Current on EGC?

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Ken9876

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I have a situation, there is a 50kva 277 pri 120/240 sec transformer feeding a panel at this location. I noticed when I had the panel cover off to correct a 30amp load on a 20 amp breaker that the neutral was connected to the EG bar in the panel, so I put a meter on it and had 7 amps on this 6 awg jumper. So I took a look at the transformer and the main bonding jumper was installed there, with the GEC. I checked the current on the EGC back to the transformer and had about 3.5 amps. I removed the incorrect jumper at the panel, and the current on the EGC dropped down, but I still have .6-.4 amps on the EGC and on the metal raceways back to the transformer. Is a small amount of current on the EGC normal? and if so what is ok. The loads on this panel vary from lighting and recs. to a UPS for a PLC system and heating and A/C loads, fire alarm...Should I turn off loads untill the current goes away?

Thanks,
Ken
 
There are several ways to locate this problem.
? Perform zero sequence readings on each branch circuit (if possible) use a True RMS amp clamp and measure the phase/phase and grounded/neutral coonductors at the same time the circuits with any amperage are the culprits.
? Measure grounded/neutral conductor voltage at the panel (should be close to ?0? millivolts) circuits with any loads should have an increase in measured voltage (between the grounded/neutral conductor and the Equipment grounding conductor (copper conductor or conduit makes no difference). If the voltage is low, similar to the reading in the PDU, that is a suspect circuit.
? During a schedule outage turn off all the branch circuit breakers, unplug loads if possible and remove the grounded/neutral conductors from the neutral termination bar. Utilizing a low voltage megger 50VDC 0r 100 VDC (I use a low voltage megger to avoid damaging equipment with 500 or 1000 VDC meggers we normally use. The neutrals that megger bad, ?0? megohms, need to be traced out and the ?SHORTED? grounded/neutral conductor will need to be replaced, repaired, lifted depending on the exact nature of the
 
brian john said:
There are several ways to locate this problem.
• Perform zero sequence readings on each branch circuit (if possible) use a True RMS amp clamp and measure the phase/phase and grounded/neutral coonductors at the same time the circuits with any amperage are the culprits.
• Measure grounded/neutral conductor voltage at the panel (should be close to “0” millivolts) circuits with any loads should have an increase in measured voltage (between the grounded/neutral conductor and the Equipment grounding conductor (copper conductor or conduit makes no difference). If the voltage is low, similar to the reading in the PDU, that is a suspect circuit.
• During a schedule outage turn off all the branch circuit breakers, unplug loads if possible and remove the grounded/neutral conductors from the neutral termination bar. Utilizing a low voltage megger 50VDC 0r 100 VDC (I use a low voltage megger to avoid damaging equipment with 500 or 1000 VDC meggers we normally use. The neutrals that megger bad, “0” megohms, need to be traced out and the ”SHORTED” grounded/neutral conductor will need to be replaced, repaired, lifted depending on the exact nature of the
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brianjohn, do you need a helper for 2 weeks?:D
i bet i could learn a lot from working with you for 2 weeks, then i would know a lot more than i know now!:D

thanks for the explanation, now, if i could just remember, what you said!:wink: i would go fix all these improper neutral to ground connections i see in panels.
 
What is the load on the panel?

0.4-0.7 ain't much. Might just be induced and not a problem at all. I don't think I'd worry about it.

If you have a N-G bond somewhere down stream it is still a parallel path same as your #6 jumper was only with a little more impedence so even it you turned off breakers one at a time it won't tell you where the extra N-G bond is.

Example, unbalanced neutral current at your panel flows back to the transformer through the neutral of course AND some amount splits off on some bonded neutral wire to the branch circuit bond then back on the branch EGC back to the panel EGB then to the transformer. Turning off the breaker to this load does not disconnect this N-G path and some portion of the total neutral load continues to take this path.

If that is the case though then I would expect the parallelled, unbalanced current you are measuring to be greater than 0.4-0.7 amps.

I suspect it is just induced current at this level.

Probably not a problem.
 
I didn't check the load on the panel as I only had my fluke T-5 on me at the time(I think that's the model). How much current would you consider a problem?

Thank you for all the info Brian, what are the possible problems from neutral ground currents?
 
Although I have no engineering basis for this, I was told years ago the rule of thumb is less than 10% of the neutral current flowing on your equipment ground back to the transformer from the panel is nothing to worry about. This means if your neutral current is 20A then 2A or less on the EGC is considered normal primarily due to induced currents. You can drive yourself crazy looking for the source and never find it. I wouldn't worrry a bit about 0.4A unless your neutral current is 0.8A, which from your original post doesn't sound like it is the case at all.

I will say this, I have taken hundreds of current measurements over they years, on systems that had green wires landed on the neutral bar in branch panels when they definitely shouldn't have as well as on new installs that I know for sure didn't have any N-G bonds other than at the transformer i.e. wired properly, and I have NEVER, EVER measured 0.0A on the EGC back to the transformer when there was load on the panelboard. Sometimes it is very very small but there has always been some amount of current flow.

Heck, depending on where this is in relation to your MDP and the utility transformer you could even be picking up some of that stray unbalanced current since the N-G bond is present in the MDP through the main bonding jumper and likely at the utility company pad mount as well. Even though you don't have an EGC running out to the utility company xfmr you have a parallel path for neutral current called the earth which is bonded to the building.

0.4-0.8A is nothing to worry about in my opinion.
 
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.4-.8 most likely not an issue, but I would not live by the 10% rule.

BUT if IMO if you think it is an issue if I were you I would CYA as noted above..
 
I learned another method from a EE at Edison.
you need a clamp on meter or accessory with Jaw large enough to go around the entire service. We used flexible probes that would go around a 3 phase set of 500's
There should be close to zero amps on the meter as you are measuring the entire feeder. If there is a current then it is not following the correct path.
Any imbalance we would trace back to each group circuits. by doing the same thing. What we would find is a ground used as a nuetral or the wrong nuetral used where there are several conductors in the same conduit.

all this Net Current as we call it can lead to exessive EMF. Use a gauss meter next time you find a problem like this. Bet ya the needle pegs.


ss
 
Sierrasparky said:
all this Net Current as we call it can lead to exessive EMF. Use a gauss meter next time you find a problem like this. Bet ya the needle pegs.
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That's actually what I got my first gauss meter for. Troubleshooting by walking around. :grin: Helps narrow down the approximate spot of the n-g short if the place is big.
 
Ken9876 said:
How much current would you consider a problem?

Thank you for all the info Brian, what are the possible problems from neutral ground currents?
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Small levels of current on the EGC can cause computer lockups because it can override the data being sent between them. If the computers are networked with fiber, then you really don't have a problem there. More elevated levels of current can cause a shock hazard of course.

I can't really give specific levels to say this current level is a problem and less than this is ok. It really depends on the customer perception of what a "problem" is.
 
Small levels of current on the EGC can cause computer lockups because it can override the data being sent between them. If the computers are networked with fiber, then you really don't have a problem there. More elevated levels of current can cause a shock hazard of course.

I can't really give specific levels to say this current level is a problem and less than this is ok. It really depends on the customer perception of what a "problem" is.[/quote}

As noted current on the EGC should be corrected but in my expierence I have not seen any PC operational issuse with high levels of ground current or high levels of neutral to ground voltage. Other than screen shake from current on the EGC, which as an issue is pretty much going away with the advent of newer monitors.
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brian john said:
Small levels of current on the EGC can cause computer lockups because it can override the data being sent between them. If the computers are networked with fiber, then you really don't have a problem there. More elevated levels of current can cause a shock hazard of course.

I can't really give specific levels to say this current level is a problem and less than this is ok. It really depends on the customer perception of what a "problem" is.[/quote}

As noted current on the EGC should be corrected but in my expierence I have not seen any PC operational issuse with high levels of ground current or high levels of neutral to ground voltage. Other than screen shake from current on the EGC, which as an issue is pretty much going away with the advent of newer monitors.
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Please note that in every instance I had where someone complained about the Net currents or weird flicker , or some weird sensation they felt in a given area or room we found gross faults in the wiring that could have resulted in harm to person of a fire. These things should be taken seriously and are not that difficult to narrow down. It does get a bit more difficult in computer rooms running on 3 phase and with high levels of harmonics.
Anyway it should not be arbitrarily dismissed .
Thats my opinion.
SS
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What I was refferring to is computer/Data centers or similar places. Many of the power supplies leak to ground. Thus add to the nuetral currents. This is mostly unavoidable. Those switching power supplies as I understand cause harmonics because of the way they work. I was told by the EE that this may cause phantom readings while troubleshooting. When all this is in the same building it makes it difficult to find the root cause.
I could be wrong.
 
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