I work at an oil refinery where we recently had a shut down on our cogen plant because a high voltage breaker tripped.The root cause failure analysis completed by my coworkers attributed the failure to use of an L.E.D. type pilot light in the trip circuit instead of an incandescent G.E. MB120.This particular trip circuit is rated at 135 volts dc. The reasoning was that a LED type lamp has zero resistance and the increased current flow caused a resistor in series with the socket to short allowing enough current to the trip coil to open the contactor. I would give my opinion but I do not want to prejudice any opinions by any one else

Kittysman,
Did you know that you require a Series Dropper resistor in line with the LED?.

AS a former tv repair man I can say that I have not seen a resistor short. They usually burn up and go open if something else shorts and draws too much current them. They act more like a fuse.
My evaluation would be similar to your coworkers. The led caused too much current to flow thus burning the resistor and tripping the coil either because the resistor went open or the current was too much.

Kittysman — I have some experience with switchgear control and protective relaying. I am a retired electrician from a site with this type of equipment that I routinely maintained. If you don’t mind, I’d like to ask a few questions. In turn, I may be able to explain typical breaker- control characteristics.

In your post, although probably incidental, is the lamp part number you reference possibly a "GE 120MB"? Another question—the terms “circuit breaker” and “contactor” are used—is there an interposing relay {device 94} in the breaker-trip circuit?

I assume that the breaker operation disconnected a synchronous AC generator stator from a three-phase AC bus. For the sake of others that may be unfamiliar with this stuff, that bus serves other equipment; often other generators, motors and transformers(s) that in turn feed other plant loads. Is that the case here?

Also, was there a “large” battery set involved? —Bjarney

I would like to thank each of you for your interest in my subject and reply to each of you. First , Trumpy could you please tell me what this Series Dropper resistor does and why you require one? The Square D lamp socket assembly had a 1000 ohm resistor in 1987 and now the new ones have 2 1000 ohm resistors. Next, jdevlin I had the same impression as you described about a resistor acting like a fuse and opening which has made me curious to say the least in my coworkers analysis. I would like to add that this LED was not DC rated . Bjarney , This lamp is a GE 120MB,I should have said contactor 4160volt , to the best of my memory there are at least 3 contacts that can close to open the contactor I think one was a 52.I will bring a copy of the print home tomorrow to be more specific .Yes the contactor does disconnect the generator from the bus. Lastly there is a large string of batteries with the UPS.I hope to hear from all of you soon. Thanks Kittysman

Examples of breaker indicator lamps used in electric utility switchgear are:
http://www.geindustrial.com/pm/catalog/test/et1617.pdf incandescent lamp
http://www.geindustrial.com/pm/catalog/test/et16.pdf led version

In a trip circuit, a series resistor external to the lamp and lamp base prevents trip initiation if the lamp fails shorted, or is inadvertently shorted out or broken during on-line maintenance.

The lampholder resistor for 125V systems is usually 2kÙ. The resistor external to the lamp and lamp socket is important—for a based-led replacement, the assembly may also fail shorted in its semiconductor junctions, although not a common failure mode. [The LED ET-16 lamp assembly has a bridge rectifier for either-polarity or AC operation.]

The [usually green] lamp-resistor assembly is paralleled across trip contacts of the manual control switch and other contacts in protective relays. If the lamp is not burning, the it warrants immediate investigation, for manual or automatic tripping may be “dead.”

Bjarney,thanks again for your information. I think that this could verify my coworkers decision to remove all LED type lamps from use in my plant as the lamp sockets in our gear are not equipped with a resistor external to the socket. I did give you a bit of misinformation in my last post,these are the relays that can trip this breaker: 50/51 and 51N. I found that the LED lamp installed was not rated for DC use nor are the sockets rated for LED use as per your information. I would hope switch gear manufacturers would warn others not to change to LED lamps unless they know the lamp and the socket are rated for LED use .In my companys case it took four hours to put the cogen back on line. Ouch ! there goes our Christmas bonus. Thanks Kittysman

Some processes don’t do well without an orderly shutdown and a scripted restart. [I dumped a station serving a running linear accelerator. People can surely get excited sometimes.]

Kittysman,
An LED (Light Emitting Diode), is a device that normally runs on a voltage as low as 2 Volts.
You cannot run an LED on AC voltage, as this will reverse bias the diode and destroy it.
The series dropper resistor is required to lower the voltage supplied to the LED, as most of the voltage, is "dropped" across this resistor.
A formula to find the value of this resistor,would go like this:
R= (E-1.7)x 1000/I
Where E= Supply Voltage(DC)
I=LED forward current in milliamperes.
With this calc, you shouldn't go wrong!.
The 1.7 in the formula, is the forward bias voltage of the LED, itself, please make changes, according to your particular LED.

[This message has been edited by Trumpy (edited 12-22-2002).]