High Impedance Ground

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BPoindexter

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I came across something I have not seen before. I have installed many HRG systems from 34.5kV to 480V and this is a first for me. We installed a Post Glover ground system on a 480/277 Wye. While looking at the unit we realized that they have the system grounded conductor coming into the main disconnect where it is connected to switched terminal on the unit disconnect along with the 3 phase conductors. This then goes to a fuse and then goes to the grounding resistor. What this means if someone turns off the detector unit or the single fuse blows in the grounding circuit then we lose our ground and this becomes an ungrounded system. Looking at the NEC there is no prohibition for the switch in 250.36 but one would infer that it should run from the X0 of the transformer to the resistor with out a switch and certainly without a fuse in the circuit. As is if someone were to open the disconnect or the one fuse were to blow then you have an ungrounded wye.

This does not seem logically correct to me given the purpose of such a system. This is the first time i have seen the grounded system conductor that is switched and fused? Typically they are run directly to the HRG CT then to the Resistor. Anyone seen this have any experience with this.
 
What's the application? I've seen switched neutrals on HRG systems for paralleled generators where only one generator would be grounded at a time in order to prevent circulating currents.

It sounds like the disconnect will interrupt power conductors as well as the neutral, while it seems unnecessary, this is not by itself a hazard.

I agree, I know of absolutely no reason whatsoever to fuse the neutral, and it sounds like a terrible idea.
 
Big john

Actually the disconnect is fed from a branch circuit on the MCC and only disconnects the branch circuit to the P G unit not the main. Opening the switch only opens the branch circuit ungrounded conductors feeding the unit while opening the main grounded system conductor for MCC. So this would be a violation of 240.22 in the case of both the disconnect and the fuse. It is not the case of switching the neutral to another source for a separately derived systems they are opening the grounded system conductor period. We all at our facility including our engineer agree that as you say this is a "terrible idea."
 
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