ECN Forum Forums General Topics of the Trade General Discussion Area Ground fault indicator lights @ old plant

Hello,

I once worked at an old mill (circa 1920's) and above the entry for the main switch gear room there were three indicator lights. These were mounted in such a way that they could be easily seen from the chief of Maintenance's office. The only time these indicators were lit is when that light's phase had a short to ground. A small 3 phase motor would barely make it glimmer. These small ones would often flicker for several hours. A shorted wire in a large branch would light it up like Christmas. Then of course that was usually followed by the sound of phase to ground "UUMMPPP" arc sound and trip.

Over the years I've often wondered how these indicators were wired. According to the head maintenance guy the bulbs had never been changed since the plant opened. He didn't know how they worked either. Any of you ever seen these type indicators before?

The only time I've ever seen these used is with an ungrounded 3 phase system. All of them were home made.

One light would be connected to each phase and the other side of the lamp would be connected to ground. Depending on the lamp wattage used, the lights would glow on the dim side. When there was a phase to ground fault, that lamp would go out and it was time to start troubleshooting.

Tom

Ground Detector lights are very common, well atleast in this part of the world.

There are many different wiring diagrams. But, the most common, for 240V ungrounded systems, uses three lamps (light bulbs). 240V lamps are connected in a wye arrangment with the center point connected to ground. This arrangement gives a voltage across each lamp of about 136V so under normal conditions the it just glows. Under a gorund connection each ungrounded lamp sees a full 240V so they are at full intensity and the faulted lamp sees 0V so it goes out. Because the lamps are running at 1/2 voltage they last forever.

Some typical variations on this circuit include: (1) a "lamp check" pushbutton installed between the lamp center point and ground, opening the switch "removes" the fault detection and all lamps will go back to be equally dim unless one of them is burnt out;(2) 2 - 240V lamps in series for use on 480V systems; (3) Transformers and 120V lamps instead of line voltage lamps.

Erickson (the metering guys) used to make a "listed" detector package.

This plant had a 575v system. The lights only lit when a ground or partial ground was evident. The stronger the current, the brighter the individual lights would light. If there was not a problem then the lights were out.

Ground detector lights are only used on ungrounded systems. A single phase to ground fault shoult not cause any real problems. The purpose of the lights is to indicate that there is a problem so that you can trouble shoot it before there is a second fault on a different phase. At that point it becomes a line to line fault.
Don

Doing this creates a resistive voltage divider under normal non fault conditions. If you got out a high impedance voltmeter (DVM for example) and measured the voltage of each phase to ground, you should find roughly the same voltage values. This drains off capacitive voltage coupling from the high voltage primaries of the transformers that source this system. This would maintain a reasonable voltage level on the system to ground while being able for the system to tolerate a single fault to ground. So the factory could avoid downtime until a convenient point of time for the electricians to fix it (like at 2AM).

[This message has been edited by wa2ise (edited 06-07-2005).]

At the wastewater treatment plant where I used to work the setup I used was 2 120volt lamps in series from each phase to ground. Under normal conditions the lamps would glow dimly (voltage to ground was about 275volts) A ground fault would extinguish the lamps on that phase and the others would go to full brightness.

We had a little discussion about these a while back.
https://www.electrical-contractor.net/ubb/Forum17/HTML/000064.html

Bjarney did a good job of 'splainin.

My guess is that it was a current transformer on each phase, like a current clamp meter, but with a small bulb instead of the meter.

Now, it sounds like a high resistance ground indicator. "The lights only lit when a ground or partial ground was evident. The stronger the current, the brighter the individual lights would light"

The lamps may have been installed to measure (indicate) the voltage drop caused by the ground current through the resistor. The higher the current, the more the voltage, and the brighter the lamps