Meter testing for neutral/ground bond

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  #1  
Old 05-02-14, 05:54 AM
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Meter testing for neutral/ground bond

In order to find if neutral/ground are touching... say a pinched neutral in a light fixture or box or maybe somebody intentionally made a bootleg ground or otherwise connected neutral to ground downstream of the panel. How would you test? Is this correct:

Disconnect the neutral at the homerun box before the panel... with neutral disconnected (one wire goes back to panel other wire goes out to rest of the circuit) check for continuity between downstream neutral and the egc (in this case the metal box/conduit).

No continuity means no neutral/ground bond downstream.

Is that an accurate measurement?

Another method Tolyn suggested was to disconnect the egc (ie conduit) at a coupling and measure VAC between the two pieces of conduit. If no VAC, then its safe to assume neutral and ground (under load) are not touching downstream.

What other method can be used?
 

Last edited by ardmi; 05-02-14 at 06:13 AM.
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Old 05-02-14, 06:20 AM
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Yes, your test is good. Some purists will say it needs to be taken with half a grain of salt -- the equipment grounding conductor (conduit or ground wires) continuing downstream needs to be proper and intact.

(The original paragraph here was found to be barking up the wrong tree.)

At some later date or time, you will want to test the upstream portion for proper absence of neutral-ground faults by doing the same test at the panel i.e. turning off that circuit and unhooking the neutral for this branch circuit from the panel neutral bus bar for a few minutes.
 

Last edited by AllanJ; 05-02-14 at 06:46 AM.
  #3  
Old 05-02-14, 06:36 AM
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why delete allan? Your replies are top notch.
 
  #4  
Old 05-02-14, 06:49 AM
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The continuity test is better than a voltage test to show that you do not have neutral to ground faults.

But to show that you have a good path (whether hot or neutral or ground) back to the panel, a voltage test under load is preferable although not always easy to stage.

Some reminders: Turn off power before doing resistance or continuity tests. At least one of the places or objects (wires, terminals, etc.) you touch the meter probes to for a resistance or continuity test must be unhooked.
 
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Old 05-02-14, 07:09 AM
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Good info as usual.

In regards to the test where the conduit path is broken and VAC is measured between the two conduits.

With a low impendence meter, a reading of .090 was rendered between the conduits.

That's miniscule of course, but does that difference of potential indicate ground and neutral are bonded?
 
  #6  
Old 05-02-14, 02:35 PM
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Just checking back to see if anyone can speculate on my volt reading...

I separated a conduit at a coupling and measured between the two pieces and got .090 VAC. My thinking is if neutral and ground were touching downstream then the conduit would be serving as part of the neutral circuits return. Then there would be current on the conduit and separating the two (removing the bond) would then allow me to measure voltage between the two.

Is the VAC reading I received something I should investigate further or is that miniscule difference of potential normal?
 
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Old 05-02-14, 04:58 PM
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Your measurement of 0.09 volts between the separated conduit ends does not prove anything. The hot wire running for several feet inside the conduit probably induced a voltage in the conduit that the meter was capable of measuring between the now isolated downstream end and the still grounded upstream end. This "phantom voltage" can happen with no metal to metal contact between the hot wire and the conduit and no metal to metal contact between the neutral wire and the conduit.
 
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Old 05-03-14, 05:20 AM
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Thanks Allan, that is what I was thinking as well. However, I used a low impendence meter that eliminates phantom voltage... so I have to assume that minimal difference in potential is real.

I have read something about "skin effect" on conductors, perhaps this is the cause for that VAC reading?
 
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