Circuit Breaker Schematic

[mull982]

I was looking at the breaker schematic for a 5kV breaker the other day when I came across something odd it the control circuit. The trip coil of the breaker (52T) has a Red indication light "R" in series with this trip coil.

The part that I found strange was the fact that the red light was lit when the breaker was in operation. Looking at this red light in the circuit at first I thought that since this light was in series with the trip coil the light would be lit but also cause the trip coil to trip the breaker (Breaker is tripped only by switch or overload contact)

After thinking about it I noticed what looks like a resistor in series with the light. I then though that maybe the light and resistor combination had a much higher impedance than the trip coil and therefore the majority of the voltage appears across the resistor and light and very little across the trip coil thus causing it not to operate. The voltage across the light in this combination is enough to power the light however.

When the switch or overload contacts close, the full voltage is seen across the trip coil and thus it will operate.

Am I right with my thinking in regards to the higher impedance of the light causing the trip coil not to trip?

 

[StephenSDH]

Looks like you are right. The current draw of the light/resister isn't enough to activate the solenoid. The solenoid provides such a low impedence that most of the voltage drop is over the light and resister.

 

[Cold Fusion]

Adding to what Stephen said, the light is part of a supervisory circuit - lets you know the trip ckt is intact and there is trip power available. Sometimes the supervisory ckt is connected to an alarm input in the relay for a remote indication if it fails.

cf

 

[zog]

Yes you are correct, this is a common practice with breakers that have remote position indication lights.

 

[72.5kv]

Common practice. Once the breaker trips, that circuit it open by the breaker "a" contact and the "b" (green light) contact is made up to give breaker open status. In this circuit the red light also monitor the trip coil. If the breaker is close and the red light is not on, then there is the possibility that that trip coil has opened up.