Measuring the Resistance from XFMR's NEUT to GND on a NGR system

[majesus]

We have a 3 wire, 600V secondary distribution fed from a 1.5MVA XFMR with a 5A NGR on the Neutral to ground. The system is designed to Alarm on Ground fault only.

Yesterday while clearing snow with the excavator, the crew ran over a 600V, 3c-#4/0 armoured cable feeding the fresh water pump house via an upstream 200A fused disconnect. This caused a Phase A to ground fault but the system did not trip as designed. However, the crew mentioned that they witnessed a large arc and then evacuated the area. "Bubbling water" from melting snow around the faulted cable was observed until the system was finally de-energized.

Prior to shut down, the electrician did measure the phases at the fused disconnect and recorded the following:
Phase A 173A
Phase B 141A
Phase C 142A

The measurements suggest that there might have been approximately 30A ground fault while the system's NGR is only 5A. The grounding system appears not to be correct. The fault has been isolated, but my questions would be:

Is there a method to measure the Neutral to ground point from the transformer to verify that the only connection is via the 5A NGR? (Hi-pot?)
Can this be done simply using a mutli-meter on the Ohm setting?

Thanks,
Majesus

 

[waross]

First check for voltage and current. If there is any voltage and current use ohms law. In the absence of voltage and current the multi-meter should work. The thought of a ground fault while you have the ohm-meter connected is a little scary.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[majesus]

Morning Waross,

The NGR is located inside the dry type unit sub. Measurements must be done the system is off. We plan to de-energize and shall try the multi-meter.

Just out of curiousity... Assuming the neutral on secondary of the unit sub XFMR was floating and the system was artificially grounded via an NGR and a zigzag. Could you measure the NGR using the mutli-meter on the unit sub XFMR's neutral to ground?