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

Few things are harder to put up with than the annoyance of a good example.