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.
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.
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.
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