finding ground loops

[junkhound]

How do you check for ground loops?

My 94 YO neighbor died in August, his wife called today that bathroom outlet was not working. I do mostly airplane power systems, not residential.

30 YO GFCI in garage was tripping, immediately upon reset. Weird way house got wired (1985) via GFCI in garage, 50 ft from bath. GFCI outlet, not breaker.

No 'detectable' leakage current with 6A clamp on.

New GFCI, works ok, so figured bad GFCI after 30 years, no problems till yesterday. Checked known outlets what were dead, all clear in the boxes, nothing plugged in, no known lights on the circuit.

Putting all cover plates back on, hear gas furnace come on. 30 seconds later hear the new GFCI click open. Furnace off, no problem.

So, OK, now need to find where the original electrician crossed some white wires.

Any residential tricks for this? Other than opening up boxes and tracing white wires?
Expect a white wire voltage drop on furnace line (different breaker) is resulting in parallel path WW leakage > 6 mA thru the GFCI thru one of the 2 white wires on the GFCI output which has a > 6 mA parallel current.

I do have accurate enough meters to detect 4 mA current when no direct loads on GFCI outlet lines so at least can isolate which of the 2 white wires are involved on the load side of GFCI, but would think that type current would be >> 6 mA, possibly just capacitively coupled wire to wire, but all is NM, no multiple white wires in same conduit or anything. I have no idea if a bunch of NM all go thru the same stud holes though. The GFCI trip was not just at Furnace start, anytime it was on; Say 2 v drop on white wire to furnace, mA trip of GFCI would need 90 uF or so -- possible that motor start cap in furnace is involved? There is an 240 Vac A/C (off) that has about that value motor run cap on the compressor, probably need to check the white wire to that location also.

I'm assuming the old GFCI did not trip for say 6 mA, with age got 'more sensitive?' BTW, the old one checks ok by itself on a different individual circuit, did not measure the trip limits, but 2 kohm resistor trips it.

Am I missing anything? Any neat tricks for checking?

Whatever it is has been there for 30+ years.

 

[GoldDigger]

If the furnace is new enough to have spark ignition and a flame sensor, that has been known to cause leakage that should be below a normal 6ma trip.
But add in enough three wire dimmers or timers that use EGC instead of neutral and you could reach the trip point.

 

[kwired]

If it has been there for 30+ years and working along with a GFCI that does still work properly, I'd say something has changed recently.

Megger may be your friend here.

 

[gar]

151105-0901 EST

At 60 Hz, 120 V, and 0.006 A the impedance is 20,000 ohms. This corresponds to about 0.13 mfd. At 100 pfd per foot of wire to wire coupling capacitance this is 1300 ft of Romex.

Remove the GFCI, or jumper across it, then measure the unbalanced current leading from the GFCI location. This provides a steady state signal to use to look for the leakage.

At the furnace open the hot wire. Does the leakage disappear?

.

 

[junkhound]

Thanks for the insight on the furnace sensor, have never worked on a gas furnace except mom's 1953 pilot light model that is still going strong, don't make'm like that anymore.

Never found the crossed white wires, 3 mA in white wire, zero in black wire with furnace off, difference jumps to 8mA with furnace on. Probably 2 places with something interconnected, never found. The furnace gas pipe runs right across the front of the junction box to furnace!, < 1/8" clearance.

Moved GFCI in garage to first bathroom, 2nd bath from that one, so baths have GFCI protection. Put a regular outlet in garage.

Also replaced 7ea 30 YO backstab outlets that lady said would not hold the vacuum cleaner plug in them any longer. A couple of them had the ground wire under the screw but never tightened! Really cheap outlets from whatever contractor did the electrical (was not the HO), 8 backstab wires in some, not even any screw terminal on side. Musta been 18 cent outlets back in 1985, which was date on signed off permit.