to arcing and de-energizing the circuit when an arc fault is detected. The arc generated will cause the AFCI to trip. Arcs normally generated from electric equipment such as a light switch or power drill will not cause the AFCI to trip.

Ground Fault Protection

A ground fault occurs when a current-carrying conductor comes in contact with ground. A faulty appliance or the presence of water in contact with a conductor are two possible ways a ground fault can occur. One way ground fault protection is accomplished is by the use of GFCI receptacles. These are installed in place of a normal receptacle.

A ground fault circuit interrupter (GFCI) compares current on the hot wire with current returning on the neutral wire. Under normal circumstances the current is equal.

When a ground fault occurs some of the current will return to the source through ground. In the following illustration, for example, a ground fault has occurred in a common household appliance. Anyone coming in contact with the appliance will become part of the circuit. The sensing and test circuit will detect that the amount of current returning on the neutral is less than the current on the hot wire. The sensing and test circuit will cause the trip coil to automatically open the circuit breaker, removing power from the appliance. GFCI devices trip between 4 to 6 milliamps. The amount of time it takes for a GFCI device to trip depends on the current. The higher the current the faster the device will trip.

Circuits providing power to certain areas of the home require ground fault circuit interrupters (GFCI).
Ground fault protection is required on the following circuits:
Bathroom receptacles,Residential garage receptacles,Outdoor receptacles,Receptacles in unfinished basements,Receptacles in crawl spaces,Receptacles within six feet of a kitchen or bar sink,Pools.

GFCI type circuit breakers have one white neutral lead which is connected to the neutral bus in the load center. The phase and load neutral are connected to lugs in the GFCI. They mount in the load center in the same way as a standard circuit breaker.

Electrical Power Residential Distribution

A distribution system is a system that distributes electrical power throughout a building. Distribution systems are used in every residential, commercial, and industrial building.

Power, purchased from a utility company, enters the house through a metering device.The incoming power then goes to a load center which provides circuit control and overcurrent protection. The power is distributed from the load center to various branch circuits for lighting, appliances and electrical outlets. Careful planning is required so that the distribution system safely and efficiently supplies adequate electric service for present and possible future needs.

The term “load center” is an industry term used to identify a panelboard used in certain applications. Load centers are typically rated 225 amps or less and 240 volts maximum and are intended for use in residential applications.

The load center (panel-board) as a single panel or group of panel units designed for assembly in the form of a single panel; including buses, automatic overcurrent devices, and equipped with or without switches for the control of light, heat, or power circuits; designed to be placed in a cabinet or cutout box placed in or against a wall, partition, or other support; and accessible only from the front

Circuit Breakers

Another device used for overcurrent protection is a circuit breaker.circuit breaker is a device designed to open and close a circuit by nonautomatic means, and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating.

Circuit breakers provide a manual means of energizing and de-energizing a circuit. In addition, circuit breakers provide automatic overcurrent protection of a circuit. A circuit breaker allows a circuit to be reactivated quickly after a short circuit or overload is cleared. Unlike fuses which must be replaced when they open, a simple flip of the breaker’s handle restores the circuit.

Like fuses, every circuit breaker has a specific ampere, voltage, and fault current interruption rating. The ampere rating is the maximum continuous current a circuit breaker can carry without exceeding its rating. As a general rule, the circuit breaker ampere rating should match the conductor ampere rating.

Generally the ampere rating of a circuit breaker is selected at 125% of the continuous load current. This usually corresponds to the conductor ampacity which is also selected at 125% of continuous load current. For example, a 125 ampere circuit breaker would be selected for a load of 100 amperes.

The voltage rating of the circuit breaker must be at least equal to the circuit voltage. The voltage rating of a circuit breaker can be higher than the circuit voltage, but never lower.

Circuit breakers are also rated according to the level of fault current they can interrupt. When applying a circuit breaker, one must be selected which can sustain the largest potential short circuit current which can occur in the selected application.

Electrical Power Circuit Protection

Circuit protection must be taken into consideration with any electrical circuit, including busway. Current flow in a conductor always generates a watts loss in the form of heat. As current flow increases, the conductor must be sized appropriately in order to compensate for higher watt losses. Excess heat is damaging to electrical components. For that reason, conductors have a rated continuous current carrying capacity or ampacity.Overcurrent protection devices are used to protect conductors from excessive current flow. Two devices used to protect circuits from overcurrent are fuses and circuit breakers. These protective devices are designed to limit the flow of current in a circuit to a safe level, preventing the circuit conductors from overheating.

overcurrent is any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault.

Circuit protection would be unnecessary if overloads and short circuits could be eliminated. Unfortunately, overloads and short circuits do occur. To protect a circuit against these currents, a protective device must determine when a fault condition develops and automatically disconnect the electrical equipment from the voltage source.

An overcurrent protection device must be able to recognize the difference between overcurrents and short circuits and respond in the proper way. Protection devices use an inverse time-current characteristic. Slight overcurrents can be allowed to continue for some period of time, but as the current magnitude increases, the protection device must open faster. Short circuits must be interrupted instantly.

A fuse is the simplest device for interrupting a circuit experiencing an overload or a short circuit. A typical fuse, like the one shown below, consists of an element electrically connected to end blades or ferrules. The element provides a current path through the fuse. The element is enclosed in a tube and surrounded by a filler material.

Current flowing through the element generates heat, which is absorbed by the filler material. When an overcurrent occurs temperature in the element rises. In the event of a harmless transient overload condition the excess heat is absorbed by the filler material. If a sustained overload occurs the heat will eventually melt open an element segment forming a gap; thus stopping the flow of current.

Short-circuit current can be several thousand amperes and generates extreme heat. When a short circuit occurs several element segments can melt simultaneously, which helps remove the load from the source voltage quickly. Short-circuit current is typically cut off in less than half a cycle, before it can reach its full value.

Nontime-delay fuses provide excellent short circuit protection. Short-term overloads, such as motor starting current, may cause nuisance openings of nontime-delay fuses. They are best used in circuits not subject to large transient surge currents. Nontime-delay fuses usually hold 500% of their rating for approximately one-fourth second, after which the current carrying element melts. This means that these fuses should not be used in motor circuits which often have inrush (starting) currents greater than 500%.

Time-delay fuses provide overload and short circuit protection. Time-delay fuses usually allow five times the rated current for up to ten seconds. This is normally sufficient time to allow a motor to start without nuisance opening of the fuse unless an overload persists.

Fuses have a specific ampere rating, which is the continuous current carrying capability of a fuse. The ampere rating of a fuse, in general, should not exceed the current carrying capacity of the circuit. For example, if a conductor is rated for 10 amperes, the largest fuse that would be selected is 10 amperes.

There are some specific circumstances when the ampere rating is permitted to be greater than the current carrying capacity of the circuit. For example, motor and welder circuits can exceed conductor ampacity to allow for inrush currents and duty cycles.

The voltage rating of a fuse must be at least equal to the circuit voltage. The voltage rating of a fuse can be higher than the circuit voltage, but never lower. A 600 volt fuse, for example, can be used in a 480 volt circuit. A 250 volt fuse could not be used in a 480 volt circuit.

Fuses are also rated according to the level of fault current they can interrupt. This is referred to as ampere interrupting capacity (AIC). When applying a fuse, one must be selected which can sustain the largest potential short circuit current which can occur in the selected application. The fuse could rupture, causing extensive damage, if the fault current exceeds the fuse interrupting rating.

Distribution Systems

A distribution system is a system that distributes electrical power throughout a building. Distribution systems are used in every residential, commercial, and industrial building.

Distribution systems used in commercial and industrial locations are complex. A distribution system consists of metering devices to measure power consumption, main and branch disconnects, protective devices, switching devices to start and stop power flow, conductors, and transformers. Power may be distributed through various switchboards, transformers, and panel boards. Good distribution systems don’t just happen. Careful engineering is required so that the distribution system safely and efficiently supplies adequate electric service to both present and possible future loads.

A feeder is a set of conductors that originate at a main distribution center and supplies one or more secondary, or one or more branch circuit distribution centers.

Commercial and industrial distribution systems use several methods to transport electrical energy. These methods may include heavy conductors run in trays or conduit. Once installed, cable and conduit assemblies are difficult to change. Power may also be distributed using bus bars in an enclosure. This is referred to as busway.

Fuse Ratings And Classifications

Each fuse has a specific ampere rating, which is its continuous current-carrying capability. The ampere rating of the fuse chosen for a circuit usually should not exceed the current-carrying capacity of the circuit. For example, if a circuit’s conductors are rated for 10 amperes, the largest fuse that should be selected is 10 amperes.

However, there are circumstances where the ampere rating is permitted to be greater than the current-carrying capacity of the circuit. For example, motor and welder circuits’ fuse ratings can exceed conductor ampacity to allow for inrush currents and duty cycles within limits.

The voltage rating of a fuse must be at least equal to the circuit voltage. The voltage rating of a fuse can be higher than the circuit voltage, but never lower. A 600 volt fuse, for example, could be used in a 480 volt circuit, but a 250 volt fuse could not be used in a 480 volt circuit.

Fuses are also rated according to the level of fault current they can interrupt. This is referred to as ampere interrupting capacity (AIC). A fuse for a specific application should be selected so that it can sustain the largest potential short circuit current that could occur in the application. Otherwise, the fuse could rupture, causing extensive damage, if the fault current exceeded the interrupting ability of the fuse.

Fuses

Circuit protection would be unnecessary if overloads and short circuits could be eliminated. Unfortunately, they do occur. To protect a circuit against these destructive currents, a protective device must determine when a fault condition develops and automatically disconnect the electrical equipment from the power source. A fuse is the simplest device for interrupting a circuit experiencing an overload or a short circuit.

A typical fuse consists of an element electrically connected to ferrules. These ferrules may also have attached end blades.The element provides a current path through the fuse. It is enclosed in a tube, and surrounded
by a filler material.

Current flowing through the fuse element generates heat, which is absorbed and dissipated by the filler material. When an overcurrent occurs, temperature in the element rises. In the event of a transient overload condition the excess heat is absorbed by the filler material. However, if a sustained overload occurs, the heat will eventually melt open an element segment. This will stop the flow of current.

Fuses have an inverse time-current characteristic. The greater the overcurrent, the less time it takes for the fuse to open. This is referred to as the clearing time of the fuse.

Short-circuit current can be several thousand amperes, and generates extreme heat. When a short circuit occurs several element segments can melt simultaneously, which helps remove the load from the power source quickly. Short-circuit current is typically cut off in less than half a cycle, before it can reach its full value.

Non time-delay fuses provide excellent short circuit protection. However, short-term overloads, such as motor starting current, may cause nuisance openings of nontime-delay fuses. For this reason, they are best used in circuits not subject to large transient surge currents. Nontime-delay fuses usually hold 500% of their rating for approximately one-fourth of a second, after which the current-carrying element melts. This means that these fuses should not be used in motor circuits, which often have starting currents greater than 500%.

Time-delay fuses provide both overload and short-circuit protection. Time-delay fuses usually allow five times the rated current for up to ten seconds. This is normally sufficient time to allow a motor to start without nuisance opening of the fuse. However, if an overload condition occurs and persists, the fuse
will open.

Why need circuit protection?

Current flow in a conductor always generates heat. The greater the current flow in a given size conductor, the hotter the conductor. Excess heat is damaging to electrical components and conductor insulation. For this reason conductors have a rated continuous current carrying capacity, or ampacity. Overcurrent protection devices, such as fuses, are used to protect conductors from excessive current flow. Fuses are designed to keep the flow of current in a circuit at a safe level to prevent the circuit conductors from overheating.

Excessive current is referred to as overcurrent.overcurrent is any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault

An overload occurs when too many devices are operated on a single circuit, or if a piece of electrical equipment is made to work harder than it is designed to work.

Every circuit requires some form of protection against overcurrent and the heat it produces. For example, high levels of heat to insulated wire can cause the insulation to break down and flake off, exposing the conductors.

When exposed conductors touch, a short circuit occurs, and the circuit resistance drops to nearly zero. Because of this very low resistance, short circuit current can be thousands of times higher than normal operating current.

The heat generated by short-circuit current can rise to dangerous levels quickly, causing extensive damage to
conductors and connected equipment. This heat-generating current must be interrupted as soon as possible after a short circuit occurs. Slight overcurrents can be allowed to continue for some period of time, but as the overcurrent magnitude increases, the protection device must act more quickly. In order to minimize costly damage, outright short circuits must be interrupted almost instantaneously.

When a short circuit occurs in an unprotected circuit, current will continue to flow until the circuit is damaged, or until the power is removed manually. The peak short-circuit current of the first cycle is the greatest, and is referred to as peak let-through current . The electromagnetic force associated with this current can cause mechanical damage to electrical components.

A properly used overcurrent protecting device will open the circuit quickly, limiting peak let-through current and energy.

Safety Switches

A switch is generally used for two purposes:
1) A disconnecting means for a service entrance
2) A disconnecting means and fault protection for motors

A safety switch is simply a switch located in its own enclosure. The enclosure provides a degree of protection to personnel against incidental contact with live electrical equipment. It also provides protection to the enclosed equipment against specific environmental conditions. Safety switches may consist of a switch only, or may consist of a switch and fuses. There are two families of Siemens safety switches: general duty and heavy duty.

Safety switches can be used in any number of applications.for example, requires that a disconnecting means shall be located in sight from the motor location and the driven machinery location.Regardless of where the safety switch is used, the function is to provide a means to connect and disconnect the load from its source of
electrical power.

With power removed the operator can safely service the machinery without coming into contact with live electrical components or having the motor accidently start.

A switch with no associated fuses is referred to as a non-fusible safety switch. A non-fusible safety switch has no circuit protection capability. It simply provides a convenient means to open and close a circuit. Opening the circuit disconnects the load from its source of electrical power, while closing the circuit
connects the load. Circuit protection must be provided by external overcurrent devices such as circuit breakers or fuses.

A safety switch can be combined with fuses in a single enclosure. This is referred to as a fusible safety switch. The switch provides a convenient means to manually open and close the circuit, while the fuse provides overcurrent protection.

LOGO Logic Module

LOGO! is a logic module used to perform control tasks. The module is compact and user friendly, providing a cost-effective solution for the end user.

In the past, many of these control tasks were solved with contactor or relay controls. This is often referred to as hard-wired control. Circuit diagrams had to be designed and electrical components specified and installed. A change in control function or system expansion could require extensive component changes and rewiring.

Many of the same tasks can be performed with LOGO!. Initial hard-wiring, though still required, is greatly simplified. Modifying the application is as easy as changing the program via the keypad located on the front of the LOGO!. Likewise, control programs can be created and tested before implementation via a PC software program. Once the program is performing per specification, the transfer to LOGO! is as simple
as plugging in a cable.

LOGO! accepts a variety of digital inputs, such as pushbuttons, switches, and contacts. LOGO! makes decisions and executes control instructions based on the user-defined program. The instructions control various outputs. The outputs can be connected to virtually any type of load such as relays, contactors, lights, and small motors.

Pressure Switches

Pressure switches are control devices that respond to changes in pressure of liquid or air. The liquid or air is referred to as fluid pressure. They open or close electrical contacts in response to pressure changes by either turning on or off a motor, opening or closing louvers, or signaling a warning light or horn. For loads up to 5 HP the pressure switches may handle the current directly.
For larger loads the pressure switch is used to energize relays, contactors, or motor starters, which then energize the load.

The basic components of a pressure switch are Electrical contacts are operated by the movement of a diaphragm against the force of a spring. The contacts may be normally open (NO) or normally closed (NC). The spring setting determines how much fluid pressure is required to operate the contacts.

Pressure switches are frequently used to maintain a specified pressure range in a storage tank. Storage tanks can be used to hold a liquid, such as water, or a gas, such as air.

Pressure switches are designed to operate within a specified pressure range, usually given in pounds per square inch (PSI).

Reverse action pressure switches cut-in on a rising pressure. They are designed to ground the ignition on gas engine driven pumps and compressors when the maximum pressure has been reached.

Timing Relays

A timing relay has two major functions: On-delay and Off-delay timing. An arrow is used to denote the function of the timer. An arrow pointing up indicates an On-delay timing action. An arrow pointing down indicates an Off-delay timing action.
On-delay and Off-delay timers can turn their connected loads on or off, depending on how the timer’s output is wired into the circuit. On-delay indicates that once a timer has received a signal to turn on, a predetermined time must pass before the timer’s contacts change state. Off-delay indicates that once a timer has received a signal to turn off, a predetermined time must pass before the timer’s contacts change state.

Control Relays

Relays are widely used in control circuits. They are used for switching multiple control circuits and for controlling light loads such as starting coils, pilot lights, and audible alarms.

 The operation of a control relay is similar to a cont-actor. In the following example a relay with a set of normally open (NO) contacts is used. When power is applied from the control circuit, an electromagnetic coil is energized. The resultant electromagnetic field pulls the armature and movable contacts toward the electromagnet closing the contacts. When power is removed, spring tension pushes the armature and movable contacts away from the electromagnet opening the contacts.

A relay can contain normally open, normally closed, or both types of contacts. The main difference between a control relay and a cont-actor is the size and number of contacts. The contacts in a control relay are relatively small because they need to handle only the small currents used in control circuits. There are no power contacts. Also, unlike a cont-actor, each contact in a control relay controls a different circuit. In a cont-actor, they all control the starting and stopping of the motor. Some relays have a greater number of contacts than are found in the typical cont-actor. The use of contacts in relays can be complex. There are three words which must be understood when dealing with relays.


Pole describes the number of isolated circuits that can pass through the relay at one time. A single-pole circuit can carry current through one circuit. A double-pole circuit can carry current through two circuits simultaneously. The two circuits are mechanically connected so that they open or close at the same time.

Throw is the number of different closed-contact positions per pole. This is the total number of different circuits each pole controls.

The following abbreviations are frequently used to indicate
contact configurations:
Single-Pole, Single-Throw
Single-Pole, Double-Throw
 Double-Pole, Single-Throw
Double-Pole, Double-Throw

Break is the number of separate contacts the switch contacts use to open or close individual circuits. If the switch breaks the circuit in one place, it is a single-break. If the relay breaks the circuit in two places, it is a double-break.

Signalling columns

Signalling columns can be mounted locally on individual machines, making it possible for the operating personnel to monitor production stations from a distance. Individual modules, or elements, are connected together. Various visual elements are available, including strobe lights, steady or flashing lights, and incandescent or LED lights. Lenses for the light elements are available in red, yellow, green, blue, and clear. Audible elements for the 8WD43 include a siren and a buzzer. Audible elements for the 8WD42 include a buzzer. In addition, a communication element is available allowing the signalling column to communicate with PLCs or computers through the Actuator Sensor Interface (ASI) network. Up to 10 elements can be used on a signalling column.

Pilot lights

Pilot lights provide visual information at a glance of the circuit’s operating condition. Pilot lights are normally used for “ON/OFF” indication, caution, changing conditions, and alarm signaling.

Pilot lights come with a color lens, such as red, green, amber, blue, white, or clear. A red pilot light normally indicates that a system is running. A green pilot light normally indicates that the system is off or d-energized. For example, a red pilot light located on a control panel would give visual indication that a motor was running. A green pilot light would give visual indication that a motor was stopped.

Selector Switches

Selector switches are also used to manually open and close contacts. Selector switches can be maintained, spring return or key operated. Selector switches are available in two-, three-, and four-position types. The basic difference between a push button and a selector switch is the operator mechanism. With a selector switch the operator is rotated to open and close contacts. Contact blocks used on push-buttons are interchangeable with those on used on selector switches.
Selector switches are used to select one of several circuit possibilities such as manual or automatic operation, low or high speed, up or down, right or left, and stop or run. The Siemens 22 mm selector switches can handle up to a maximum of 6 circuits

Pushbuttons

A pushbutton is a control device used to manually open and close a set of contacts. Pushbuttons are available in a flush mount, extended mount, with a mushroom head, illuminated or non illuminated. Pushbuttons come with either normally open, normally closed, or combination contact blocks. The Siemens 22 mm pushbuttons can handle up to a maximum of 6 circuits. The Furnas 30 mm pushbutton can handle up to a maximum of 16 circuits.

Normally Open Pushbuttons:
 are used in control circuits to perform various Pushbuttons functions. For example, pushbuttons can be used when starting and stopping a motor. A typical pushbutton uses an operating plunger, a return spring, and one set of contacts. The following drawing illustrates a normally open (NO) pushbutton. Normally the contacts are open and no current flows through them. Depressing the button causes the contacts to close. When the button is released, the spring returns the plunger to the open position.

Normally Closed Pushbuttons:
 Normally closed (NC) pushbuttons are also used to open and close a circuit. In the push-button’s normal position the contacts are closed to allow current flow through the control circuit. Depressing the button opens the contacts preventing current flow through the circuit. These types of pushbuttons are momentary contact pushbuttons because the contacts remain in their activated position only as long as the plunger is held depressed.


Pushbuttons are available with variations of the contact configuration. For example, a pushbutton may have one set of normally open and one set of normally closed contacts so that when the button is depressed, one set of contacts is open and the other set is closed. By connecting to the proper set of contacts, either a normally open or normally closed situation exists.

Magnetic Contactors and Starters

Most motor applications require the use of remote control devices to start and stop the motor. Magnetic contactors, similar to the ones shown below, are commonly used to provide this function. Contactors are also used to control distribution of power in lighting and heating circuits.

Magnetic contactors operate utilizing electromagnetic principles. A simple electromagnet can be fashioned by
winding a wire around a soft iron core. When a DC voltage is applied to the wire, the iron becomes magnetic. When the DC voltage is removed from the wire, the iron returns to its nonmagnetic state. This principle is used to operate magnetic contactors.

There are two circuits involved in the operation of a contactor : the control circuit and the power circuit. The control circuit is connected to the coil of an electromagnet, and the power circuit is connected to the stationary contacts.

The operation of this electromagnet is similar to the operation of the electromagnet we made by wrapping wire around a soft iron core. When power is supplied to the coil from the control circuit, a magnetic field is produced magnetizing the electromagnet. The magnetic field attracts the armature to the magnet, which in turn closes the contacts. With the contacts closed, current flows through the power circuit from the line to
the load. When the electromagnet’s coil is deenergized, the magnetic field collapses and the movable contacts open under spring pressure. Current no longer flows through the power circuit.

Contactors are used to control power in a variety of applications. When applied in motor-control applications, contactors can only start and stop motors. Contactors cannot sense when the motor is being loaded beyond its rated conditions. They provide no overload protection. Most motor applications require
overload protection. However, some smaller-rated motors have overload protection built into the motor (such as a household garbage disposal). Overload relays, similar to the one shown below, provide this protection. The operating principle, using heaters and bimetal strips, is similar to the overload relays.

Contactors and overload relays are separate control devices. When a contactor is combined with an overload relay, it is called a motor starter.

Overload protection(2)

Fuses and circuit breakers are protective devices used to protect circuits against short circuits, ground faults, and overloads. In the event of a short circuit, a properly sized fuse or circuit breaker will immediately open the circuit. There is, however, a dilemma that occurs when applying fuses and circuit breakers in motor control circuits. The protective device must be capable of allowing the motor to exceed its full load
rating for a short time. Otherwise, the motor will trip each time it is started. In this situation it is possible for a motor to encounter an overload condition which does not draw enough current to open the fuse or trip the circuit breaker. This overload condition could easily cause enough heat to damage the motor.

Overload Relays
Overload relays are designed to meet the special protective needs of motor control circuits. Overload relays:
• allow harmless temporary overloads, such as motor starting, without disrupting the circuit
• will trip and open a circuit if current is high enough to cause motor damage over a period of time
• can be reset once the overload is removed.

Bimetal Overloads:
 Overload protection is accomplished with the use of a bimetal strip. This component consists of a small heater element wired in series with the motor and a bimetal strip that can be used as a trip lever. A bimetal strip is made of two dissimilar metals bonded together. The two metals have different thermal expansion characteristics, so the bimetal bends at a given rate when heated.


Under normal operating conditions the heat generated by the heater element will be insufficient to cause the bimetal strip to bend enough to trip the overload relay.

As current rises, heat also rises. The hotter the bimetal becomes, the more it bends. In an overload condition the heat generated from the heater will cause the bimetal strip to bend until the mechanism is tripped, stopping the motor.

Some overload relays that are equipped with a bimetal strip are designed to automatically reset the circuit when the bimetal strip has cooled and reshaped itself, restarting the motor. If the cause of the overload still exists, the motor will trip again and reset at given intervals. Care must be exercised in the selection of this type of overload as repeated cycling will eventually damage the motor.

Overload Protection(1)

Overload Protection
  Current and Temperature Current flow in a conductor always generates heat due to resistance. The greater the current flow, the hotter the conductor. Excess heat is damaging to electrical components. For that reason, conductors have a rated continuous current carrying capacity or ampacity. Over-current protection devices
are used to protect conductors from excessive current flow.

Thermal overload relays are designed to protect the conductors (windings) in a motor. These protective devices are designed to keep the flow of current in a circuit at a safe level to prevent the circuit conductors from overheating,excessive current is referred to as over-current.

over-current is any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault

Short Circuits:
 When two bare conductors touch, a short circuit occurs. When a short circuit occurs, resistance drops to almost zero. Short-circuit current can be thousands of times higher than normal operating current.

Overload Conditions:
 An overload occurs when too many devices are operated on a single circuit or a piece of electrical equipment is made to work harder than it is designed for. For example, a motor rated for 7 amperes may draw 15, 30, or more amperes in an overload condition. In the following illustration a package has become
jammed on a conveyor causing the motor to work harder and draw more current. Because the motor is drawing more current it heats up. Damage will occur to the motor in a short time if the problem is not corrected or the circuit is not shut down by the overload relay.

Temporary Overload Due to starting current:
 Electric motors are rated according to the amount of current to Starting Current they will draw at full load. When most motors start, they draw current in excess of the motor’s full-load current rating. Motors are designed to tolerate this overload current for a short period of time. Many motors require 6 times (600%) the full-load current rating to start. Some newer, high-efficiency motors may require higher starting currents. As the motor accelerates to operating speed, the current drops off quickly. The time it takes for a motor to accelerate to operating speed depends on the operating characteristics of the motor and the driven load. A
motor, for example, might require 600% of full-load current and take 8 seconds to reach operating speed.

Control Circuits

controller is a device or group of devices that serves to govern, in some
predetermined manner, the electrical power delivered to the
apparatus to which it is connected.

Control, as applied to control circuits, is a broad term that means anything from a simple toggle switch to a complex system of components which may include relays, contactors, timers, switches, and indicating lights. Every electrical circuit for light or power has control elements. One example of a simple control circuit is a light switch used to turn lights on and off.

Motor control  can be used to start and stop a motor and protect the motor, associated machinery, and personnel. In addition, motor controllers might also be used for reversing, changing speed, jogging, sequencing, and pilot-light indication. Control circuits can be complex: accomplishing high degrees of automatic and precise machine operation.

Control is considered to be manually operated when someone must initiate an action for the circuit to operate. For example, someone might have to flip the switch of a manual starter to start and stop a motor.

While manual operation of machines is still common practice, many machines are started and stopped automatically. Frequently there is a combination of manual and automatic control. A process may have to be started manually, but may be stopped automatically.

The elements of a control circuit include all of the equipment and devices concerned with the circuit function. This includes enclosures, conductors, relays, contactors, pilot devices, and over current-protection devices. The selection of control equipment for a specific application requires a thorough understanding of controller operating characteristics and wiring layout. The proper control devices must be selected and integrated into the overall plan.

Contact symbols are used to indicate an open or closed path of current flow as an example NO(normally open)contact, NC(normally closed)contact, switch and Push-button.

Circuit Breakers

 Circuit breakers supply a manual means of energizing and de-energizing a circuit. In addition, circuit breakers provide automatic over-current protection of a circuit. Siemens residential circuit breakers are available with current ratings from 15-125 amps and a voltage rating of 120/240 volts. In residential applications, single-pole breakers protect 120 volt circuits; two-pole breakers protect 240 volt circuits.

circuit breaker/surge arrester mounts in a load center similarly to a conventional circuit breaker. This
device protects (defend)electronic equipment, such as televisions or computers, from electrical surges on the system. Surges can come from electrical equipment, switching, or lightning.

The ground fault circuit interrupter (GFCI) is required on certain residential receptacles, such as bathroom receptacles, receptacles located within six feet of a kitchen sink, and outdoor receptacles. The GFCI is designed to interrupt a circuit when a ground fault occurs. Often the GFCI is mounted at the
receptacle. When this is not feasible, a Siemens GFCI circuit breaker is installed in the load center.

GFCI devices are designed to protect a person from getting a shock when touching an ungrounded appliance. Arc Fault Circuit Interrupters (AFCI), in comparison, protect against a fire being started from an unintended arc. An arc fault occurs when a current-carrying conductor has an arching condition to ground
or another conductor. An AFCI device is intended to provide protection from the effects of arc faults by recognizing the characteristics unique to arcing and de-energizing the circuit when an arc fault is detected. The arc generated will cause the AFCI to trip. Arcs normally generated from electric equipment such as a light switch or power drill will not cause the AFCI to trip.

Electrical Power

Power, originating at a power generating plant, is distributed to residential, commercial, and industrial customers through various transmission lines and substations.

Power Sources :
There are several sources used to produce power. Coal, oil, and uranium are fuels used to convert water into steam which in turn drives a turbine. Some utilities also use gas turbines, or
both gas and steam turbines, for combined cycle operation. The output shaft of the turbine is connected to an AC generator. The AC generator is rotated by the turbine. It is the AC generator which converts the mechanical energy into electrical energy.

Hydroelectric Power:
 Hydroelectric power plants use mechanical energy from falling
water to turn the turbine.

AC Generators:
 AC generators operate on the theory of electromagnetic induction. This simply means that when conductors are moved through a magnetic field a voltage is induced into the conductors. A basic generator consists of a magnetic field, an armature, slip rings, brushes, and some type of resistive load.
An armature is any number of conductive wires (conductors) wound in loops which rotate through the magnetic field.

If the rotation of the AC generator were tracked through a complete revolution of 360°, it could be seen that during the first quarter of a revolution voltage would increase until it reached a maximum positive value at 90°. Voltage would decrease during the second quarter of a revolution until it reached zero at 180°. During the third quarter of a revolution, voltage would increase in the opposite direction until it reached
a maximum negative value at 270°. During the last quarter of a revolution, voltage would decrease until it reached zero at 360°.
This is one complete cycle or one complete alternation between positive and negative. If the armature of the AC generator were rotated 3600 times per minute (RPM) we would get 60 cycles
of voltage per second, or 60 hertz.


Energy Transfer :
The role of the generator just described is to change mechanical energy into electrical energy. In order for this energy to be useful, however, it must be transmitted to the utility’s customers via transmission lines. The most efficient way to do this is to increase the voltage while at the same time reducing
the current. This is necessary to minimize the energy lost in heat on the transmission lines. These losses are referred to as I2R (I-squared-R) losses since they are equal to the square of the current times the resistance of the power lines. Once the electrical energy gets near the end user, the utility will need to step down the voltage to the level needed by the user.

Programmable logic controller(PLC)

The purpose of a PLC was to directly replace and substitute electromechanical relays as logic elements, substituting instead a solid-state digital computer with a stored program, able to emulate the interconnection of many relays to perform certain logical tasks.

A PLC has many "input" terminals, through which it explain"high" and "low" logical states
from sensors and switches. It also has many output terminals, through which it outputs "high"
and "low" signals to power lights, contactors,solenoids, relays, small motors, and other devices lending
themselves to on/off control. In an effort to make PLCs easy to program, their programming
language was designed to resemble ladder logic diagrams. Thus, an industrial electrician or electrical
engineer accustomed to reading ladder logic schematics would feel comfortable programming a PLC
to perform the same control functions.

PLCs are industrial computers, and as such their input and output signals are typically 120 volts
AC, just like the electromechanical control relays they were designed to replace. Although some
PLCs have the ability to input and output low-level DC voltage signals of the magnitude used in
logic gate circuits, this is the exception and not the rule.

Signal connection and programming standards vary somewhat between different models of PLC,
but they are similar enough to allow a "generic" introduction to PLC programming here. The
following illustration shows a simple PLC, as it might appear from a front view. Two screw terminals
provide connection to 120 volts AC for powering the PLC's internal circuitry, labeled L1 and L2. Six
screw terminals on the left-hand side provide connection to input devices, each terminal representing
a different input "channel" with its own "X" label. The lower-left screw terminal is a "Common"
connection, which is generally connected to L2 (neutral) of the 120 VAC power source.

Inside the PLC housing, connected between each input terminal and the Common terminal, is an
op-to-isolator device (Light-Emitting Diode) that provides an electrically isolated "high" logic signal
to the computer's circuitry (a photo-transistor interprets the LED's light) when there is 120 VAC
power applied between the respective input terminal and the Common terminal. An indicating LED
on the front panel of the PLC gives visual indication of an "energized" input.

Output signals are generated by the PLC's computer circuitry activating a switching device
(transistor, TRIAC, or even an electromechanical relay), connecting the "Source" terminal to any
of the "Y-" labeled output terminals. The "Source" terminal, correspondingly, is usually connected
to the L1 side of the 120 VAC power source. As with each input, an indicating LED on the front
panel of the PLC gives visual indication of an "energized" output.

The actual logic of the control system is established inside the PLC by means of a computer pro-
gram. This program dictates which output gets energized under which input conditions. Although
the program itself appears to be a ladder logic diagram, with switch and relay symbols, there are
no actual switch contacts or relay coils operating inside the PLC to create the logical relationships
between input and output. These are imaginary contacts and coils, if you will. The program is
entered and viewed via a personal computer connected to the PLC's programming port.

Equally important to understand is that the personal computer used to display and edit the
PLC's program is not necessary for the PLC's continued operation. Once a program has been
loaded to the PLC from the personal computer, the personal computer may be unplugged from
the PLC, and the PLC will continue to follow the programmed commands. I include the personal
computer display in these illustrations for your sake only, in aiding to understand the relationship
between real-life conditions (switch closure and lamp status) and the program's status ("power"
through virtual contacts and virtual coils).


The true power and versatility of a PLC is revealed when we want to alter the behavior of a
control system. Since the PLC is a programmable device, we can alter its behavior by changing the
commands we give it, without having to recon figure the electrical components connected to it. For
example, suppose we wanted to make this switch-and-lamp circuit function in an inverted fashion:
push the button to make the lamp turn off, and release it to make it turn on. The "hardware"
solution would require that a normally-closed pushbutton switch be substituted for the normally open switch currently in place. The "software" solution is much easier: just alter the program so
that contact X1 is normally-closed rather than normally open.

SOLID-STATE RELAYS

As versatile as electromechanical relays can be, they do suffer many limitations. They can be expensive to build, have a limited contact cycle life, take up a lot of room, and switch slowly, compared
to modern semiconductor devices. These limitations are especially true for large power contactor
relays. To address these limitations, many relay manufacturers offer "solid-state" relays, which use
an SCR, TRIAC, or transistor output instead of mechanical contacts to switch the controlled power.
The output device (SCR, TRIAC, or transistor) is optically-coupled to an LED light source inside
the relay. The relay is turned on by energizing this LED, usually with low-voltage DC power. This
optical isolation between input to output rivals the best that electromechanical relays can offer.

Being solid-state devices, there are no moving parts to wear out, and they are able to switch on
and off much faster than any mechanical relay armature can move. There is no sparking between
contacts, and no problems with contact corrosion. However, solid-state relays are still too expensive
to build in very high current ratings, and so electromechanical contactors continue to dominate that
application in industry today.

One significant advantage of a solid-state SCR or TRIAC relay over an electromechanical device
is its natural tendency to open the AC circuit only at a point of zero load current. Because SCR's
and TRIAC's are thyristors, their inherent hysteresis maintains circuit continuity after the LED is
de-energized until the AC current falls below a threshold value (the holding current). In practical
terms what this means is the circuit will never be interrupted in the middle of a sine wave peak.

Such untimely interruptions in a circuit containing substantial inductance would normally produce
large voltage spikes due to the sudden magnetic field collapse around the inductance. This will not
happen in a circuit broken by an SCR or TRIAC. This feature is called zero-crossover switching.

One disadvantage of solid state relays is their tendency to fail "shorted" on their outputs, while
electromechanical relay contacts tend to fail "open." In either case, it is possible for a relay to
fail in the other mode, but these are the most common failures. Because a "fail-open" state is
generally considered safer than a "fail-closed" state, electromechanical relays are still favored over
their solid-state counterparts in many applications.

Protective relays

A special type of relay is one which monitors the current, voltage, frequency, or any other type of
electric power measurement either from a generating source or to a load for the purpose of triggering
a circuit breaker to open in the event of an abnormal condition. These relays are referred to in the
electrical power industry as protective relays.

The circuit breakers which are used to switch large quantities of electric power on and of are
actually electromechanical relays, themselves. Unlike the circuit breakers found in residential and
commercial use which determine when to trip (open) by means of a bimetallic strip inside that bends
when it gets too hot from over current, large industrial circuit breakers must be "told" by an external
device when to open. Such breakers have two electromagnetic coils inside: one to close the breaker
contacts and one to open them. The "trip" coil can be energized by one or more protective relays,
as well as by hand switches, connected to switch 125 Volt DC power. DC power is used because it
allows for a battery bank to supply close/trip power to the breaker control circuits in the event of
a complete (AC) power failure.

Protective relays can monitor large AC currents by means of current transformers (CT's), which
encircle the current-carrying conductors exiting a large circuit breaker, transformer, generator, or
other device. Current transformers step down the monitored current to a secondary (output) range
of 0 to 5 amps AC to power the protective relay. The current relay uses this 0-5 amp signal to power
its internal mechanism, closing a contact to switch 125 Volt DC power to the breaker's trip coil if
the monitored current becomes excessive.


Likewise, (protective) voltage relays can monitor high AC voltages by means of voltage, or
potential, transformers (PT's) which step down the monitored voltage to a secondary range of 0
to 120 Volts AC, typically. Like (protective) current relays, this voltage signal powers the internal
mechanism of the relay, closing a contact to switch 125 Volt DC power to the breaker's trip coil is
the monitored voltage becomes excessive.
There are many types of protective relays, some with highly specialized functions. Not all
monitor voltage or current, either. They all, however, share the common feature of outputting a
contact closure signal which can be used to switch power to a breaker trip coil, close coil, or operator
alarm panel. Most protective relay functions have been categorized into an ANSI standard number
code.