An undervoltage (U/V) release mechanism is fitted to all generator breakers and some main feeder circuit-breakers.
Its main function is to trip the breaker when a severe voltage dip (around 50%) occurs.
This is achieved by lifting the mechanical latch (which keeps the contacts closed) to allow the trip spring to function which opens the breaker contacts.
The U/V release on a generator circuit-breaker also prevents it being closed when the generator voltage is very low or absent.
Undervoltage relay, which may be magnetic or electronic, also provides back-up protection to short-circuit protection.
As an example, suppose during generator paralleling procedures, an attempt was made to close the wrong circuit-breaker e.g. the breaker of a stopped and dead generator.
If this circuit-breaker was closed, the dead generator would be the equivalent of short-circuit fault on the bus-bars and cause a blackout.
The undervoltage relay prevents the closure of the circuit-breaker of the dead generator.
Undervoltage protection is also required for motor starters.
The starter contactor normally provides this protection as it drops out when the supply voltage is lost or is drastically reduced.
The starter circuit will not normally allow the motor to re-start when the voltage supply is restored except when special automatic re-starting facilities are provided.
Undervoltage protection can be electro-magnetic or electronic.
Checking and calibration of generator undervoltage relays can only be done accurately by calibrated voltage injection.
A known variable voltage is directly applied to the undervoltage relay to check:
- The voltage at which the relay pulls-in
The voltage at which the relay drops-out
Generator U/V relays are usually slugged to allow a time-delay which prevents spurious tripping during transient voltage dips (typically 15%) caused by large motor starting currents.