Power surge

Status
Not open for further replies.
I

inspector 102

Senior Member
Location
Northern Indiana
Can anyone give me a good way to describe what happens when a power surge goes through a residential panel. I believe that a homeowner removed some of the grounded conductors under a load that created a power surge back on the other branch circuits. This in turn cause their computer to short out and be ruined. Any thoughts?
 
Re: Power surge

You may be referring to an open neutral in a multi-wire circuit.

When a three-wire circuit (two "live" conductors and a common neutral) is used to supply outlets, the loads on one live wire are actually in series with the loads on the other live wire. (Example - the 10 ohm load drawing 12 amps, and the 30 ohm load drawing 4 amps in Diagram A.)

The neutral carries the unbalanced load (difference in the two currents), as well maintaining the correct voltage (120 volts) across both loads.

If the neutral is opened, the total 240 volts is divided between the two loads in proportion to their resistance.
This can subject the higher resistance load to a damaging over-voltage.(Diagram B)

This also illustrates the importance of balancing the loads on multi-wire circuits, as much as possible.

Ed

3wire3.gif


[ February 13, 2004, 04:39 PM: Message edited by: Ed MacLaren ]
 
Re: Power surge

I am not sure the event you described could be classified as a "Power Surge", IMO an over voltage. Depending on where the neutral was lifted, and what was left connected, it is possible for the PC voltage at the plug to go to 240 VAC.
 
Re: Power surge

What kind of inspector are you ??? Electrical ??? If so you should know what an open neutral can do !!! Or the dreaded H.I. :D
 
Re: Power surge

A.Wayne, I am a combination inspector who was looking for some guidance. That is why I enjoy this format. I have gained alot of knowledge from participants on this site. I requested some opinions on the explaination for the homeowner. I understand the importance of a properly installed system and do everything I can to make sure that the code is complied with. This situation was an older service that I condemned after I became aware of problems. Know it is being reviewed by an insurance company because of damage done to a computer by a resident. I was simply asking for another way to explain the potentials of a power surge.
 
Re: Power surge

I`m not really sure what a combination inspector is ???But I apologize for being rude,you are right there is allot of very good info here.
To give an explanation as to what might have happened is almost impossible with the details given.But if there are 2 circuits on opposite phases that share a neutral or have neutrals tied together somewhere and one neutral is removed,or the sole neutral is removed,well computers,micros,televisions,etc. have a problem with 240 hitting them.Did anyone check neutral to ground for voltage ??? Or was a licensed Electrician called ???
 
Re: Power surge

Allen, it is a myth that loosing a neutral would or could apply leg to leg or phase to phase voltage on "one" load.

Look at Ed's diagrams and notice that in diagram (B) there has to be "two" resistors for anything to happen. With that said, there has to be voltage drop across both resistors, so full circuit voltage can not be present at any time on "one" resistor.

Roger
 
Re: Power surge

Roger,your missing something.Yes 2 resisters but one might be 1 ohm and the other 100 ohms.So it won't be 240 but might be 237 .Same results

[ February 17, 2004, 07:18 PM: Message edited by: jimwalker ]
 
Re: Power surge

Originally posted by roger:
it is a myth that loosing a neutral would or could apply leg to leg or phase to phase voltage on "one" load.
Click to expand...
Thanks Roger, I am glad you say that.

I think what throws some people is that if you remove the 10 ohm load and insert your digital volt meter in Ed's drawing you will see a reading very close to line to line voltage.

But having such a large difference in loads IMO is unusual.
 
Re: Power surge

Jim, how is that missing something, in your math is 237 the same as 240? I didn't say how much difference did I?

Read the posts closely Jim.


Roger

[ February 17, 2004, 07:28 PM: Message edited by: roger ]
 
Re: Power surge

The exact numbers depend on whats on each phase.
No 237 isn't 240 but i dought it will make any differance to that fried computer.Just how unbalanced do you think it needs to be before things fry
 
Re: Power surge

Jim did I say anything about it not frying a computer? Are you posting just to post again?

Roger
 
Re: Power surge

Originally posted by jimwalker:
Roger,your missing something.Yes 2 resisters but one might be 1 ohm and the other 100 ohms.So it won't be 240 but might be 237 .Same results
Click to expand...
So you picture a circuit with 120 amps on one side and 1.2 amps on the other?

A little bit unusual I would think.

I will not say I have never seen equipment damaged from lost neutrals, but many more times I have seen equipment not damaged by lost neutrals.
 
Re: Power surge

Bob,
So you picture a circuit with 120 amps on one side and 1.2 amps on the other
Click to expand...
definatly not the best balancing design. :)

Roger
 
Re: Power surge

The chances of it being that bad are slim.But should there be a motor or 2 one one side and only a small load like a 60 watt lamp on the other,then that bulb will be gone in a few seconds.The first thing people tend to do before calling sparky is try everything.And then even more stuff pops.Full voltage no but when we get 170 and 60 things are starting to go fast.
It would be volts unbalanced not amps that cause the problem.
And design ? Who designs around whats pluged in or what switch is on or off

[ February 17, 2004, 07:48 PM: Message edited by: jimwalker ]
 
Re: Power surge

Jim,Jim,Jim,
But should there be a motor or 2 one one side and only a small load like a 60 watt lamp on the other,then that bulb will be gone in a few seconds.The first thing people tend to do before calling sparky is try everything.And then even more stuff pops.
Click to expand...
"When the bulb is gone in a few seconds" (your words) there is no circuit, so regardless of what "people tend to do" (your words again) there wouldn't be any additional damage beyond the 60 watt lamp.

Roger

[ February 17, 2004, 07:57 PM: Message edited by: roger ]
 
Re: Power surge

Just a question:
With resistance being current limiting the voltage fall off curve is at the amperage the resistance is able to carry.

With that thought in mind a 100 ohm load on one leg of a service will supply near full voltage until 1.2 amps or 144 watts is reached on the other leg? then the voltage will flip flop or as some say sea saw to the other leg. I have seen this happen when the lower resistance load is switched off before the lost neutral problem was discovered. this would be very similar to voltage drop in a wire as the current level of the resistance is reached the voltage drops dramatically.
A plot on a chart looks like a shallow down grade ramp until the thresh hold is reached then just falls. This would support what Jim is trying to say wouldn't it?
 
Re: Power surge

Wayne,
kind of. If there were more lights and switches.
But, in Jim's scenario, there was only one 60watt light, so by Roger's post, once that light bulb blows, it's over. There are no more lights to effect it (in this scenario). (I think) :confused:
 
Status
Not open for further replies.
Top