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.