Switching Impedance Grounded system to solidly grounded system

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seanybonbon

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USA
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Engineer
Have a new-to-me application. Designing for a supervised location on a 480V, wye, 3ph, 3 wire system with a high resistance ground. There has been instances in the past where maintenance needs to make repairs to a HRG panel and/or resistor enclosure which would normally require an outage of the transformer. I want to install a double throw switch to transition the system from impedance grounded to solidly grounded in order to isolate the HRG equipment for online maintenance. NEC requires unspliced conductors for the SBJ (250.30(A)(1)) which would disallow the switch from my understanding. Is it effectively impossible by code to transition out of a solidly grounded system?
Other option is to go from impedance grounded to ungrounded and forget about the SBJ.
 
Why was the HRG originally chosen? Has that changed?

A simple SPST switch or contactor could bypass a resistor.

Ungrounded systems require ground-detection systems.
 
It is a new installation and HRG is the company standard for LV systems in order to provide fault tolerance to operations as well as make faults easier to locate and repair. New part is trying to design for the ability to do online maintenance on the HRG. The bypass switch/contactor effectively has the same problem where it becomes a part of the SBJ when closed which then isn't "unspliced".
 
seanybonbon said:
It is a new installation and HRG is the company standard for LV systems in order to provide fault tolerance to operations as well as make faults easier to locate and repair. New part is trying to design for the ability to do online maintenance on the HRG.
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Fora new system, I would get the original designer/engineer involved.

seanybonbon said:
The bypass switch/contactor effectively has the same problem where it becomes a part of the SBJ when closed which then isn't "unspliced".
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I would say that depends on where you consider the GEC termination to be.
 
I am the engineer. Just trying to figure out if there are allowances available for isolation of neutral resistors without de-energizing the transformer. The GEC would be connected from the ground ring to the ground pad on the transformer tank. I would take the SBJ from that pad to the switch that transitions the neutral from HRG to solidly grounded. The more I look into this, it seems like HRG to ungrounded is the way to meet code requirements since the impedance grounding conductor is not required to be unspliced. I can install the ground indication system and associated labeling for the time period where the system is ungrounded.
 
seanybonbon said:
It is a new installation and HRG is the company standard for LV systems in order to provide fault tolerance to operations as well as make faults easier to locate and repair. New part is trying to design for the ability to do online maintenance on the HRG. The bypass switch/contactor effectively has the same problem where it becomes a part of the SBJ when closed which then isn't "unspliced".
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How often do you expect to have to do maintenance on the HRG?
 
zbang said:
May be common but is it appropriate to the circumstances? (Might be... don't & can't know; we see lots of boilerplate specs that don't make sense.)
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IMO the only real issue with an HRG service is the lack of 277V for lighting.
The HRG resistor is not prone to failure, however the ground detection components might be 'cheap' just like some GF components are.
 
jim dungar said:
The HRG resistor is not prone to failure, however the ground detection components might be 'cheap' just like some GF components are.
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I don't see why one couldn't put isolation switches in place to isolate the detection components but leave the resistor in place. You could then work on the detection components without shutting the system down, you just won't have indication while doing these repairs, but you weren't going to have this indication either way. At least you still have the HRG while operating, if an undetected fault occurs equipment continues to run. If a fault on another phase conductor occurs you still will have unexpected shut down.

I agree with some that question whether the HRG is justified here or not. They are mostly intended for when you have a process that may be more dangerous to suddenly shut down when a fault would occur than to have indication of the fault and then shut down the process in an orderly fashion.

I've heard of facilities that have such indication for HRG or ungrounded systems that the indication of a fault is there for days or even almost all the time. Kind of defeats the whole purpose to keep going any longer than absolutely necessary because if there should be a fault on another phase things are going to abruptly shut down rather than having operators shut things down in proper manner.
 
kwired said:
....

I agree with some that question whether the HRG is justified here or not. They are mostly intended for when you have a process that may be more dangerous to suddenly shut down when a fault would occur than to have indication of the fault and then shut down the process in an orderly fashion.

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While that can be the case, it is often just the cost of the shut down...Could be tens of thousands of dollars of lost product. One continuous sheet glass manufacturing plant I worked at, a very short outage typically resulted in turning all of the glass produced in the next 24 hours in to cutlet. (broken and crushed to be used as part of the next glass batch)

A friend of mine was the chief electrical engineer for a design firm that specialized in sewage treatment plants...the used HRG for all of the larger plants.
 
I would be reluctant to switch to solid-ground unless I knew that all of the EGC were installed and sized for full fault current and that ground fault protection was provided. If this will be a very short duration, I'd be more tempted to simply operate with the neutral ungrounded for a short period of time, or preferably, just take the outage and do what you need to do. I'm not quite sure how you could safely install a bypass system with the system energized, but I haven't thought about too hard.
 
Elect117 said:
Why not?

250.136(2)
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I replied to your mention of "grounding lights".
The typical 'three lights in a wye' system only works on ungrounded systems. A HRG is always grounded. Impedance grounded system use some type of relay, usually voltage sensing, to indicate when too much current is flowing on the ground.
 
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