neutral to ground resistance

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bunge

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What is an acceptable amount of resistance between neutral and ground?

The Amphitheater i work at recently upgraded the the switchgear and ever since i get up to 6 ohms of resistance between ground and neutral on some of the disconnect switches.

since it is now protected by GFP, is a slightly higher resistance to ground a good thing? of does it sound like the neutral/ground bond at the service entrance just a poor connection?
 
bunge said:
What is an acceptable amount of resistance between neutral and ground?

The Amphitheater i work at recently upgraded the the switchgear and ever since i get up to 6 ohms of resistance between ground and neutral on some of the disconnect switches.

since it is now protected by GFP, is a slightly higher resistance to ground a good thing? of does it sound like the neutral/ground bond at the service entrance just a poor connection?
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What size neutral conductor and EGC? At what distance (wire length) from service? When you say some disconnects, does this mean not all, or only the ones you have checked? If only some of the ones checked, and distance, size of wire does not justify the difference, perhaps the resistive connection is not at the service.

Higher resistance to ground is seldom a good thing. Some systems are designed with high resistance to intentionally limit ground fault currrent... but yours is likely not one of them.
 
Smart $ said:
What size neutral conductor and EGC? At what distance (wire length) from service? When you say some disconnects, does this mean not all, or only the ones you have checked? If only some of the ones checked, and distance, size of wire does not justify the difference, perhaps the resistive connection is not at the service.

Higher resistance to ground is seldom a good thing. Some systems are designed with high resistance to intentionally limit ground fault currrent... but yours is likely not one of them.
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the worst reading is off a 200amp 3phase disconect. the tails i have tied into that are 2/0 awg. its about 150-200 feet away from the building switchgear room.

i have checked other disconnects on the stage and gotten readings of .3 to 5 ohms resistance at varying times.

The Venue is a outdoor ampitheatre, and the Ground fault protection at the service entrance is new to it with this renovation, which is why i was wondering if the limiting of ground fault current was by design.

i've also got calls out to the contractor that did the work, and the engineer who designed the renovation. i'm just doing my own homework as well.
 
bunge said:
the worst reading is off a 200amp 3phase disconect. the tails i have tied into that are 2/0 awg. its about 150-200 feet away from the building switchgear room.
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Assuming a #6 EGC and 250' I would expect to see less than 1/2 ohm if you measure the resistance between the EGC and the grounded conductor.
 
bunge said:
the worst reading is off a 200amp 3phase disconect. the tails i have tied into that are 2/0 awg. its about 150-200 feet away from the building switchgear room.

i have checked other disconnects on the stage and gotten readings of .3 to 5 ohms resistance at varying times.

The Venue is a outdoor ampitheatre, and the Ground fault protection at the service entrance is new to it with this renovation, which is why i was wondering if the limiting of ground fault current was by design.

i've also got calls out to the contractor that did the work, and the engineer who designed the renovation. i'm just doing my own homework as well.
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Possible... and that's all I can say without knowing the system specifics.

But first, I have to note I'm far from an expert on high-impedance grounded systems. Second?and this is the rub in your scenario?high-impedance grounded systems (HIGS's) are not permitted to serve any line-to-neutral (L-N) loads. I can't see why you would have a neutral in a 200A disconnect if it didn't serve any L-N loads??? If there is a HIGS for the facility, there is likely also a low-impedance or solidly-grounded system for L-N loads...
 
The vast majority of HRG's (high resistance ground) are sixed with an impedance that linits the current to about 5A (a few are as high as 10A), so at 480V this equates to a 57.7ohm resistor. I will state pretty emphatically you do not have an intentionally impedance grounded system.

The Ground Fault protection, has to do with the NEC requiring GF protection on all service >1000A @ 480Y/277. These GF systems simply monitor the amount of current flowing out of and then back into the main gear. These have no impact on the neutral to ground resistance.
 
my bad for not clarifying,

the building is a 120/208 3phase Y system.

all the disconects on stage(1 100a, 1 200a, 4 400a, and 5 600amp) are used as temporary power for the shows coming in, or the buildings in house theater lighting system.

the racks are fed via #2 to double runs of 4/0awg depending on the loads via camlock connectors



the distribution and dimmer racks that the lighting, sound, and video mostly want 3pahse 5 wire hookups, they only use 3pahse chain motors to get the equipment to hang in the air. everything else is single phase 120/208 loads for lights, speakers, and other assorted stage equipment.




Smart $ said:
Possible... and that's all I can say without knowing the system specifics.

But first, I have to note I'm far from an expert on high-impedance grounded systems. Second?and this is the rub in your scenario?high-impedance grounded systems (HIGS's) are not permitted to serve any line-to-neutral (L-N) loads. I can't see why you would have a neutral in a 200A disconnect if it didn't serve any L-N loads??? If there is a HIGS for the facility, there is likely also a low-impedance or solidly-grounded system for L-N loads...
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bunge said:
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the building is a 120/208 3phase Y system.
...
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In addition to what Jim stated, high-impedance grounded neutral systems (NEC terminology) are only permitted for systems operating at 480-1000 volts. So I concur.

I'd expect numbers well below 0.5 ohm at disconnects closer to service equipment. Ranging to 5 and 6 ohms at other locations says you have some not-quite-solid, resistive connections... likely not limited to the main bonding jumper at the service, but it's a good place to start. I highly suggest verification after any rework.
 
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