Ground currents on grounding electrode conductor

[rossfox]

Hello all. I have been asked by a local manufacturer plant, to look into ground currents that are measured by their maintenance man on the service grounding conductor - specifically the grounding conductor to the building steel, which is a 3/0 copper. The service is an underground in PVC 3000 amp, 480/277 wye, paralleled service. The neutral is not being used in this plant for anything, but it has been brought to the service gear, and terminated on the neutral bar. The neutral bar is then connected by a buss bar to the equipment grounding bar, which in turn has the grounding conductor going to building steel and cadwelded. There is also a grounding conductor going to a ground rod and then unbroken on to a second ground rod 8 feet away. Their electrician tells me he is measuring from 160 amps to 230 amps on this grounding conductor. He goes on to say that as he turns things off in the plant, the current on the grounding conductor to building steel diminishes to around 30 amps. With all of the breakers off in the service gear he says he is reading around 30 amps. I am thinking that maybe one of the paralleled phase conductors underground in damaged and leaking current to ground, which is finding its way back to building steel, thru the grounding conductor, to the equipment ground bar, the neutral bar and back to the pad mount. Any opinions? rossfox

 

[Cavie]

Turn off all the breakers. Disconnect the GEC. Watch for an arc. Measure the voltage each leg to ground rods.

 

[electricalist]

A 230 amp load should be easy to find.Did he check the amps on the neutral?
Not 1 120 v plug in the whole plant, how does he charge his phone while googling loose neutral?

 

[ptonsparky]

Seems to me if the current goes down as you shut breakers off it is not coming from a faulted service conductor. The 30 amps at 277 is quite a load for a faulted conductor and would be getting worse every minute.

Ground currents for residential can be several amps depending on the neighborhood. IDK about large buildings such as yours.

 

[winnie]

With a 3000A service, I presume the conductors are composed of parallel sets of cables.

How are these cables arranged?

With PVC the parallel conductors could have separate phases in each pipe. This can create a magnetic field which could couple with the neutral conductors.

If there is a ground bond at the supply transformer and a neutral-ground bond at the service equipment, and if there is a separate metallic path between the two, then you could have a ground loop and transformer coupling between the phase conductors and the neutral.

-Jon

 

[ActionDave]

Couple of guesses......Fault that's not clearing, bad POCO neutral, wiring error in some equipment or transformers....something other than a bad underground wire.

Did the electrician of record check anything besides amps on the 3/0 building steel conductor? Voltage phase to phase and phase to neutral? Amps on all the phases? Amps on the neutral? Anything? What made him decide to put his amp meter on the 3/0 conductor in the first place?

 

[meternerd]

Per Code, the neutral wire must be run to the service if it's a grounded 277/480 system. If it's not being used, the neutral bus should have no other conductors on it other than the bonding and grounding wires. Have you measured the neutral current? Hard to see why there'd be current back to the transformer neutral unless you have a leak to ground. Let us know what you find. What kind of meter are you measuring with?

 

[rossfox]

Reponses so far

Reponses so far

Hey guys - all good points at this time. The local utility has now got involved with installing monitoring at the plant. We are having meeting Monday with the utility, plant engineer, maintenance man, TVA engineer and myself. I am not sure at this point what exactly the maintenance man is doing to come up with what he is saying. I will let everyone know what the deal is as soon as the situation becomes more clear. Not too sure of the maintenance man's knowledge at this point. rf

 

[electricalist]

Couple of guesses......Fault that's not clearing, bad POCO neutral, wiring error in some equipment or transformers....something other than a bad underground wire.

Did the electrician of record check anything besides amps on the 3/0 building steel conductor? Voltage phase to phase and phase to neutral? Amps on all the phases? Amps on the neutral? Anything? What made him decide to put his amp meter on the 3/0 conductor in the first place?

Why check amps at bldg steel. I was wondering the same thing.

 

[ActionDave]

Why check amps at bldg steel. I was wondering the same thing.

I hope we get to find out. Too often when a juicy troubleshooting question like this gets posted we never find out how it was resolved.

 

[electricalist]

Probably why I love electrical work.
Many layers of possibilities within a system and when it's working it seems simple enoygh. Chip one of the layers and we could end up with an amp probe on the bldg next door.

 

[kwired]

Hello all. I have been asked by a local manufacturer plant, to look into ground currents that are measured by their maintenance man on the service grounding conductor - specifically the grounding conductor to the building steel, which is a 3/0 copper. The service is an underground in PVC 3000 amp, 480/277 wye, paralleled service. The neutral is not being used in this plant for anything, but it has been brought to the service gear, and terminated on the neutral bar. The neutral bar is then connected by a buss bar to the equipment grounding bar, which in turn has the grounding conductor going to building steel and cadwelded. There is also a grounding conductor going to a ground rod and then unbroken on to a second ground rod 8 feet away. Their electrician tells me he is measuring from 160 amps to 230 amps on this grounding conductor. He goes on to say that as he turns things off in the plant, the current on the grounding conductor to building steel diminishes to around 30 amps. With all of the breakers off in the service gear he says he is reading around 30 amps. I am thinking that maybe one of the paralleled phase conductors underground in damaged and leaking current to ground, which is finding its way back to building steel, thru the grounding conductor, to the equipment ground bar, the neutral bar and back to the pad mount. Any opinions? rossfox

30 amps with everything off could be current from outside the plant, but 160 to 230 amps tells me to look at separately derived systems and make sure they are grounded/bonded properly. You surely have some 120 volt receptacles at very least on at least one separately derived system.

 

[electricalist]

I'm glad you commented Mr.Kwired I hope my op didn't come across as being rude. I was thinking what you know.

 

[kwired]

I'm glad you commented Mr.Kwired I hope my op didn't come across as being rude. I was thinking what you know.

No offense - how often does one have a facility with no 120 volt circuits? Limited loads like a pumping station maybe - but now NEC requires a 120 volt receptacle within 50 feet of service equipment in 210.64 (new in 2014) so even a single 480 volt pumping station with no other loads will now require a system with 120 volts capability to be incorporated some how. :roll:

I hate that change - all the irrigation equipment I work on (over 99% of it is 480 volt systems) we use 120 volts from generators while constructing it and seldom ever need 120 volts again. Those that do regular work on such systems (even non electrical work) have portable welder-generators on their service trucks and have no real need for a permanent 120 volt receptacle on the site, and seldom need it within 50 feet of the service equipment - they are often up to 1400 feet away from service equipment when working on such a machine.