Changing breaker indicator lamps tripped a substation!

[Associate Engineer]

An electrician was replacing indicator lamps for Breaker Status, Auto Transfer Ready, 86T Healthy lights on a (480V Main – Tie – Main) Switchgear lineup. When he was installing the Auto Transfer Ready light on Bus 2, the Main 2 Breaker opened, It appears that we have installed the incorrect lamps and somehow managed to energize the trip coil on the main breakers. I have attached the control schemtic.
How to calculate the correct lamp resistance ?
And how installing wrong lamp energized the trip coils?
Thank everyone!

 

[David Castor]

The red light is in series with the trip coil. This is to provide visual indication that the trip coil has continuity and is not burned out. Normally, the resistance of the lamp plus the resistor in the indicating light assembly limits the current to a value low enough to pass through the trip coil without actually tripping the solenoid. The indicating lights should have a voltage rating equal to the control power voltage. A resistor is normally integral to the indicating light assembly. The lamp itself probably has a lower voltage rating. If only the lamp was replaced, it would be surprising that there would be enough current to trip the breaker. If the entire light was replaced, it's certainly possible. The simplest approach is to replace the lamps with the same model that is being replaced.

 

[Associate Engineer]

The red light is in series with the trip coil. This is to provide visual indication that the trip coil has continuity and is not burned out. Normally, the resistance of the lamp plus the resistor in the indicating light assembly limits the current to a value low enough to pass through the trip coil without actually tripping the solenoid. The indicating lights should have a voltage rating equal to the control power voltage. A resistor is normally integral to the indicating light assembly. The lamp itself probably has a lower voltage rating. If only the lamp was replaced, it would be surprising that there would be enough current to trip the breaker. If the entire light was replaced, it's certainly possible. The simplest approach is to replace the lamps with the same model that is being replaced.

Thank you for the details reply, we were only replacing the lamp when the trip happened. I don’t have any info about the old burned lamp, how do i calculate the right lamp resistance were it wont cause a trip again? Or do i just select the highest resistance lamp available where it will limit my current to the trip coils.

 

[David Castor]

Simplest approach is to determine the recommended lamp for the indicating light. You would need specific data on the trip coil to determine how much current it will pass before pulling in the solenoid. There's no standard. There should be model number on the indicating light. I'd start there. Or check the lamp resistance on a similar light in the same switchgear. I have to admit that I'm a little skeptical simply using a different lamp would cause a trip. Google "GE ET-16" for a typical indicating light unit. Originally these were incandescent lamps - same lamps were used for 48 V up to 250 V dc - the resistors were changed based on the voltage. There was also a neon lamp version ET-17. Now there are LED versions. You really need to know what lamp was in there or get the model number for the indicating light. Look inside the door for spare lamps.

 

[qcroanoke]

I know that if you use LED marker, turn and brake light bulbs in cars where incandescent bulbs were strange things will happen..