Locating an underground fault

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sparky 134

Senior Member
Location
Joliet, IL
A customer is having issues with the power arriving at the meter socket from the POCO transformer. At first the voltages were 120, 120, 314 and this is on a 120/240volt 3phase 4wire system. POCO is called back to the site and the crew discovers the transformer is wired incorrectly. After rewiring the voltages were 120, 120, 208. I informed the customer the third phase should be 240volt.

Fast forward a week and I received a call saying the compressor that is fed from this service will not start. The voltages recorded by the service guy are 120, 95, 208. POCO says the problem lies with the customer owned underground cable which was installed +/- 14 years ago.

I'm thinking there is a fault in the cable and the B phase is leaking to ground. Is there a tool I can use to determine the location of the fault ?

The service was existing. The extent of my work is limited to the raceway/wire from the meter socket to the equipment.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
After rewiring the voltages were 120, 120, 208. I informed the customer the third phase should be 240volt.
That's incorrect; the expected voltage on the high leg is 208v. This is a high-leg Delta.

Fast forward a week and I received a call saying the compressor that is fed from this service will not start. The voltages recorded by the service guy are 120, 95, 208.
There's a problem with the line with the 95v. No doubt, the voltage varies with load on that phase.

POCO says the problem lies with the customer owned underground cable which was installed +/- 14 years ago.
That's entirely possible. It has to be narrowed down by measuring voltages every place the wires are accessible (while the service is under load).

I'm thinking there is a fault in the cable and the B phase is leaking to ground. Is there a tool I can use to determine the location of the fault ?
If current was "leaking to ground", it would cause excess current on that conductor, but wouldn't lower the voltage.

There are such instruments, but I'm not that familiar with them, but others here are.

The service was existing. The extent of my work is limited to the raceway/wire from the meter socket to the equipment.
If the reduced voltage occurs at the meter terminals, then the problem is ahead of the meter.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
A faulted wire to ground, meaning earth not the equipment ground, does not neccesarily result in excessive current flow. Not even when they start arcing phase to phase. The resulting poor connection through the residue of left over aluminum does cause lower voltage and that may cause motors to overload.
 

hunt4679

Senior Member
Location
Perry, Ohio
We have run into this about a dozen times a year on these resi jobs when the laterals get knicked. We purchased a ground fault locator that pin points the problem within 3" square. Its a bit pricey but it gets used for everything from UF cable thats been hit by an edger (of course installed by some handyman or landscaper 3" below grade) to service laterals
Tempo - Progressive PE2003 Pulser Underground Cable Fault Locator
This link should work but I think we got ours for about $700 ish from the supply house www.stayonline.com/detail.aspx?ID=170
 
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