110-7 Neutral Ground Test

[dshipcott]

When testing branch circuits to assure that there does not exist a connection between the neutral and ground conductors, what is the acceptable resistance allowed between the conductors?
What is the proper instrument to use for this measurement?
I have received different opinions from testers, inspectors and engineers.
In addition to your opinions, I am looking for documentation that will define the requirement for performing the test.

 

[bennie]

Re: 110-7 Neutral Ground Test

An ohmmeter. Open all circuit breakers. Lift the neutral from the bus put test leads between the bus and the neutral. Infinity is good, anything else is a problem.

[ July 16, 2003, 07:33 PM: Message edited by: bennie ]

 

[Nick]

Re: 110-7 Neutral Ground Test

I have done this test many times and it's not that straight forward. Metal Halide ballasts read from neutral to ground sometimes. No rime or reason just some do and some don't. These are new fixtures that work perfectly. So how do you certify the system as neutral to ground free at this point?

 

[tom baker]

Re: 110-7 Neutral Ground Test

What kind of ohmmeter? This test should be done with a megger. An electronic ohmeter has too high an input impedance for this use. At least use an analog meter, like the good old simpson 260.

 

[bennie]

Re: 110-7 Neutral Ground Test

According to GE the metal halide ballast is an autoregulator type. A NG connection indicates a failure of insulation.

 

[brian john]

Re: 110-7 Neutral Ground Test

The proper method is to turn off or disconnect all loads, isolate the neutral/grounded conductor from the equipment ground and Megger at 1000 VDC for 600-volt cable. The reading should be 1 mega ohm or higher, we prefer 5 mega ohms. We also start at 250 VDC to avoid damaging any equipment someone inadvertently left connected. TVSS should also be removed from the circuits

 

[roger]

Re: 110-7 Neutral Ground Test

Aside from faults in appliances, (fixtures in this case) the original post would indicate wiring only. Lets open all switches and remove all connected loads then use any test method mentioned before.

Roger

 

[bennie]

Re: 110-7 Neutral Ground Test

We are not looking at a high voltage between the NG. A flashlight continuity tester will likely prove the circuit. I have a 9 volt buzzer that works.

The load can remain. All that is necessary is to open the branch circuit breakers. If any part of this circuit is in contact with the ground, the light will go on.

If the L-1 side is grounded the breaker should trip, if the L-2 side is touching the ground the light or buzzer will indicate same.

Don't use a high pot or megger across an unknown load.

 

[karl riley]

Re: 110-7 Neutral Ground Test

To determine if there is a N/G connection without having to disconnect anything or flip any breakers is simply to go in the panel, clamp an ammeter around the hot and neutral together. If the reading is zero there is no N/G connection in the circuit.

If you get a reading, then measure the hot/s and then the neutral. If neutral is less than the hot/s then there is either a N/G connection in the circuit or else two neutrals from different circuits are connected. If so, by repeating this test on the other circuits, you will find a neutral with too much current.This is the one connected to the first neutral, usually in a junction box.

The full tracing technique is spelled out in my book, (Tracing EMFs in Building Wiring and Grounding) but that's the way to identify the circuit. To trace out in the circuit and find the N/G fault the fastest way is to use a gaussmeter or detecting coil to an audio amplifier (Magstick). And/or a clamp-on ammeter.
The gaussmeter, by the way, tells you which circuit has the problem even before using the ammeter.

Another instrument will tell you if the N/G fault is within 10' of a receptacle, if it happens to be a recept. circuit: SureTest Circuit Analyzer.
Karl

 

[don_resqcapt19]

Re: 110-7 Neutral Ground Test

Karl,
The clamp on won't work unless there is a load actually on the circuit with the neutral to ground fault. Also is a standard ammeter sensitive enough for this application? It seems that the physical placement of the wires within the amp clamp will make a difference in the reading.
Don

 

[bennie]

Re: 110-7 Neutral Ground Test

Karl: Where is the net current to create the EMF?

 

[dshipcott]

Re: 110-7 Neutral Ground Test

This is some great information. Thanks for your input.
What I am looking for is information I can use to define the testing requirement and methods to satisfy the requirement that many city inspectors are enforcing.
The last situation I delt with the inspector wanted a reading of 100 megaohms or or greater on a ac ohm meter. Another inspector insisted we use a greenlee 6701 tester.
I am told that 1 megaohms is the NETA standard.
I am trying to gather the information to develop an procedure my electricians an use to complete this testing process. I would like to be able to review the information with the inspector prior to the testng so we can agree on the proper method.
Note: The testing I am talking about is for new branch circuit wiring tested prior to be energized.

 

[bennie]

Re: 110-7 Neutral Ground Test

I stand by my first answer...Open the breaker, lift the neutral and measure to ground. Use an ohmmeter.

 

[dshipcott]

Re: 110-7 Neutral Ground Test

AC or DC meter?
Is only infinity acceptable?
What is your opinion based on?
Remember I need to sell this to inspection authorities.
Thanks again for your help.

 

[bennie]

Re: 110-7 Neutral Ground Test

I have a Micronta digital multimeter. 10 megohms input resistance.

This meter will indicate over range when the neutral to ground is open. For me, this is all I need. For your inspectors, good luck.

Current won't flow on an open circuit, voltage is not an issue. Common mode voltage is a different ballgame.

 

[dshipcott]

Re: 110-7 Neutral Ground Test

That make sense.
What is common mode voltage?

 

[bennie]

Re: 110-7 Neutral Ground Test

Common mode voltage is...The noise voltage that appears equally from current carrying conductor to ground.

In other words...It is the difference in voltage between the neutral and the equipment ground.
With no electrical connection there is only common mode voltage. With a connection, common mode current will flow.

 

[karl riley]

Re: 110-7 Neutral Ground Test

Don, in my work all circuits are energized, such as in a busy office environment. I don't have to shut down anything. Usually the misconnection is in the lighting circuits. The magnetic field I see on the gaussmeter tells me as soon as I walk into the area that there is a neutral wiring problem.

The clamp ons I use show milliamps, though the readings are usually in the amps. Digital. Several sizes are needed to get at small and large conductors. Position of conductors in the clamp makes no difference. The measurements are precise, which helps in the trouble shooting. 0.66 amps net current on a circuit will be found as 0.66 amps on wherever the neutral was diverted to.

If the circuits are originating in a subpanel, a quick clamp-on around the feed hots and neutral together tells you right away whether to expect N/G either in the panel or in a circuit. A quick clamp around any equipment grounds verifies what is happening.

In residential work I have all lights turned on before I take measurements. I may plug a portable load into some receptacles if needed.

Bennie, "net current" applies to current on a single conductor such as a water pipe, which produces a high field because it is uncancelled by any other conductors. The same net current and the same field will be found on the circuit which has been robbed of that amount of its neutral.

Thus in a building where some of the neutral is going out the water pipe, the exact same net current and field is found on the service drop.
Karl

 

[karl riley]

Re: 110-7 Neutral Ground Test

Since the question was how to measure N/G connections before energizing, I have this to add: If you remove a neutral from the bus and test resistance to the box, and do not get infinity there are two possible causes: either neutral is connected to grounding conductors somewhere, or the neutral is connected to another neutral from another circuit out in some junction box. This other neutral will still be grounded to the bus, so this method works only if you remove all neutrals! What a pain!

Of course if you do the testing before connecting the neutrals to the bus it would be a lot easier.
Karl

 

[dereckbc]

Re: 110-7 Neutral Ground Test

For new installations where there is no load, or the load can be turned off and disconnected a DVM ohmeter test is good. The high imput impedance is not a problem on the resistance test, only voltage measurements where the circuit is open. Even a Simpsom 260 will work fine.

I do not suggest using any type of meggar because it could damage load equipment if connected. Otherwise it can be used, but only if certain there is no load equipment connected.

For receptacle testing there is no easier or better tool than the Sure Test. It will tell you everything about the circuit. The only time it could mis-lead you if you are extremely close to the N-G bond and give you a false neutral-ground bond.

For in service testing I like Karl's idea of clamping the phase and neutral and look for 0 amps. Any indication would indicate leakage into ground. However it takes a very sensitive clamp-on meter to do this, and would not tell the isolation impedance of greater than anything. Only pass or fail.

Another in service test I use is simply measure the voltage between the ground and neutral conductor on a known connected load. If you see a voltage drop then you have a good idea the circuit is OK. Again this method has some drawbacks. There must be enough load far enough away from the N-G bond to develop a voltage drop. It is simular method used by the Sure Test meter. The difference is the Sure Test puts either a known 15 or 20 aqmp load on the circuit, and a meter measurement load current is unknown unless measured by a clamp on.

Also don't forget you can verify by simply using a clamp-on meter on the MJB is it is wire or cable. If it is a bus, it will not work. If you see current you know you got a problem.

Good Luck.. Dereck

[ July 17, 2003, 02:31 PM: Message edited by: dereckbc ]