Installing HRG

[adamscb]

I had a few questions about installing HRG systems and how to wire the neutral resistor. I’ve always been under the impression that this resistor has to be wired in place of the neutral to ground bonding conductor, but I just saw an online video saying you can install it anywhere in the power system. Can someone explain this a little further?

Also I know that on an HRG system, you can use a clamp-on ammeter around all three phases in conduit to help locate the fault. The ammeter will sense that not all three legs are equal, thus a ground fault somewhere. But what if the ground wire is in the conduit as well? The ammeter will always show zero amps because the ground wire is included in the measurement. Am I correct in saying that? I don’t want to mislead electricians when they troubleshoot.

 

[mikeames]

I don't know enough to opine about your first question, but as far as reading current around all phase conductors and ground its certainly possible. If equipment faults to the building steel on any phase then the EGC will not have fault current. So your correct but, fault current has multiple paths other than the conductor in the conduit. Thus an imbalance, so current can be measured around a conduit with all phase conductors.

 

[jim dungar]

I had a few questions about installing HRG systems and how to wire the neutral resistor. I’ve always been under the impression that this resistor has to be wired in place of the neutral to ground bonding conductor,

You are correct. The neutral grounding resistor replaces the neutral to ground bonding jumper.

The video may have been describing an artificial neutral or grounding transformer, which can be installed anywhere on an 'ungrounded' system. The HRG would then be connected at this newly derived neutral point.

 

[dy11]

Check out 250.36. Older electronics like UPSs and VFDs may not be able to handle the full L-L voltage to ground as would be experienced during a ground fault. We've seen some newer units that can be field configured for use on an HRG system. Cable insulation ratings are usually not an issue for LV systems but any connected device should have an insulation rating to ground that exceeds the L-L voltage.

Not endorsing any of these products but there are lots of good educational materials available here for choosing a resistor size, performing conversions and locating faults using the pulsing feature most of these products have. The Eaton white paper has some good diagrams that address your question about how a clamp on ammeter will read with a ground fault.

I-gard | Application Guides

i-gard.com

Literature Library - Post Glover Resistors

www.postglover.com

Eaton White Paper

 

[kwired]

The EGC could be carrying little or no fault current or it could be carrying all the fault current. Just depends on the situation.

clamping around all but the EGC should give you a net zero if no current is returning via some other path. This is basically same concept that GFCI and GFPE work from.