Lost neutral

[tpeterson]

I need some help here. I work for a city which has there own power company. Recently we had an incident where a tree had rubbed through a service line neutral burning it clear and frying T.V.s and clocks in the house. I assume that the damaged equipment was due to voltage going from 120 to 240 to the equipment. The power company is telling me that if the house was properly grounded the grounding electrode would act as the neutral return path through the ground and not fry the appliances. I have always been taught that if you loose your neutral you get 240 volts to the 120 volt circuits in the house. My question is could your grounding electrode really serve as your neutral Conductor through the ground? (Depending on resistance maybe)

 

[don_resqcapt19]

My question is could your grounding electrode really serve as your neutral Conductor through the ground? (Depending on resistance maybe)

Most electrodes have too much reistance to provide any protection from this type of problem. The one execption would be a common metal water piping system that is bonded to other services in the area. In that case you have a metallic path and may not even notice that the srevice neutral is open.
Don

 

[winnie]

The grounding electrode system, combined with grounding at the transformer, creates a parallel path for the service neutral.

If the resistance of this parallel path is low enough, then in theory it could serve as a 'backup' for the service neutral.

However grounding electrode systems are not designed to perform this service, and the standards, such as they exist, for a 'properly installed grounding electrode system' permit resistance that is about 100-500x greater than that necessary for the parallel path through the earth to provide the capability of a service neutral.

Grounding electrodes can have low enough resistance to serve as the neutral for distribution voltages, but I rather doubt that you have 24kV coming into your house. *grin*

As Don mentions, a common metallic underground water system can provide an alternate metallic path back to the transformer, but this really isn't a path through the soil. As he mentions, the return path to the transformer is via your neighbor's service.

-Jon

 

[megloff11x]

And watch how fast your pipes corrode from this or develop a Voltage and zap you or your neighbors...

You actually have 240V across the parallel loads on one line, all in series with the parallel loads on the other line. They are in series because the neutral bus, now disconnected from the utility, nonetheless connects them together.

If these loads were balanced, you wouldn't fry a thing.

One good thing about the switching supplies used in computers and many other appliances is that they can often accept any Voltage between 85-264VAC RMS (100VAC - 15% for Japan and 240VAC +10% for other parts of the world). They won't be as likely to cook, but if the Voltage jumps around as it will (as other stuff cooks) they may suffer too.

Matt

 

[roger]

As already stated, there are Grounding Electrode Systems that might provide a neutral current return path but that would be an exception not the rule, the majority of NEC compliant GES's will not and are not intended to.


Roger

 

[allenwayne]

I need some help here. I work for a city which has there own power company. Recently we had an incident where a tree had rubbed through a service line neutral burning it clear and frying T.V.s and clocks in the house. I assume that the damaged equipment was due to voltage going from 120 to 240 to the equipment. The power company is telling me that if the house was properly grounded the grounding electrode would act as the neutral return path through the ground and not fry the appliances. I have always been taught that if you loose your neutral you get 240 volts to the 120 volt circuits in the house. My question is could your grounding electrode really serve as your neutral Conductor through the ground? (Depending on resistance maybe)

If they really expect a ground rod or a ufer to act as a neutral, they need to stop smoking what they are smoking .For those of you that have encountered the effects of a lost neutral you already know that as stated a all metallic water piping system might serve as a neutral in a sense.Not a good idea though. I have seen some really strange things occur from a lost neutral.But have not seen a grounding system eleviate the lost neutral problem.

 

[jeff43222]

I have always been taught that if you loose your neutral you get 240 volts to the 120 volt circuits in the house.

Not always. It depends on how the branch circuits are wired. If every circuit in the house is a 120V circuit with its own neutral, losing the service neutral would only cause everything to stop working because the circuit would have no return path. However, if those 120V circuits are part of multiwire branch circuits, i.e., sharing their neutrals with other 120V circuits, then losing the neutral would turn all of those 120V circuits in parallel into 240V series circuits. That would indeed cause the TVs, clocks, etc. to fry.

This is one reason I've never been a big fan of multiwire branch circuits, although I have installed them in some cases.

 

[winnie]

Not always. It depends on how the branch circuits are wired. If every circuit in the house is a 120V circuit with its own neutral, losing the service neutral would only cause everything to stop working because the circuit would have no return path.

This is not correct.

Even if every circuit is wired with its own dedicated neutral, all of these neutrals, from _both_ supply legs, are connected to the neutral bus.

In this case, if the service neutral is lost, the 240V series circuit would be from leg A ungrounded conductors, through the loads, through the leg A grounded conductors, _through the neutral bus_, through the leg B grounded conductors, through the leg B loads, and then through the leg B ungrounded conductors.

-Jon

 

[allenwayne]

This is not correct.

Even if every circuit is wired with its own dedicated neutral, all of these neutrals, from _both_ supply legs, are connected to the neutral bus.

In this case, if the service neutral is lost, the 240V series circuit would be from leg A ungrounded conductors, through the loads, through the leg A grounded conductors, _through the neutral bus_, through the leg B grounded conductors, through the leg B loads, and then through the leg B ungrounded conductors.

-Jon

I agree 100%. That is as long as the service is a 240 volt serviceElectricity will find a return path even if it via another phase leg and through your TV,Micro,surround system .......................etc.

 

[jeff43222]

I stand corrected. I forgot about the neutrals all being tied together at the panel.

I rarely see lost/loose service neutrals, but I see poorly done wiring on MWBCs fairly regularly. Losing the neutral on a MWBC would fry the equipment, but that wouldn't happen on a branch circuit with a dedicated neutral.

Perhaps I shouldn't be so reluctant to install MWBCs.

 

[realolman]

I'm not going to speculate about what kinds of loads might be connected, but if they were all equal resistors, wouldn't you still drop half the voltage across each load?

2 loads connected from each phase to neut. 1000 ohms each would still drop 110 v across each one, if you lost the neutral.


Like I said, I wouldn't try to speculate about what kinds of loads, or what impedance they would all add up to be... but my point is: it might not fry anything.

 

[Pierre C Belarge]

I'm not going to speculate about what kinds of loads might be connected, but if they were all equal resistors, wouldn't you still drop half the voltage across each load?

2 loads connected from each phase to neut. 1000 ohms each would still drop 110 v across each one, if you lost the neutral.


Like I said, I wouldn't try to speculate about what kinds of loads, or what impedance they would all add up to be... but my point is: it might not fry anything.

this is correct. The voltages will divide according to Ohms law.