Memorable Breaker Trip

[wiigelec]

Working on a near total outage in one of our large plants in order to upgrade DCS. After upgrade in process of bringing plant back up, testing equipment, interlocks, etc. Walk into control room and ask if there is anything they need us to look at and the entire plant suddenly goes dark.

To make a long story short, "Breaker Closed" lightbulb on Cutler-Hammer 1200A 480V breaker was burned out and being replaced. Apparently the lamp is in parallel with the NO trip coil contacts and is a high impedance lamp. Replacement lamp was incorrect impedance and shunted trip contacts tripping the breaker which fed the lighting transformers for the plant. Good news is we found a UPS (dead battery holder) that was no good. Talk about exciting...

 

[SG-1]

The indicator light is serving double duty as both a "Closed Breaker" indicator & a trip coil monitor. I would bet the most excitement was for the poor guy changing the bulb.

 

[richxtlc]

That is a poor design, usually there is a high Z resistor in series with the bulb to prevent just what you had from happening. Sometimes this resistor is a twist in kind and can be replaced with a metal slug if it does not have a tripping function. We almost lost a substation as a result on having a metal slug instead of a 220Kohm resistor.

 

[SG-1]

I agree using a full voltage bulb with no resistor is a bad idea. Yet some customers will actually specify this.