MechEdetour
Member
- Location
- NY, USA
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?
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?