There are six areas of helplessness regarding the dependable tripping of a circuit breaker. These are failure of:
The dc supply
The tripping contact
Wiring runs
The circuit breaker auxiliary contact
The trip coil
The circuit breaker mechanism
A trip circuit supervision relay is provided on each circuit breaker unit. it monitors the trip circuit and its power supply continuously ,both when the circuit breaker is closed and also when its open.
A trip circuit supervision schematic diagram in show as below figure. The trip circuit supervision relay consists of three elements: ‘a’, ‘b’, ‘c’.The trip circuit is monitored by passing a small current derived from the tripping supply through the trip coil and all associated trip circuit wiring in series with the coils of relay ‘a’ and ‘b’ or both.This current is sufficient to operate relays ‘a’ and ‘b’ even when they are connected in series but being limited by resistors, is not high enough to trip the breaker .Relay ‘a’ and ‘b’ are energised individually when the contacts across which they are connected are not closed.Realy ‘c’ is a hand rest flag relay with alarm contacts which are closed if the trip circuit is healthy.
If the circuit breaker is open, both relays ‘a’ and ‘b’ are energised in series. If the circuit breaker is open but a trip initiation signal is present, say from a hand reset tripping relay, then relay ‘b’ alone is energised .When the circuit breaker is closed ,relay ‘b’ is de-energised and only relay ‘a’ monitors the trip circuit.
If the trip supply fails or the breakers fails to open when a trip signal is initiated, then both relay ‘a’ and ‘b’ are de-energised and an alarm is initiated when relay ‘c’ releases. This happens when
The trip supply fails
Any parts of the trip circuit is open-circuited
The breaker fails to open
The trip circuit supervision auxiliary supply fails.
The opening of the contacts of relays ‘a’ and ‘b’ delayed about 400ms to allow for transient dips in the 110vdc supply and to allow time for the circuit breaker to open without initiating an alarm.
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