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HRG system grounding w/ failed resistor

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MechEdetour

Member
Location
NY, USA
Occupation
Design Engineer
Hi.

Seems like my life is filled with never ending thought experiments on grounding, and now I've come across another.

I'm pretty well versed in HRG (high-resistance ground) systems and how they operate, but I found myself in a discussion the other day that left me questioning some things.

To put my thoughts into words:

During normal operation on an HRG system (let's assume a 480/277Y secondary), there is no current flowing through the neutral/grounding resistor. Ideally there would be some, but let's also assume the capacitive charging current is negligible, err zero. Now lets assume that the grounding resistor has failed open (from a failed resistor element per say).

The system would then behave like an ungrounded system, would it not?

On an ungrounded delta system, the first L-G fault inadvertently grounds the system, causing it to behave like a corner grounded delta. So on the first L-G fault the system continues to operate. Second L-G fault results in a L-L fault which is a problem.

Is there any reason this logic would be any different if the system has a 480/277Y secondary instead of 480V delta? My conundrum: If I have an ungrounded wye secondary, and A-phase shorts to ground, system would still continue to operate and the only difference would be that VB-Gnd and VC-Gnd would now be 480V instead of 277V right? Similarly to the delta system, second fault on the wye system would also result in a L-L fault.

During this aforementioned conversation, the other individual was telling me that in the event the resistor fails open on an HRG system, persons would be exposed to a shock hazard on the first fault if they came into contact with the faulted phase. Like if a motor winding failed and the case was at the "same potential" as the faulted phase. I don't see how this would be a shock hazard as I just look at it as an ungrounded system without the resistor in the circuit.

Can someone set me straight?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
On a first fault it would be possible for a person to be acting as the grounding resistance.

I need to think on this a little more.
 

MechEdetour

Member
Location
NY, USA
Occupation
Design Engineer
On a first fault it would be possible for a person to be acting as the grounding resistance.

I need to think on this a little more.

Still, that would be no different than an ungrounded delta being "grounded" through the first fault right? For HRG the resistor is as close as possible to the neutral point on the secondary. So if the neutral (resistor) does open, it would be ungrounded. Or at least that's my entire argument.

I am only considering the hazards of the circuit downstream of the open resistor.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Still, that would be no different than an ungrounded delta being "grounded" through the first fault right? For HRG the resistor is as close as possible to the neutral point on the secondary. So if the neutral (resistor) does open, it would be ungrounded. Or at least that's my entire argument.

I am only considering the hazards of the circuit downstream of the open resistor.
You are correct. An open neutral point would create an ungrounded Wye system, same as an ungrounded delta.
 

MechEdetour

Member
Location
NY, USA
Occupation
Design Engineer
You are correct. An open neutral point would create an ungrounded Wye system, same as an ungrounded delta.

So to circle back around to my original point. With the resistor open, there would be no shock hazard to the user on the first fault.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
So to circle back around to my original point. With the resistor open, there would be no shock hazard to the user on the first fault.
There would be no significant additional hazard.
You would still have some problems, especially if the first fault is an arcing type fault that continually charges the coupling capacitance. Parallel paths, involving humans, almost always have some potential hazard.
 

MechEdetour

Member
Location
NY, USA
Occupation
Design Engineer
There would be no significant additional hazard.
You would still have some problems, especially if the first fault is an arcing type fault that continually charges the coupling capacitance. Parallel paths, involving humans, almost always have some potential hazard.
Thanks for chiming in. Looks like we're on the same page.
 

CoolWill

Senior Member
Location
Alabama
Occupation
Electrical Contractor
Changing out the failed resistor would be hazardous if the electrician happened to think he could do it with the system energized, as there would be 277 volts across it.
 
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