Spark from ground to neutral???

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KP2

Senior Member
Location
New Milford, CT
During a project in our lab we saw a tiny spark between the ground and neutral conductors. This building has a 3 phase 208 / 120 service and at least 70% of the load today is flourescent, (no A/C). When the students asked I felt it had to do with the non-linear loads in the building. We measered it to be 5 mA.

Any thoughts?
 

brian john

Senior Member
Location
Leesburg, VA
What voltage are you reading? Line to ground, neutral to ground.

How far are you from the transformer when you saw the arc?

Was there load on the circuit you saw the spark on?

Have you read the 3 phase voltage and neutral to ground?

Is the system properly bonded?
 

Chamuit

Grumpy Old Man
Location
Texas
Occupation
Electrician
I'm wondering if there isn't something tied to the ground instead of the neutral somewhere.

Or, you have a better path for current (at that point) back to your main bonding/grounding point.
 

KP2

Senior Member
Location
New Milford, CT
What voltage are you reading? Line to ground, neutral to ground.

How far are you from the transformer when you saw the arc?

About 275'.

Was there load on the circuit you saw the spark on?

Yes about 12 amps of lighting.

Have you read the 3 phase voltage and neutral to ground?

I will in the morning.

Is the system properly bonded?

I will look at that too.

Thanks
Kevin
 

brian john

Senior Member
Location
Leesburg, VA
Give me a call when you graduate, a four handed electrician could be a very fast electrician. :grin:

I'll vote for improper N-G bond(s) past the MBJ.

NOPE, that would result in less chance of a spark. A better chance of sparking with a properly bonded system, long length of conductor with load, resulting in a difference of potential between neutral and ground.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
It may simply be voltage drop along the neutral, past the bonding point.

This does point out why we don't use a grounded conductoir for grounding.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
NOPE, that would result in less chance of a spark. A better chance of sparking with a properly bonded system, long length of conductor with load, resulting in a difference of potential between neutral and ground.

I have to agree with Brian on this point. As other pointed out unbalanced loads could be the culprit. I also see good reason for LarryFine's conclusion. Hard to guess without specific knowledge of runs, bonding point, loads. I think the only significant point that the students should gather from this situations is that as much as there are near universals rule of thumbers in this beezwax of electricity there are also situations where outcomes that can have different causes that are very dependent on specifics on the system.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
How about checking for a voltage difference between neutral and ground?

Besides, 5ma is almost nothing, considering that a meter set to measure current has an extremely low input impedance.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Back in the old days...witnessing a small spark between N and EG at a sub panel made me realize why we separate them. That small spark indicates that you now have a parallel path for the neutral and the current is now flowing on every other piece of metal in the building back to the SE. I happened to see it when my screw driver slipped from the neutral bar & accidentily made the connection to EG. The room was fairly dark so the spark was evident. I wondered what was happening & got my new very low tech Amprobe clamp out & experimented.

Since OSHA, we know working with energized equipment is a big safety risk so none of you should ever witness this happening and never attempt to duplicate a bare screwdriver shank making intermittent contact between the neutral bar & can at a properly installed sub. Did not take much of a load.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I wondered what was happening & got my new very low tech Amprobe clamp out & experimented.
Do you recall the voltage difference?

... never attempt to duplicate a bare screwdriver shank making intermittent contact between the neutral bar & can at a properly installed sub.
And don't always assume you can touch the neutral bar as you can the EGC.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I don't think you can say this loud enough. Neutrals can hurt you bad, usually when you least expect it.


I agree. If you take a neutral wire that has current flowing through it, and say you disconnect it from the panels neutral bus, that wire is suddenly a hot wire.


Steve
 

gorms400

Member
First post here, and I couldn't help but post after reading this!

A little off the topic, but I have seen and FELT a strong arc from a ground wire to building steel. I was but a wee apprentice at the time, and it was the EBJ for an MRI room, which is basically a steel and concrete vault supposedly isolated from all other steel for shielding outside frequency. We had to move a 150 amp 480V three phase feed for the equipment in the room, which the disconnecting means was actually in "normal" space outside of the MRI room. The plans called for something along the lines of a 2/0 ground from the RF filter panel (a stainless steel access panel to inside the room that all electrical RF filters connect to mechanically) to building steel. The ground from this panel had been disconnected for about an hour, and the journeyman working with me warned me to treat the ground wire like it was hot. I thought he was kidding, but sure enough once I was well grounded I grabbed the ground wire and got a pretty good static belt. Even after leaving it disconnected for only a few seconds, you could see a visible arc as it was connected to ground.

I know it was a little off topic, but I thought it was interesting.

Actually in the same building we got a call for computer systems frying up, come to find out the 120/208Y transformer that fed all 120V outlets was never bonded. We shut down, bonded the neutral, and the problem cleared itself. But before I could take a meter and read 124V between the neutral and ground anywhere in the building. Yikes!
 
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