Disconnected Neutral

[Jeffrey R. Pope]

It's been a while since I've seen this condition, and I've always listed it as a "significant" defect. Can one of you remind me of the potential consequences of this condition?

[Bruce A. King]

The feeder legs will pass voltage through light bulb filaments and cause voltage to be present on related neutrals.

Even with a ground rod, some voltages can show up on grounds which include equipment cabinet metal. Very scary.

If no ground rod is present, you have a death trap for sure.

Too many variables including what is present in the house and how it is used but as a whole, just very dangerous.

[Jeffrey R. Pope]

That's basically what I remembered, but I needed a "refresher."

In this case, I could not find the grounding electrode - no rod and no visible water line connection. It was a 1960 build, so it's unlikely there was a UFER anywhere.

[Bruce A. King]

I always wanted to find that condition so I could make some voltage checks to see firsthand how everything acts. Not curious enough to insert the problem at home though....

I would probably make the client leave the house if I found that one to be safe.

[Marc D. Shunk]

Read this story:

http://www.mikeholt.com/mojonewsarch...d~20040506.php

[gbeaumont]

Originally Posted by Marc D. Shunk

Read this story:

http://www.mikeholt.com/mojonewsarch...d~20040506.php

Hi Mark,

can you explain how an open Neutral can cause an electrical fire, I'm pretty sure I understand whats going on, but would like to understand the issue better and no doubt many other members would as well.

Regards

Gerry

[Greg Fretwell]

The neutral carries the unbalanced current on the 2 hot phases. If everything was perfectly balanced an open neutral has little effect. The problem comes when one side is heavily loaded and the other side is lightly loaded. The voltage on the lightly loaded side will be proportionately higher than the other side.
An open neutral can cause equipment on the "high side" to burn up and start a fire. Usually it is just equipment destruction though.
Part of the problem is, as things blow up, the load is less, the voltage goes up some more and more stuff blows up.

[rbrady]

Another possibility is if only one circuit is bonded to ground (e.g. the furnace through the gas pipe), then all the unbalanced neutral load could be going through that one 12 gauge neutral wire.

Also in the article it sounds like the transformer had the neutral disconnected, so who know what was going on with that!!

Edit: I re-thought about the one circuit to ground comment and realized that the current would have to travel through earth/soil as a return path to the utility, which would greatly reduce the current capacity.

[Marc D. Shunk]

Originally Posted by gbeaumont

Hi Mark,

can you explain how an open Neutral can cause an electrical fire, I'm pretty sure I understand whats going on, but would like to understand the issue better and no doubt many other members would as well.

Regards

Gerry

Yes, consider the following diagram. It's for an open neutral on a multiwire branch circuit, but the same applies to the home's service. The home's service is, indeed, one big multiwire branch circuit from the power company. In the second diagram, the neutral is open. You'll notice that the TV is receiving a much higher than normal voltage, because it is only buffered by the resistance of the hair dryer, in this example. This higher voltage experienced during a neutral failure, in a home, could be up to a full 240 volts. Appliances and equipment normally powered at 120 volts may not be able to tolerate these higher voltages without catestrophic failure, sometimes involving a fire. An open service neutral is one of the very few genuine electrical emergencies. It's one of the few things that stands a real good chance of burning the place down. The best thing for a homeowner to do in such a case is to turn off the main(s) and call the power company.

[Jeffrey R. Pope]

Great info. Thanks Marc.

[Peter Doane]

Originally Posted by jpope

It's been a while since I've seen this condition, and I've always listed it as a "significant" defect. Can one of you remind me of the potential consequences of this condition?

Nice find Jeff. Just curious. Do you do the roof first like I do. I knew there was a good reason/ method to my madness. I do exterior, roof, interior etc.
After finding that I wouldn't be to anxious to check the homes electrical system and components. Hope business is good down in sunny Cal.

Peter

[Bruce A. King]

Another good item to mention is how this same situation can be caused by double tap branch neutrals (two neutrals from opposite feeder circuits).

When someone loosens the neutral bus bar screw (or it is loose already) AND the two neutrals are touching each other but not the neutral bus bar you will have 240V dropped across both loads just as in the 2nd picture above.

Good picture to show people who don't think double tap neutrals can be a big deal. I agree, they are rarely a big deal but its not our job to assign low risk factors.

[Jeffrey R. Pope]

Originally Posted by pdoane

Nice find Jeff. Just curious. Do you do the roof first like I do. I knew there was a good reason/ method to my madness. I do exterior, roof, interior etc.
After finding that I wouldn't be to anxious to check the homes electrical system and components. Hope business is good down in sunny Cal.

Peter

Yep. I do exterior, roof and then interior.

In this case (bank owned REO) there were no appliances, fixtures, or switches, and only an occassional receptacle. Not much chance for anything to go wrong. . .

Oh, and yes. Very busy here

[brian winkle]

Originally Posted by rbrady

Edit: I re-thought about the one circuit to ground comment and realized that the current would have to travel through earth/soil as a return path to the utility, which would greatly reduce the current capacity.

Not necessarily. The gas pipes are normally bonded to the water pipes, which are bonded to the electrical system. In your example, with an open neutral, and the grounds and neutrals all connected at the main panel, neutral current would back up onto the grounding system, and find it's way back to the transformer thru the piping systems and the neighbors neutral connection. This path would likely be high enough in resistance to cause problems.

[Marc D. Shunk]

Originally Posted by brian winkle

... neutral current would back up onto the grounding system, and find it's way back to the transformer thru the piping systems and the neighbors neutral connection.

You're assuming way, way, way too much. Consider a rural property with no TV cable, no municipal water piping, and no natural gas service. This describes millions and millions of homes. The only "path", if you even want to call it that, is by way of the rod electrodes. No path at all, really, at the voltages we're talking about.