ungrounded system - insulation monitor

[snorkles]

Hi. Our 480 volt ungrounded system utilizes ground detection lights and also an insulation monitor with a Megohm meter.This insulation monitor unit is hard wired to only one of 3 bus phases. My question: How does an insulation monitor work?

 

[Skogsgurra]

There are different methods to do this. All of them rely on the fact that the system is fed from a transformer secondary and that the DC resistance between the phases is in the milliohms range, which makes low frequency measurement from one phase to ground possible.

The Bender system is in wide use in ungrounded industrial systems in Europe and elsewhere. Bender uses a low frequency square wave current that is injected in the (mostly fictive) floating zero. The resulting voltage is measured and the total system-to-ground resistance calculated as the classic U/I quantity.

The Bender system measures with a 40 V maximum peak value and has 180 kohm internal resistance, which reduces maximum injected current to 220 microamperes.

The low frequency (lower than 1 Hz) makes measurements in systems with high capacitance between system and ground possible. Standard setting allows uo to 150 microfarads and the devices can be set to handle up to 500 microfarads.

There are other makes. But the Bender systems are the ones that I know best.



Gunnar Englund

http://www.gke.org