Utility "fix it solution" for stray voltage on manhole plates/lighting

[ghostbuster7]

The utility is being plagued with their L.V.wires(less then 600 volts) in connection boxes corroding off and then making electrical contact with their sidewalk metal cover plates,metal lighting poles,streetwalk signs and metal push button controls .Their fix it solution is to install a 1:1 isloation transformer,leave the secondary side ungrounded and refeed these defective electrical systems.Their argument is there is no direct return ground path,therefore dogs and people will no longer receive electrical shocks.
Any comments??

 

[Skogsgurra]

A second fault on the opposite phase (phase A) will ground it. And then you have the same situation again if phase B touches a manhole or cover plate that isn't grounded.

It is easily demonstrated that such a situation is for real and that it will hurt or kill.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

At least put some form of ground fault detection on the secondary of this transformer so you can detect the first failure.

 

[jghrist]

How is an unintentional ground detected? Probably not for the first one. If one conductor corrodes and makes contact with a ground in one location, then the system is grounded (poorly) but will still operate. A second ground contact in another location with the other conductor will create a shock hazard.

A better solution would be to run a grounding conductor with the circuit and bond all metal parts that might get connected with the conductor. This would provide a path for fault current so that it the protective device will operate.

 

[waross]

On a 120:240 Volt system, the possible touch voltage with two grounds may be 240 Volts rather than 120 Volts.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[marks1080]

I know of a utility in this area that fixed the same problem by installing plastic buckets over the wiring in the manholes. Much cheaper option than the 1:1 isolating transformer.

 

[resqcapt19]

This issue is a result of one of the major differences in the rules found in the National Electrical Safety Code that the utilities use and the National Electrical Code that most others are required to use. The former does not require an Equipment Grounding Conductor for the circuits in question and the latter does.

 

[dpc]

I know of a utility in this area that fixed the same problem by installing plastic buckets over the wiring in the manholes.

I can imagine some poor utility engineer explaining in court why this seemed liked an acceptable solution after someone gets electrocuted.

The real fix is to start installing grounding conductors with the hot wires, as jghrist and rescapt19 have mentioned.

David Castor

http://www.cvoes.com

 

[stevenal]

NESC 384 requires grounding or guarding. 311 requires maintenance to the rules (nothing similar in NEC).

This is not an inadequate code problem, it is one of inadaquate enforcement.

Isolation transformers address neither grounding nor guarding.

 

[dpc]

I agree that NESC requires local grounding, but that does not always eliminate the potential shock hazard. There needs to be a low-impedance return path for the ground fault current. Grounding a light pole to a local ground rod does not provide this.

David Castor

http://www.cvoes.com

 

[Electic]

Are you certain this is the applied solution for the given problem?

Usually 1:1 isolation transformers are used to reduce "stray voltage" originating on the utility neutral conductor: voltage rise caused by neutral current returning to the substation. "Stray voltage" by this definition is a nuisance to farm animals that might be sensitive to the few volt rise that is typical. If stray voltage of this definition were being sensed by users of cross walk pushbuttons, the 1:1 transformer would be a good fix. It seems like that would be the application your utility is using these in.

The corroded conductors making contact seems like someone's misunderstanding of the problem. Most conductors in underground distribution systems are completely insulated in waterproof insulating systems. because of the insulation, the conductors are not prone to corrosion, and because the insulators are generally some kind of plastic neither is the insulation prone to 'corrosion'. In years gone by some exposed common neutral strand was left bare on the exterior of URD cables, that was sometimes prone to corrosion and resulting high 'stray voltage'; it seems like someone is mixing up this diagnosis.

 

[LionelHutz]

What's wrong with a transformer combined with some form of low current leakage detection? As long as the ground fault is detected and the circuit is tripped then it would be safe.

It sounds like right now the resistance of the safety grounding is high enough that the ground fault current will not trip the circuit. So, set-up equipment that detects and trips the ground fault current.

LittelFuse Startco has some different modules for detecting ground faults.

The SE-502 offers Class A GFCI trip levels and times for a 120V/208V 3-phase system. It wires to the neutral of a Y connected transformer and uses a dc current to measure the resistance to ground which avoids tripping due to the system capacitance. I installed one in our training room and it trips with about 4mA of leakage to ground due to a resistor but running motors does not trip it.

A SE-703 can be set to trip at 25mA to 500mA of leakage current. It uses a zero sequence CT to measure the leakage current. It would be used with a neutral grounding resisor if you wanted to limit the fault current.

 

[ghostbuster7]

Lionelh& Electic

I have included a typical picture of 1 type of problem found.The steel sidewalk access plates are not grounded.The wires are initially "marred" together.This connection is coming apart due to salt corrosion etc. and the live wires then make contact with the underside of this steel plate.The result is dogs etc. receive electrical shocks when one of their paws comes in contact with this metal
plate.Humans can also receive shocks.A very large U.S.utility is installing these isolation tx.s on any defective ccts. found.The fault is not corrected eg.faulted pole lighting etc.

 

[LionelHutz]

Well, I don't believe just installing a tranformer is good enough. There can still be the same shock hazard probem if a second failure on another phase of the same circuit occur.

 

[stevenal]

Never seen a ground on a manhole cover. I suspect they would be very difficult to maintain. I suppose this would generally qualify as "guarded", but certainly not in this case. Picture is pretty fuzzy when zoomed in to see the connection. What type of splice is this, and what do you mean by "marred?"

I'm sure the state(s) utility regulator(s) would be interested in this.

 

[Electic]

I do not believe 1:1 transformers are a satisfactory solution to this problem. Actually it seems there are several problems causing this potentially lethal situation.

1) In general I do not believe in grounding manhole lids but locally our inspectors are not making this call. But if the manhole lid IS grounded, that should eliminate the shock hazard. I would investigate better methods of maintaining the ground connection: attach a robust lug or two to the manhole receptacle that is not prone to come apart.

2) If the manhole lids are becoming energized, that is a another challenge of failed insulation and connections off energized conductors (not ground conductors) That needs to be fixed as a separate project, and the remedies investigated for durability.

There are many good grounding methods, and cable connection methods with corrosion resistant materials. I would consult with a grounding materials vendor rather than a isolation transformer. And I would absolutly be consulting with an cable connector vendor rather than an isolation transformer.