triple D

That is exactly correct Joe, your trouble shooting skills are right on. The neutral slowly burned itself the rest of the way apart over time. The breaker would of never held, from the time the screw was installed. Was this person a licensed electrician by chance?

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jsbeckton

He is not a licenced electrician. I still am having trouble understanding how a open neutral can cause it to be at 120V?

Also, should the breaker have tripped?

john4153

Let me try to explain why one can see full voltage or almost full voltage from the com to ground, when com is open. Refer to the following diagram. I have represented each load as a resistance.



It is important to note that the voltmeter draws almost no current. Consider first the switched load. If it is switched on, there is a direct path from hot to the com side. Since there is no current drawn by the voltmeter, there is no voltage drop across that load resistance. Thus, one sees the full voltage when measuring to ground. Now assume the switched load is off. Why can one still see a voltage in most cases? Well there is always some leakage which may be a very high resistance and might be measured. But more commonly in modern equipment, there is going to be some real, but very small load, such as an LED (whether seen or not), maybe a defrost timer, etc. In that case, the no-current voltmeter will see the voltage. That is why I suggested applying a load across the points being measured.

Now, are there instances in which an open com may result in no voltage reading from com to ground? I am sure there are, but considering the complexity of the wiring in our houses and the number of potential non-switched loads on the com lines down-stream of the break, it might be uncommon. Also, given the close proximity of the com wire to the hot wire, coupling to the field around the hot line to com (think of a radio antenna) could give a voltage reading. Just look at how the value on an AC voltmeter will tend not to be zero when one lead is grounded and you touch the other or just leave it in free air.

Regards, John

jsbeckton

If the com was not open wouldn't you still have an open path between the hot and the com? Why does it read 120V if its open but ~0 if it is closed.

So would there be any reason for the breaker to trip?

john4153

I am not sure I understand the point of confusion, but let me try to exlain.

First case: Assume the com is not open and the switched load off. Com-to-ground voltage will be real close to zero (I'll just call it zero from now on), because com is grounded.

Second case: Assume com is not open and the switched load is closed (on). Com-to-ground voltage will still be zero. Why isn't it 120 measured through the load, as happens in the case of an open com? Because, there is a load (current) and the voltage drop across that load (i.e., between hot and com) will be 120 V (Ohm's law). Com is grounded, of course, and the only reason current flows to give that voltage drop is that the grounded conductor provides a return path.

John

jsbeckton

So basically you are saying the open neutral is no longer grounded ands thus retains the voltage when the switch is closed because there can be no current flow back to the ground?

So there would be no reason for the breaker to trip if there is an open neutral?

Thanks

john4153

I think we are saying the same thing. John

triple D

If the nail or screw had touched the hot to the neut as you earlier indicated causing some kind of 120volt anomoly on the neut, you would blow the breaker in less than a second. you would never have had the chance to even measure voltage.

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