We have 3 chargers for DC power supplies (125VDC battery banks) at our facility and had a contractor come in and test them. They said there is a 'ground fault' on 2 of them in their report. The only evidence they show in the report is from voltage readings between the (+) terminal to ground, and also the (-) terminal to ground. Voltages between terminals is fine.
The readings they show are:

battery 1: +/ground= 110Vdc -/ground= 20Vdc

battery 2: +/ground= 26Vdc -/ground= 108Vdc

I took readings with a handheld fluke multimeter and for battery 1 I get 0Vdc for both terminals in reference to ground. So the multimeter has a much lower resistance than exists already on the floating power supply to ground. This makes sense, but I don't see how they are saying there is a ground fault...

Battery 2 I do measure +/ground= 26Vdc -/ground= 108Vdc just like the report. Now, that shows me there is a very low resistance between the power supply and ground (because the internal resistance of the multimeter is able to read the potential), somewhere in the system, correct?

Or at least lower than 10Mohm. And I should isolate components until the issue is found?

Furthermore, is this an actual issue that could damage the power supply?

I haven't been able to find anything wrong by visual/operational inspection of the components operating on the DC bus.

  • \$\begingroup\$ A test isn't meaningful unless it defines what expectation is being tested against. \$\endgroup\$ Mar 22, 2019 at 15:23
  • \$\begingroup\$ I agree. Maybe my question to the contractor should have been 'so what?' \$\endgroup\$ Apr 1, 2021 at 14:01

1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Figure 1. Earth fault tester.

Try wiring two 120 V lamps as shown in Figure 1. If the system is floating both lamps will light at half brightness. If one lights more brightly it indicates an earth fault on the other line.

Retake your voltage readings with the lamps connected. Their lower impedance will give a more reliable "pull" than your high impedance meter.


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