it did not make sense

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Ponchik

Ponchik

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Location
CA
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Electronologist
I was troubleshooting two pole lights that I think on the incoming circuit conductors has a ground fault or phase to phase short that is intermittent. Lights are 208V

I use my fluke T5-1000. When I have the red test lead on the ground and the other lead to the phase conductor I get a zero reading. When I have the black lead on ground and the red on the same phase conductor the reading goes away.

When I have the leads on the incoming circuit conductors I read approximately 400-500 ohms. But When I reverse the leads the reading goes to zero. :?:?

Any Ideas is much appreciated.

Thanks
 
Little Bill said:
Sounds like you are reading through a diode.
Click to expand...

Where would I find diode on the circuit conductors (between the lighting contactor and the load) with the load disconnected?

Could there be a wrong reading similar to ghost voltage but for diode reading?
 
If the load were a relay coil, I could see a series diode to make a DC relay work from AC.
If there is a solid state ballast/driver in the load it may have circuitry which looks like a diode to low voltage but conducts both ways at higher voltage.

Sent from my XT1585 using Tapatalk
 
GoldDigger said:
If the load were a relay coil, I could see a series diode to make a DC relay work from AC.
If there is a solid state ballast/driver in the load it may have circuitry which looks like a diode to low voltage but conducts both ways at higher voltage.

Sent from my XT1585 using Tapatalk
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When you say "Load" you are referring to the lighting fixtures as "load"? If yes, the fixtures were disconnected from the circuit when I was getting the readings.
 
ptonsparky said:
Does the breaker trip after you have removed the fixtures from the circuit?

You really need a different type of meter to check for ground faults.
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No photo cell.
The breaker did trip twice with the fixtures disconnected.
 
Cu oxide can act as a rectifier or diode and permits current flow in one direction vs the other. That is why reversing the leads changes your readings. I’ve noticed it on the rare occasion when looking for faulted copper conductors with testers similar to yours.

A megaohm meter or Megger is what you need for testing faulted conductors.

Add:
That does assume Cu. IDK about Al.
 
ptonsparky said:
Cu oxide can act as a rectifier or diode and permits current flow in one direction vs the other. That is why reversing the leads changes your readings. I’ve noticed it on the rare occasion when looking for faulted copper conductors with testers similar to yours.

A megaohm meter or Megger is what you need for testing faulted conductors.

Add:
That does assume Cu. IDK about Al.
Click to expand...

Thank you. The conductors are CU. I will look into the megger testing.
 
edward said:
I use my fluke T5-1000.

When I have the leads on the incoming circuit conductors I read approximately 400-500 ohms. But When I reverse the leads the reading goes to zero. :?:?

Any Ideas is much appreciated.

Thanks
Click to expand...

edward said:
Thank you. The conductors are CU. I will look into the megger testing.
Click to expand...


Just my opinion. If you read zero ohms of resistance with a regular meter between conductors with both ends disconnected you know that you have a short. You can megger the conductors but they are not going to get any better.

What you need to know is how to located the short for repair or repull the conductors. Are the cables in conduit or direct buried? If in conduit and the conduit is not crushed then you are lucky ( time to buy lottery tickets).

To make sure of short you can hook the conductors back up with no load and use an ammeter to mearsure current flow where there shouldn't be any.
 
It does show ZERO with my meter. But what confused me is when I change test lead orientation the reading changes as well.

It is a 50 year old under ground conduit system but can not tell if metallic or PVC. It is buried under a driveway.

I am planning to bypass the supply circuit conductors with SO cord and do a temporary set up to further isolate the problem. But I am leaning towards a fault, either phase to phase or ground fault.
 
milldrone said:
The only ohm reading I would trust from a T5, is testing the ohms on a fuse (out of circuit).
Click to expand...

GoldDigger said:
FWIW, any residual voltage, especially DC, will screw up a resistance measurement with most meters.
Not nearly as much effect on a Megger measurement.
Click to expand...

Two conductors run in conduit with both ends disconnected should read infinity (with a little ohm meter). Anything less than that lets you know there is a problem. If I didn't trust my meter I would use a second meter to test, I never go out without at least two different meters.

A megger is used for insulation leakage. Normally high resistance. This is not insulation leakage it's a short. Electricians have been finding shorts with regular ohm meters for a 100 years.

If you think there is any residual voltage causeing a problem you can always short the ends of the cable you are measureing to ground.
 
ptonsparky said:
Cu oxide can act as a rectifier or diode and permits current flow in one direction vs the other. That is why reversing the leads changes your readings. I’ve noticed it on the rare occasion when looking for faulted copper conductors with testers similar to yours.
Click to expand...
Not calling you a liar, but I don't believe you.

A megaohm meter or Megger is what you need for testing faulted conductors.
Click to expand...
Agree

That does assume Cu. IDK about Al.
Click to expand...

An ohm meter can identify oxides?
 
To confirm any diode action there, meggar it first. The applied voltage of meggar (500V) would destroy the diode, if any. Now again check with ohmmeter. I am sure you would not get different readings this time.:)
 
edward said:
It does show ZERO with my meter. But what confused me is when I change test lead orientation the reading changes as well.

It is a 50 year old under ground conduit system but can not tell if metallic or PVC. It is buried under a driveway.

I am planning to bypass the supply circuit conductors with SO cord and do a temporary set up to further isolate the problem. But I am leaning towards a fault, either phase to phase or ground fault.
Click to expand...
I don't know when PVC raceways were introduced, but I don't recall seeing any used on original wiring installed before about 1975 or so, leading me to think anything 50 years or older likely won't have PVC raceway.
 
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