Ground Detector:

The ground detector is an instrument which is used to detect conductor insulation resistance to ground.  An ohm meter, or a series of lights, could be used to detect the insulation strength of an ungrounded distribution system. Many power distribution systems in use presently are of the grounded variety; therefore, a few ungrounded systems still exist.

Within the ohm meter method, a DC voltage is applied to the conductor.  If a leakage path exists among the conductor insulator and ground, a current will flow by the ground to the ohm meter proportional to the insulation resistance of the conductor.

Figure: Simple Ohm Meter Ground Detector

Within the ground detector lamp method was show in above, a set of three lamps linked by transformers to the system is used.  For check for grounds, the switch is closed and the brilliance of the lamps is observed. No ground exists and all the lamps receive the similar voltage if the lamps are equally bright. If anyone lamp is dark and the other two lamps are brighter, the phase in that the darkened lamp is in is grounded.  Within this case, the primary winding of the transformer is shorted to ground and receives no voltage.

Figure: Ground Detector Lamp Circuit