Grounding grid needed??

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massfd

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I have a site that has a grounding potental problem. First a brief overiew of the site.

(1) property aprox 300 feet deep
(2) front property line is a road
(3) rear property line is a rail road with DC tracton supply 3rd rail that also has the local POCO high voltage feeds to the substations overhead.

Located on a concrete pad aprox 50 feet from the railroad and 40 feet from the POCO overhead we have a 8 X 40 foot steel fire training trailer.

Service is from a pole mounted service located at the road, the service has 2 rods and the ground is bonded to neutral at the service. From the service there is aprox 200 feet of #6 (trailer needs 22A, #6 is for voltage drop) 4 wire (2hot 1 neutral 1 ground) It terminates at the trailer in a "mobile home" pedastal 4 wire and the trailer plugs in from there. In the trailer the neutral and ground are seperated as the service is already bonded once.

At the trailer today in the pouring rain there was a complaint of a slight tingle between an aluminum ladder that was in a puddle of water and the roof of the trailer (ladder was insulated from trailer by a thick coating of paint)

I took voltage readings with a fluke DVOM and got 9 volts AC. Looked at the grounds and they are all good, ground to neutral is in the MV range. Decided to drive a ground rod and bond it to the trailer frame, it made very little differance.

I started to take some readings between the puddles on the ground and the trailer frame. At the front I got 5 volts, middle of trailer 9 volts and at the rear (the end closest to the RR and POCO)13 volts. I opened the service disconnect to the site (200 feet away) and there was no change at all in the readings.

I am thinking that this site needs to be excavited for at least 4 feet all the way around the trailer slab and a grounding grid constructed and bonded to the trailer. Kind of like a in ground pool.

What do you guys think, I need to dry out and think about this some more.
 
I'm wondering if this is poco primary neutral current and if it would stabilize if some bonding took place at the concrete pad. :smile: Just a thought.
 
I think you need to locate the source of the voltage, trying something that may only aggravate the issue is a waste of time in my opinion. Trouble shoot the issue at hand do not try band aids.
 
What Brian said.
Sorry, but the readings you took were meaningless. Everything you were reading was likely phantom voltage. There is no effective way of measuring from the earth itself and an object the way you did.

This is coming from somewhere. Find out where and you found your problem.
 
wbalsam1 said:
I'm wondering if this is poco primary neutral current and if it would stabilize if some bonding took place at the concrete pad. :smile: Just a thought.
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I think you are correct, If I could get to the rebar in the slab I would try it but it is to late now that is why I was thinking about going out 4 feet with a grid and bonding it to the trailer. The voltage that was first noticed was from a puddle to the trailer via a ladder.

I do not know what kind of answer I would get from the RR or POCO but I think they would attempt to push it off on us to solve the problem.

If the ground is still wet in the morning I may go back and take some readings from the service ground to earth (puddle) working my way to the back of the site. I will see what potental develops in the span of 300 feet, I already know it goes from 5 to 13 volts in about 40 feet at the rear of the property.
 
did you check the trailer siding and roof to see if they are energized ? I agree with others - you need more investigation before making any conclusions.
 
speedypetey said:
What Brian said.
Sorry, but the readings you took were meaningless. Everything you were reading was likely phantom voltage. There is no effective way of measuring from the earth itself and an object the way you did.

This is coming from somewhere. Find out where and you found your problem.
Click to expand...

I aggree and understand that a DVOM presents no load so it will show a phantom at times where a WIGGY will not but this one you can feel. It's there and can be duplicated even when all power to the site is off. It is a potential that is developing across 2 driven ground rods that are 200 feet apart. The only other source that is close is the RR and POCO substation feeders.
 
How much current flows on the EG when the trailer is plugged in, power off? Unplug it. Check voltage at trailer & current flow on EG again.

I believe, by NEC, the pedistal should have 2 rods connected to the EG. This should reduce the step potential in the immediate area of the pededistal, but the current flow on the EG will probably go up. You will just be masking the real problem of stray current. I think you need to find the source before you apply to many solutions that just move the problem around or create bigger ones.
 
Stoped by for a quick check today, I connected 300' of #14 to a gate post at the front of the lot and drove a rod at the back of the lot. I got 14 volts on a DVOM and swear I got a spark when I connected directly to the rod., main to the site was open and I was connected to metal that had no electrical connection to the service or the trailer. I did not have my amp clamp so I did not check to see if there was current flow.

I know DVOM readings can be misleading so I will be going back on monday to see if any current is flowing between rods. Then we will get POCO and the RR involved.
 
What you have here is that the trailer metalwork potential is the same as the ground rod by the transformer, which is 200 feet away from the ladder soil potential, and over 200 feet of soil there is going to be potential difference.

If you drive enough ground rods at the trailer (ie if you can get the impedence low enough) you will get the PD to near zero.

If you build a ground grid (or a circle of ground rods) bonded to the trailer you should be able to get the whole lot equipotential, and thus remove the shock hazard.

The safest approach is to have an isolating transformer at the trailer, so you have a SDS, but with no bond from the secondary side to the supply side and use local ground rod or two, so there is no connection to the pole ground 200 foot away. But.... I'm pretty sure you can't do that in a code compliant manner.

Oh yeah - measuring the current flow with your 300 ft bit of wire; the current you'll measure is entirely dependent on how low you can make your circuit impedance, which depends on the impedance of the wire, and the impedance of your test rods to ground. Get them low enough I suspect you wont need a clamp, just the wire getting hot will be all you need to know :)
 
dbuckley said:
Oh yeah - measuring the current flow with your 300 ft bit of wire; the current you'll measure is entirely dependent on how low you can make your circuit impedance, which depends on the impedance of the wire, and the impedance of your test rods to ground. Get them low enough I suspect you wont need a clamp, just the wire getting hot will be all you need to know :)
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I took my 300 foot wire and went from the service ground rod to the chain link fence at the rear of the property and got 14 volts. When connected to the fence 2.7 amps flows. To prove to the people on site what was happening I took a 12 volt 55w sealed beam lamp and put it in series with my wire, it glowed like it was on about 10 volts. I also tryed clamping my 300 foot wire to the ground rod of the building next to the problem property and got the same results.

Both services tested are independant (their own transformers on the poles) the problem property is 200A 3Phase Wye, the other building looks to be 200A 3Phase Delta and may be corner grounded.

I can also light the lamp between the trailer and the rear property fence but this is expected as the circuit powering the trailer has a #6 ground from the service up front. The trailer pedastal (250 feet back from the service) has 2 rods driven 14 feet apart and the connection of the rods to the pedastal has .3 amps flowing.

This is all with the main breaker to the site OFF.

In the morning the engineers for the owner will be out, I expect to have the POCO on site later in the day. I am open to any other ideas before the finger pointing starts in the morning.
 
dbuckley said:
You may have missed this, so I'll say it very clearly... this is the way it's supposed to be; there's nothing wrong...
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So we are saying that if I plug a drill into an outlet in this trailer and operate it while standing on damp earth I should get a shock, I do not buy it. Even a GFCI can not protect you from this as it is on the ground wire.

I have seen many sites larger than this one and have never found a ground potential that you could light a bulb with.

Do you say it's normal due to the proximity of the POCO HV lines and the RR or it is normal everywhere??

The same "light the bulb" also exists between the lot lighting poles and the rear lot fence that is 4 feet from the pole. If someone was to touch both at the same time they will feel it if they are wet. I cannot say this is alright even if the entire installation complys with the NEC (and it does)
 
I just can't help but think that this is primary neutral current returning thru the ground to the sub station. It seems, (of course, I'm unsure) that creating an equipotential bonding grid would help reduce this problem to acceptable levels. The grounding ring you mentioned earlier seemed like a good move. Sorry I can't be of more help. :smile:
 
Have a read of the document entitled The Hazardous Multigrounded Neutral Distribution System And Dangerous Stray Currents (PDF) downloadable from this very site.

Note the second word is 'hazardous', which describes perfectly your statement "So we are saying that if I plug a drill into an outlet in this trailer and operate it while standing on damp earth I should get a shock".

And you are right; GFCIs cant protect against ground to ground shocks. The only ways around this problem are the use of an equipotential zone, or removal of foreign ground reference.
 
I just looked at the link you posted, lots of info but it looks to me like it was written by someone who has an axe to grind with current utility practices. I will read it futher in the morning when I am on the clock.

In the mean time I have been directed to involve the POCO in the morning. Will keep you posted as to the outcome.

I see a grid in my future but am I just going to put it around the trailer, what do I do with the site lighting poles that are within the reach of the fence??
 
Well POCO was out today, first lineman arrived in a bucket truck. He did some quick voltage readings between the rear fence and a short rod a few feet away and was geting readings in the 5 volt range . He then looked at my light bulb between the rear fence and the site lighting pole and called his supervisor. The supervisor said he would be right over he wanted to see this.

The supervisor arrived and both him and the lineman took various readings. They wanted to have some of the overhead shut down so they could see if that was where it was coming from but that could not be done without orders from higher up. They then broke out some maps and found that 1 of the risers on our side goes under the rail road and comes back up on the other side of a highway. They went to the highway side of the riser to do some checking and found that the steel sheld that protects the cable going up the pole had 21 amps flowing on it.

They are now reworking several splices and making arangments to have a missing neutral that crosses the highway with the 3 phase primary that fell 6 months ago replaced.

They are very concerned about libility so I think they will get to the bottom of it. Will keep you posted
 
ptonsparky said:
How much current flows on the EG when the trailer is plugged in, power off? Unplug it. Check voltage at trailer & current flow on EG again.

I believe, by NEC, the pedistal should have 2 rods connected to the EG. This should reduce the step potential in the immediate area of the pededistal, but the current flow on the EG will probably go up. You will just be masking the real problem of stray current. I think you need to find the source before you apply to many solutions that just move the problem around or create bigger ones.
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The NEC only requires two ground rods if one has a measured resistance of more than 25 ohms.
A ground rod will not reduce the step potential, due to the resistance of the earth, the voltage falls off rapidly, so there is 90 volts at 3 ft from the rod, with 120 volts and 25 ohms resistance.
 
We have had about 24 hours of POCO here, they got here yesterday at 3pm and worked out on the Highway till after midnight. They returned today and where still there this evening.

They got the 14 volts that I had from fence to pole base down to 7 volts. I can still make the filiment in my 12 volt bulb glow but it loads the voltage down to 4 volts. They are headed in the right direction.

I cannot beleve that the problems they are finding are 800 to more than 1500 feet from my site and they have the effect that they do.

I was woried about the POCO passing the buck or blowing us off completly but that has not been case at all. They want to clear this up so they have no liabilty. They do not want to be in the news as the citys utility was when they killed a few dogs and shocked their masters while walking over manhole covers and street boxes.
 
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