Ground Fault Protection on Temporary 480V Systems

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mdw228

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Location
Baton Rouge, LA
There was an incident at the refinery that i work at a couple of years ago that involved an individual wiring one phase to ground of a welding machine during a shutdown. Because of our high resistance grounding that is on the secondary side of all of our 480 V transformers, this ground fault was not detected. Consequently, every time the welding machine was used, people were getting a nice buzz when they touched a nearby stairwell (no one was hurt, thank goodness).

A recommendation has come about to look into providing ground fault protection for 480 V equipment that is brought into the plant to use on a temporary basis during shutdowns. All of our step-down transformers to 480 are D-Y with a high resistance ground. Is it possible to give ground fault protection on a four wire system with a high resistance ground? Everything that I've found online is a three wire system, which I'm guessing monitors for an imbalance between phases.

Let me know if any other details are needed. I'm a young dumb engineer. So, I may be leaving some pertinent information out.
 

raider1

Senior Member
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Location
Logan, Utah
Welcome to the forum.:)

Are you using the neutral conductor as a circuit conductor on the secondary of the delta wye 480 volt transformers?

If you have 277 volt loads you are not permitted to use a high impedance grounded system and need to solidly ground the secondary of these transformers. (See 250.20(B)(2))

Chris
 

jim dungar

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Staff member
Location
Wisconsin
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PE (Retired) - Power Systems
First, somethings need to be clarified.

In my mind Ground Fault Protection means that a device will be tripped off-line automatically, while Ground Fault Detection means that no tripping will occur. So, what are you hoping to achieve?

Are you worried about the condition of the temporary equipment or the method in which it is attached to your system?

Ideally, your high resistance ground systems already contain a detection/annunciation feature.
 

mdw228

Member
Location
Baton Rouge, LA
This is stricly to ensure the safety of those working around the area, so we want to trip the device. Which answers your second question, we are concerned about how it is attached to our system, so that it isn't shocking people touching the stairwell. When it comes down to it though, the reason I'm looking into this is because someone screwed up. Everyone messes up, but this is more of a training issue. Nevertheless, I've been asked to look into it. Our system has ground fault detection in the substation.
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
The connection of one phase to ground at the welding machine should cause your ground fault detection to indicate a ground fault.

Exactly how was the phase connected to ground at the welding machine?
 

hurk27

Senior Member
The connection of one phase to ground at the welding machine should cause your ground fault detection to indicate a ground fault.

Exactly how was the phase connected to ground at the welding machine?

Ground fault protection will not work on circuits on the secondary side of a transformer if the ground fault detection is on the primary side, which is why you can bond the X/O's on the secondaries.

GFP, and GFCI's will also have this problem.

as a GF on the secondary will only produce a line to line current on the primary side.

in post 5 it was said the GFD was at the substation.

A 3-wire GFCI would only work if there is enough current able to pass to ground.

how are these 480 volt secondaries wired, if they were able to ground a phase conductor then you might want to look into bonding the X/O's in this area, if you have high resistance grounding of the X/O then a gfp that monitors the X/O current can be set to trip if someone takes a phase to ground, but just bonding the X/O will do the same thing, a 3-wire GFCI might not provide the correct protection on a high resistance X/O and I know of no GFP that would either, at least till the second phase went to ground, and grounding a phase on a resistance grounded system will put current on the grounding, which is where I suspect the voltage on the stairs came from, some of the mills around here stopped allowing outside contractors from doing their own hook up because of problems like this, and welders should never be allowed to do there own hook ups.
 
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mdw228

Member
Location
Baton Rouge, LA
bender makes 480v GFCIs, could always use those on utilization equipment....

I looked into the bender website and found what you are talking about, I think it will work for the application that I'm looking into. Thank you!

As far as bonding the X0, I'm going to have to look into our standards on what we usually do. It would have to be a plantwide change if we're not currently doing that though, which is probably not going to happen.
 

wawireguy

Senior Member
I'm looking into this is because someone screwed up. Everyone messes up, but this is more of a training issue. Nevertheless, I've been asked to look into it.

You can't completely engineer in safety for a setting like this. SKILLED people need to working on equipment and do the electrical work. . If you don't have policies in place limiting who does electrical work, the procidures for doing it and logging who does what work you will keep having problems like you described.
 

mull982

Senior Member
how are these 480 volt secondaries wired, if they were able to ground a phase conductor then you might want to look into bonding the X/O's in this area, if you have high resistance grounding of the X/O then a gfp that monitors the X/O current can be set to trip if someone takes a phase to ground, but just bonding the X/O will do the same thing, a 3-wire GFCI might not provide the correct protection on a high resistance X/O and I know of no GFP that would either, at least till the second phase went to ground, and grounding a phase on a resistance grounded system will put current on the grounding, which is where I suspect the voltage on the stairs came from, some of the mills around here stopped allowing outside contractors from doing their own hook up because of problems like this, and welders should never be allowed to do there own hook ups.

What are you referring to as the X/O bonding here. If there is a NGR then the neutral will not be bonded to ground. Can you explain what you meant?
 
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