sparking on a neutral

[allenwayne]

OK guys, I had a situation that has me at a loss.

Scenario : Added a quad in a store last night(after hours).I tested for voltage with my Fluke (none) But when I tightened the screws on the receptacle, my screwdriver came in contact with the grounded box and sparked.I tested for neutral to ground continuity and it was there(using the ohm meter on my fluke)

The circuit wasn`t traced out aside from what was in the general area.I assumed that since there was several neon ballasts on the circuit that might be the cause.I tested for a solid neutral and a ground to neutral connection, all OK.What could cause this situation?????Also when the PC was plugged back in there was an arch when prong contact was made.

The service is a single phase 200 amp CH panel and all is in conduit and MC.Any ideas ???????

I also used the fluke to see if there was any voltage between the ungrounded conductor and a ground and grounded conductor but there was none.

OK I am at a loss ????????

 

[brian john]

Did you check for current?

Were you moving to fast and the speed of your screw driver........

 

[A/A Fuel GTX]

I had a similar situation recently also. I was replacing some old duplex recepts at a residence and noticed that there was a very slight arc between the EGC and the neutral. No other signs of anything wrong. The only thing I can come up with is that somewhere downstream from the panel, the EGC and neutral are tied together, kind of a parallel path scenario.

 

[ghostbuster]

Welcome to the neutral/ground current problem.Perform net current measurements to locate the souce of this problem.

 

[don_resqcapt19]

Tom,

I was replacing some old duplex recepts at a residence and noticed that there was a very slight arc between the EGC and the neutral.

While that can be an indication of an illegal neutral to grounding connection, it also will happen in a code compliant system if there is load on the neutral. You create a parallel path when you connect to neutral to the grounding conductor and some current will flow on this path. It takes very little current to make a slight arc that you can see.
Don

 

[pismo]

Where's the location of the grounding electrode?

 

[ptonsparky]

I agree, the first time I installed a panel with isolated neutral and the screwdriver made contact with the box and neutral bar helped with understanding those illicit returns.

 

[LarryFine]

Allen, plug a 3-wire cord into a known-properly wired receptacle, preferably at or near the service, preferably one that is the circuit's only load. This provides you with a reference hot, neutral, and EGC in your hand with which to test voltages against.

Carry the female end with you to the offending outlet, and use a solenoid-type tester (for the loading as compared to a high-impedance volt meter) to check the absolute voltages on each conductor. At the outlet, you don't know whether it's the neutral or EGC that has voltage it shouldn't.

 

[allenwayne]

I went back last night to double check.Fluke in hand.........Without opening things up what I found was that the line voltage was 119 volts line to neutral and line to ground.The reading on my fluke was 17 volts neutral to ground.

I again tested my continuity from neutral to ground.It is there.What started this whole thing is that when I was tapping into the existing circuit ,I loosened the neutral screw on the existing receptacle attached to a RS cover and there was a quite evident spark.

The wiring method from what I can see without going deeper is MC and conduit.Appears that in the boxes I opened that all grounds are made up and neutrals are too.

There are 2 pc`s on this circuit with apu`s.but when this happened they were unplugged.Also when I found the sparking the circuit was de energized and the first thing that came to mind was a loose neutral somewhere.

 

[ptonsparky]

What is the current of the circuit while testing and the distance from the service? Ghost voltage? What are the characteristics of the other branch circuits or feeders close by?

 

[Hambone24]

Nebraska

Nebraska

This is electrically unrelated but Im sorry I cant let Pismo bash my cornhuskers. They are one of the classiest teams out there. Okay now back to electrical!

I do have one question, when you speak of a solenoid tester, what exactly is that? I dont think I am familiar with that.

 

[ceknight]

I do have one question, when you speak of a solenoid tester, what exactly is that? I dont think I am familiar with that.

http://www.squared.com/us/products/machine_safety.nsf/unid/58870E11543C976885256D500050927B/$file/wiggy.htm

 

[sparkzilla]

Here is another brand of tester that I use.
http://www.knoppinc.com/

 

[allenwayne]

I didn`t use a wiggy selonoid tester.I used a digital fluke meter.Ghost/fantom voltage came to mind but when I was hooking up the extended circuit to the receptacle mounted to the RS cover and when the screwdriver touched the cover as I was tightening the neutral the spark was really obvious.

What gets me is that when I ohmed the neutral to ground it was there and ungrounded to grounded/ and grounding conductors showed 119 volts.So if the unbgrounded is solid and the grounding is solid why is it that when I touched neutral to ground there was a quite obvious spark ????

 

[ptonsparky]

I....So if the unbgrounded is solid and the grounding is solid why is it that when I touched neutral to ground there was a quite obvious spark ????

When you touched the neutral to box via screwdriver you created a parrallel path for the neutral load back to the transformer through the EG. They are tied together at the service, hopefully, The neutral & EG were not parralled until you made the second connection at the box, creating the spark and allowing current to flow on both conductors.

 

[ptonsparky]

I don't think I have ever spelled "parralled" correctly or the same in my life....forgive me.

 

[hardworkingstiff]

Without opening things up what I found was that the line voltage was 119 volts line to neutral and line to ground.The reading on my fluke was 17 volts neutral to ground.

These readings don't seem possible. 119 volts to two different reference points that have a difference of potential between them (and they have been electrically connected at a previous point in the circuitry).

 

[don_resqcapt19]

[FONT=&quot]Allenwayne,

Without opening things up what I found was that the line voltage was 119 volts line to neutral and line to ground. The reading on my fluke was 17 volts neutral to ground. I again tested my continuity from neutral to ground.

You have a floated or unconnected EGC. I know that you said that you checked for neutral to ground continuity, but you can't really do that when you have a 17 volt difference between the two conductors under test. This voltage will cause many meters to show continuity where there really isn't any. It would be very unlikely to have 119 volts hot to neutral and 17 volts neutral to ground on a code compliant installation. When you measure the voltage between the hot and neutral on a code complaint installation, all you are really measuring is the voltage drop on the neutral. I would not expect to find 119 volts hot to neutral on a circuit that has a 17 volt drop on the neutral.
Don
[/FONT]

 

[Fulthrotl]

I went back last night to double check.Fluke in hand.........Without opening things up what I found was that the line voltage was 119 volts line to neutral and line to ground.The reading on my fluke was 17 volts neutral to ground.

my experience with digital voltmeters is that they have a VERY high
volt/ohm ratio, and during troubleshooting will read floating, static
and ghost voltages erratically enough to make you want to drink
cheap whiskey and write bad checks.

picking the correct fluke from the toy bag is no fluke. (sorry)
the fluke 12 is a bulletproof meter that was fluke's answer to
the wiggy back in 1991. it has been discontinued, but NOS is
still available here and there. i prefer it to the meter fluke
introduced to replace it to the extent that i bought two of them
as spares, so i have enough of them to last till i retire.
it contains a 2000 ohm shunt load, to load the circuit slightly,
and prevent ghosts and floating voltage, which it sounds like
your 17 volts amounts to... a transient.

however, i have personally observed two ground rods 8'
apart, that had 31 vac between them, with nothing else
connected to them, and when a jumper was connected
between them, a clamp on ammeter measured 85 amps
flowing between them.

in that same area, (230KV switchrack) a 10' long conductor
laying on the ground parallell to cablebus 30' overhead had 95
volts on it, and was able to power a drillmotor.

isn't inductance a glorius thing?


randy

 

[ramdiesel3500]

When you touched the neutral to box via screwdriver you created a parrallel path for the neutral load back to the transformer through the EG. They are tied together at the service, hopefully, The neutral & EG were not parralled until you made the second connection at the box, creating the spark and allowing current to flow on both conductors.

Yep, I agree with Ptonsparky! That neutral current will take any path it can. If it has a long road to travel from the point of concern back to the panel, then there will definitely be a measurable voltage between the neutral and ground at that remote location so long as there is a fairly large level of current in the neutral conductor at or near that location.