Simplex 4020 Negative Earth Ground Fault

My dad and his maintenance team have been working to diagnose an intermittent ground fault that has existed on their 4020 system for roughly 2 years.

They are insisting that the trouble is originating from a Simplex 4009 NAC Extender out in the plant, but the trouble message on the LCD seems to contradict that assumption. Simplex came out and ran tests on both the 4009 and the Power Supply card on the 4020, and both apparently checked-out fine. However, the panel was not in trouble when they ran the tests, so the results don’t do much justice.

Can anyone shed some light on this issue based on the LCD readout? Thanks!

First thing to do is check between system negative and chassis ground with a voltmeter. A completely normal system will show around 13 to 15 volts. If that is the reading there is not much to do.

If the voltage is substantially lower there is somewhat of a ground fault. The system will trigger the negative ground fault trouble at around 7 volts. If the voltage is above 7 but much lower than normal, circuits can be disconnected while monitoring that voltage. You should be able to find a circuit that brings the voltage back to normal or much closer to normal.

I always checked if the system has a PIV. Since those are outside water can get in the switch or the conduit. Conduit to a PIV can also be damaged by lawn mowers, etc. where it comes out of the ground. A PIV may have (should have) a suppressor where the wiring enters the building. These can be compromised by nearby lightning and be leaking to ground.