Stray Voltage

[satcom]

When we have a stray voltage problem, the first thought is it's comming from the utility, and very often this is the cause, what we have found is other problems may cause stray.

We had a call, when i get in the shower, i get a shock, i had two other electricians here and they din't find anything, after checking the grounding and bonding, we pulled the meter, still stray at the shower, we went outside and disconnected the cable coax incomming line, shower stray gone, we measured 90 volts from the cable shield to ground.

Another intresting call was from a women telling me she was getting a shock from the back yard kiddie pool, the small 5 ft round type you blow up,
i took some readings near the pool and sure eniough the ground had stray, after a few readings, i was near the neighbors property line, i noticed a shead in the rear of the neighbors property, it had a flood light on the front, so my best guess was it had a feed line going to it, as we were standing by the fence, looking into the yard, the neighbor came out, and inquired, was there a problem?, I explained what we were doing, and this gent was very helpful, yes i put the line out there myself, and i had to connect the two pieces just about here, the maint electrician at work gave me two pieces of cable, and some connectors, we turned off the shed circuit, and pulled the cable from the ground, no more shocks for the kiddie's and the dog.

[ July 09, 2005, 11:24 PM: Message edited by: satcom ]

 

[hurk27]

Re: Stray Voltage

Stray current can come from many sources, As in the pulling of the meter, It could have still been a lost MGN at the transformer if it fed more than one building as the pulling of the meter only disconnects the two hots to this house and doesn't disconnect the grounded conductor, But in your case the problem was most likely a lost grounded conductor at the power supply for the cable on the pole, or a lost MGN somewhere down the road, this will cause the cable to be energized by returning current through your service. I will say to be very careful when disconnecting these wire as the full voltage of the primary could be present if the grounding electrodes for the pole are disconnected or damaged leaving all the return current and voltage to return through the house grounding your working on, This was one of the reasons the water company's were so up in the air about us bonding to them for the GEC. I have seen where the POCO had used the primary neutral connection of a transformer to attach the neutral to the house to, then it has a bad connection where it connects the the main MGN and now you have a transformer pulling all its power through your grounding electrode system. Not good. Now if you were to remove any of your electrode connections you would end up with the full primary voltage across that connection!
Then about 8 months later we get this call that the stray voltage had come back and again people was getting shocked in basement apt,s showers. we again determined it was a lost primary neutral (by haveing the voltage on the transformer case) and again called out the utility. the first crew didn't remove the bad MGN connection and just butt spliced the new concentric to the cut off of the old concentric. after some butt chewing by there super it was fixed and we never got another call on that problem.

 

[satcom]

Re: Stray Voltage

Wayne,

Like finding a needle in a hay stack, it takes time to sort things out, and local stray can be the hardest to find, some utilities are upgrading their systems, and even after they do the necessary upgrades, they may still find excessive stray.
With the voltage from the cable coax shield , it was generated by faulty wiring at a cable line amp, which was mounted on a pole three blocks away from the site, the utility did a full isolation at the site, after the owner had called two electricans to find the problem, the utility also missed, checking the cable feed, slow process, and as you say, you have to be very careful when disconnecting, and testing.

[ July 10, 2005, 04:44 PM: Message edited by: satcom ]

 

[hurk27]

Re: Stray Voltage

we measured 90 volts from the cable shield to ground.

This was the reason I said that it most likely was a lost grounded conductor at a power supply was because these line amps only use 60vdc and are supplied by power supply's mounted on utility poles about every mile or so, with just a hot and a neutral that runs to a transformer or existing street light feed. I couldn't see a 60vdc voltage showing up as a 90 volt ac voltage on the cable but having a lost grounding conductor would since it is bonded to the DC- of the output, which the 60 volt DC is sent through the cable.
This current could also have caused the weak neutral connection at the pole to open but it would be hard to prove.

 

[satcom]

Re: Stray Voltage

Wayne,

Yup, 90V AC, for sure, so never assume what the cable may carry, best to test, cable tech told us 120 leg was wedged between cabinet and floating output block, careless wire routing in amp cabinet, energized every customer on that block, the 32 or 60 vdc you are refering to is the coax supply voltage from center conductor to outer sheild.
I hope we hear form others on their stray finds, it is intresting to hear about all the places stray can come from.

[ July 12, 2005, 10:44 PM: Message edited by: satcom ]

 

[karl riley]

Re: Stray Voltage

Wayne, the corrosion of the ground rods you describe would have to be from DC, as far as I understand it. How did the utility explain this, or do you have an idea? I know that some electronics put out some DC, but would that be enough to corrode the rods?
Karl