I was in school today...

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zappy

zappy

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And the teacher was talking about checking outlets with your voltmeter and he said to check neu to grd. and he said if you see 1-3 volts thats ok but if you see more than that you have a overload.I dont understand what he talking about.I dont see the reason to check neu to grd. can someone explain why you should do this.Thank you.
 
he was saying to check neutral to ground cause you shouldn't have any voltage between those two and if you do.....that means a neutral is bonded to ground somewhere or vice cersa....which means you have a problem and need to find it
 
The neutral and ground eventually terminate at the same point (main bonding jumper) so there should be no voltage between them. In reality, there will a slight voltage due to the way grounds and neutrals are run and landed. He's right in saying there shouldn't be any more than a couple of volts difference. If there is I wouldn't buy the overload, I would tend to think along the lines of an open or poor connection somewhere.
 
When you read the voltage between the neutral and the grounding conductor in a correctly installed circuit, you are actually measuring the voltage drop on the grounded conductor. This is about half of the voltage drop on the circuit so a voltage reading over a couple volts can be an indication of excessive current, excessive circuit length, or a poor connection on the grounded conductor.
 
byourdesky said:
he was saying to check neutral to ground cause you shouldn't have any voltage between those two and if you do.....that means a neutral is bonded to ground somewhere or vice cersa....which means you have a problem and need to find it
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A neutral to grounding bond would reduce the voltage between the neutral and the grounding conductors...not increase it.
 
I only do commercial, and with pipe on bar joists, the ground is almost always at a lower potential than the neutral, so the voltage reading on the neutral will be several volts (the voltage drop on the wire between the point you are measuring and the main bonding jumper). In addition, I think there is often some inductive measuremnet going on (I generally only use my digital meter). Maybe in your next class you could get your instructor to explain his reasoning more clearly and re post it, I would like to more clearly understand what he was saying
 
081216-1000 EST

zappy:

don_resqcapt19 gave a very good description.

To help you understand this:

The EGC (equipment grounding conductor - ground pin in the outlet) when correctly wired is connected to the neutral only at at the service entrance. Under normal conditions there is no load current in this EGC. There may be some small currents from electronic filters, but these currents are not normally large compared to the EGC wire size. Thus, from the connection of the EGC at the service entrance to any point on the EGC there should be virtually zero voltage drop.

This means that at any point on any EGC you have the equivalent of a long meter probe going back to the neutral at the service entrance. Or the equivalent of getting a wire, connecting it to the neutral at the service entrance, and running this external wire to one terminal of your meter wherever the meter is.

Therefore the the voltage read between the outlet ground pin and the neutral pin is the voltage drop scross the neutral conductor from the service entrance to the point of measurement.

Suppose you have a 100 ft length of #12 neutral wire with 20 A load current thru the neutral. The resistance of 100 ft of #12 copper wire is 0.16 ohms. The voltage drop is 20*0.16 = 3.2 V. Half that length is 1.6 V, or half the current at 100 ft is 1.6 V.

Use basic electricaal theory and you can answer many of these questions yourself.

If by accident or error there is a connection of the EGC and the neutral at some point in the circuit away from the service entrance, then the voltage reading would likely be less than calculated.

.
 
gar said:
081216-1000 EST

zappy:

don_resqcapt19 gave a very good description.

To help you understand this:

The EGC (equipment grounding conductor - ground pin in the outlet) when correctly wired is connected to the neutral only at at the service entrance. Under normal conditions there is no load current in this EGC. There may be some small currents from electronic filters, but these currents are not normally large compared to the EGC wire size. Thus, from the connection of the EGC at the service entrance to any point on the EGC there should be virtually zero voltage drop.

This means that at any point on any EGC you have the equivalent of a long meter probe going back to the neutral at the service entrance. Or the equivalent of getting a wire, connecting it to the neutral at the service entrance, and running this external wire to one terminal of your meter wherever the meter is.

Therefore the the voltage read between the outlet ground pin and the neutral pin is the voltage drop scross the neutral conductor from the service entrance to the point of measurement.

Suppose you have a 100 ft length of #12 neutral wire with 20 A load current thru the neutral. The resistance of 100 ft of #12 copper wire is 0.16 ohms. The voltage drop is 20*0.16 = 3.2 V. Half that length is 1.6 V, or half the current at 100 ft is 1.6 V.

Use basic electricaal theory and you can answer many of these questions yourself.

If by accident or error there is a connection of the EGC and the neutral at some point in the circuit away from the service entrance, then the voltage reading would likely be less than calculated.

.
Click to expand...
Thank you for your help and everyone else too.I think i should take a theroy class because some things like harmonics and frequnzy,potinal difference,equal potinial, and some of the weird stuff electricity does just puts a blank stare on my face!
 
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Boy i could sure use one of those nifty pictures right now!

Boy i could sure use one of those nifty pictures right now!

don_resqcapt19 said:
When you read the voltage between the neutral and the grounding conductor in a correctly installed circuit, you are actually measuring the voltage drop on the grounded conductor. This is about half of the voltage drop on the circuit so a voltage reading over a couple volts can be an indication of excessive current, excessive circuit length, or a poor connection on the grounded conductor.
Click to expand...

Question 1.excessive current on the neutral only or the hot too?

Question 2.excessive lenth couldn't you check the outlet and see what the voltage is hot to neu. to see if you are getting enough volts for your loads?

A poor neu. connection wouldn't loads be going on and off or doing weird things? I'm just wondering if there are other ways to check also.thank you for your help.
 
A1. Excessive current on the neutral would imply excessive current on the hot also, but in the case of an incorrectly wired circuit that may not always be the case. Also in the case of a multiwire branch circuit the current on the neutral can be much less that that on the hots.

A2. Yes that will give you the same information and will include the voltage drop on the hot as well as the grounded conductor. The test between the grounded conductor and the grounding conductor only gives you the voltage drop on the grounded conductor.

A3. If the voltage drop from a poor connection is not excessive you probably won't see any problems with the load.
 
why ask why

why ask why

the best reason to check neutral to ground is to make sure the neutral isn't hot (carrying 120-volts)
it can show you if hot and neutral are reversed
it can show you if your ground is hot

except for the exceptions, of course
 
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