Problem with GFP on 1600amp 277/480v 3p 4w main

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I have been asked to look into a problem which exists on a commercial building. It has a 1600amp 277/480v 3ph 4w main breaker which is ground fault protected in accordance with NEC 230.95. In the past it has tripped when a maintenance man was changing a light bulb and shorted wires together. Most currently and frequently it trips when an air conditioning compressor shorts out (another problem our company is looking into!). The setting on the GFP main breaker is set to it's maximum allowable setting. Is a complete fault current study needed to correct this problem? Any help and advice will be appreciated as always.

Coastalguy1 :confused:
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
When you say maximum allowable setting, do you mean thats what the engineer's study came up with? Or the settings are just maxed out? If they are set at the max, then either you have some severe problems, or a improper bonding or neutral issue making the ground fault think there's a problem. But it sounds like it's actually doing it's job with the bad compressor.
 

hurk27

Senior Member
I have been asked to look into a problem which exists on a commercial building. It has a 1600amp 277/480v 3ph 4w main breaker which is ground fault protected in accordance with NEC 230.95. In the past it has tripped when a maintenance man was changing a light bulb and shorted wires together. Most currently and frequently it trips when an air conditioning compressor shorts out (another problem our company is looking into!). The setting on the GFP main breaker is set to it's maximum allowable setting. Is a complete fault current study needed to correct this problem? Any help and advice will be appreciated as always.

Coastalguy1 :confused:

Knowing what the brand name and model of the MDP or main can go along way to get the correct advice, most GFP's have more then one setting, and depending upon which one is turned to max can be a key, generally if it is set to the max (x10) that would require a 16ka ground fault to trip this main, I highly doubt a GFP would trip on a local branch circuit short if the GFP setting is set at X10, if this is the case then there is a problem with the GFP or the CT, the only other thing I have seen is where sub panels was using the EGC for a neutral or didn't have neutral EGC separation, while this neutral current might no be enough to trip the GFP, it will subtract from the amount of current the GFP will trip at, might be the reason the GFP was turned all the way up, GFP settings should never be changed without an engineers stamp of approval, to do so can cause allot of damage and get someone hurt, this can be a quick way to land in court.

Since it seems that this GFP setting has been changed at some point in time, a complete fault current study should be completed by an engineer, or if you could get hold of the original fault study if the loads haven't been changed, you might have something to go by, but if it is still tripping at the max setting something is wrong.
 
When you say maximum allowable setting, do you mean thats what the engineer's study came up with? Or the settings are just maxed out? If they are set at the max, then either you have some severe problems, or a improper bonding or neutral issue making the ground fault think there's a problem. But it sounds like it's actually doing it's job with the bad compressor.

The settings are max'd out. I do not know at this time if an engineering study was done.
 
Knowing what the brand name and model of the MDP or main can go along way to get the correct advice, most GFP's have more then one setting, and depending upon which one is turned to max can be a key, generally if it is set to the max (x10) that would require a 16ka ground fault to trip this main, I highly doubt a GFP would trip on a local branch circuit short if the GFP setting is set at X10, if this is the case then there is a problem with the GFP or the CT, the only other thing I have seen is where sub panels was using the EGC for a neutral or didn't have neutral EGC separation, while this neutral current might no be enough to trip the GFP, it will subtract from the amount of current the GFP will trip at, might be the reason the GFP was turned all the way up, GFP settings should never be changed without an engineers stamp of approval, to do so can cause allot of damage and get someone hurt, this can be a quick way to land in court.

Since it seems that this GFP setting has been changed at some point in time, a complete fault current study should be completed by an engineer, or if you could get hold of the original fault study if the loads haven't been changed, you might have something to go by, but if it is still tripping at the max setting something is wrong.

Thank you Wayne for your reply. I will be doing a field study next week and will get all MFG information.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
It is extremely rare that you will be able to have a main GFP fully coordinate with any branch circuit device larger than about 60A. Understanding the limitations of the coordination will help in choosing which trade-off to make.

The best solutions to 'nuisance' tripping of main GFP include:
No live work. (The most common thing I hear is the "main tripped when someone's screwdriver slipped").
Coordinate GFP to at least 30A branch devices.
Provide multiple levels of GFP so the area affected is smaller (just like in hospitals)
Repair the equipment that constantly shorts to ground.
Properly manage multiple neutral to ground connections (correct the illegal ones, design for the legal ones)
 

G._S._Ohm

Senior Member
Location
DC area
Tripping is go/no-go thing; you need a meter that gives an intensity reading. Fluke makes a meter that shows GF current imbalance.

Maybe it's already running near the trip threshold and these incidents push it over.
 

brian john

Senior Member
Location
Leesburg, VA
What size AC units? Depending on the size of the units it may be impossible to avoid this from happening, short of installing a SDS for the HVAC units.

A light bulb as in a 120 VAC lamp? You have neutrals crossed between systems. Anything happening on the 208/120 VAC system should not affect the 480/277 VAC GFP.

If the maximum setting of 1200 amps at 1 second (fairly typical maximum and maximum per the NEC),

And the GFP is still tripping I would.

Check the neutral ground bond for current.

With an outage

Have the GFP tested at all factory settings for current and at the as found setting for time.
Verify the CT's (Current sensors) are not having any issues. I have seen several split core CT's with connection issues at the split.
Remove the neutral ground disconnect link and megger the neutral to ground, Clear all shorts located.
 
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