This question appeared on the Mike Holt forum and they gave him a lot of advice to "call an electrician":
"I have (had!) an above the stove microwave which plugged in to a receptacle above it in a cabinet. It's been there for over 3 years and operated without any problems. A few weeks ago the microwave stopped working. I replaced the fuse in the microwave and the electronics functioned properly but when it was actually ran, it completely stopped working. I purchased a new microwave, mounted it and the second I plugged it in, there was a large popping sound and a large spark traveling down the backside of the microwave. Needless to say it spooked me a bit."
I would like to expose this question to a wider audience in the hope of finding a cause for this catastrophe.
The only thing I can think of is perhaps a 240 volt receptacle but I somehow doubt it. Has anyone had a similar experience?
~Peter

Your typical microwave comes with a NEMA 5-15 plug, so it shouldn't plug into a NEMA 6-15/20 outlet... I'd start with a voltage reading from the outlet under load... Something smells of open neutral/multiwire circuit.

Oh, Bob.....

MX you were the first person I thought of when I typed that

Thanks Randy!! Now we sit back and wait...

Tony all kidding aside two wire circuits have problems as well.

If you don't want to use the type of circuit that delivers the electricity to your home, that is up to you.

But I will tell you I am tiring of your unsupported posts that multi wire circuits are inherently bad.

You have based your opinion on anecdotal evidence and not science.

Your are certainly entilitled to give your opinion but enough is enough.

Bob

[This message has been edited by iwire (edited 01-03-2007).]

FWIW I would also be looking for an open neutral condition somewhere.

It could even be at the utility and the Micro was just the first victim.

I will also jump on this bandwagon.

Tony, if it scares you so much then by all means don't use the method, for those of us who do use it and never have problems with it, we will continue to do so. Your bad experiences can probably be attributed more to sorry workmanship of the installer(s)than to the wiring method itself.

If it's so terribly bad you should lobby to have all residential services redone with individual grounded conductors for each ungrounded conductor worldwide, after all they are MWC's.


Roger



[This message has been edited by Roger (edited 01-03-2007).]

"Your bad experiences can probably be attributed more to sorry workmanship of the installer(s)than to the wiring method itself."

I am no fan of multi wire circuits, being used just for the purpose of saving a few bucks, but that does not make them bad, or dangerous to install, they have their place, and i would use them without worry, the problems usually arise, when they are installed by someone that does not understand them.

Consumers should call an electrician any time a replacement item fails out of box.

Though my work is in regulatory approvals / product safety, I say this because one day I may be an electrician...

Peter,
I'd also say to start at the receptacle. Look for an open neutral and also check to make sure the polarity is correct.

About 4 years ago, I plugged in a brand new Bosch raised-vent exhaust and saw that same flash. Turns out the factory pinched a hot wire between two metal frame parts and when I plugged it in I got a flash and the breaker tripped.

Nothing a pair of pliers can't fix...

Ian (Pulling a permit to rewire a Levittown House! <not> ) A.

I agree with you on the multi wire branch circuit. If done correctly the have there merit. I pesonally limit my use of them in my are. As so many here do not understand them. I have seen many burned nutreals due to have both circiuts on the same phase. I once had a whole loadcenter done with multi wires and all were on the same phase. All blacks on 1 side all reds on the other opposite each but on the same phase. Looked good thou.


Ob(Phil)

Who knows this could be endemic all over the hose and the guy just thinks its normal to have some lights get brighter and some get dimmer every time he turned the old on on.

Had one like that last week. POCO neutral had obviously been on the way out, or been loose for some time from what I could tell be a breif interview of the guy. Lights had been doing the hokey pokey since he bought the house last year... (Home Inspectors said it was normal.... - I did a little experiment with dimming/raising some as an example - far from normal!) Anyway the guy gets a new plasma, and DVD - and a new dryer for himself for X-mas. He tells me as he is taking a break in his first movie on the thing, he throws some clothes in the new dryer - comes back out and the DVD and plasma are on fire.... He calls the cable company to complain.... They tell him to call an Electrician - he calls us - we tell him to shut off the main and we'll be there shortly - he calls back after fifteen minutes to cancel saying his neighbor has the same problem.... The guy turns his main back on to find out how to contact Comcast - blows up his computer.... (This is after warning the guy to keep the power off - numb-nuts still thinks it has to do with his cable line, as his neighbors TV also caught fire.) Smarter neighbor calls PG&E - they get there and fix the lost neutral - and find that his hacked together main panel has no bond - tell him to call an electrician - he calls back, and I go out there to find all 2-wire circuits - including the gas dryer - Carpenters doing foundation work had moved the water bond to a painted portion of pipe - no rods - but there was a bond at the main.... Turns out PG&E is buying TV's and other crap for most of the block.

I totally agree. But the consequences of an open neutral on a two wire circuit are usually that it simply stops working, right? No harm to the connected loads, usually. Now what happens when you open the neutral on a MWBC? Or for that matter, the service?


For branch circuits, absoultely. I have no MWBC's and any place I live in won't. But the service is a factor not in my control, and:


What you all have lost sight of, and if you've read my other posts on this topic carefully, I have no issues whatsoever with service conductors and connections being set up as multiwire.

For cryin out loud, even I know that the laws of electricity make the single neutral concept a "fact of life."

Where I take issue and frankly am a bit offended is when:


..since in those other discussions I have readily conceded that in other lines of work and installations, MWBC's are not necessarily a bad thing and can indeed offer benefits.

But to make such proclamations when one has not had the same experiences and deal with the type of equipment I have for the last 20 years is downright rude. I would not presume to tell anyone here that how they wire High-bay lighting, process MCC's or a factory for that matter (all types of installations where I have no experience) is "unsupported" and "anecdotal."

Again, if one cares to carefully review the topics where we have had this discussion before, the main things I need to make clear here are:

One, my position is based on the use of MW branch circuits only in my line of work;

Two, any details I have related are NOT unsupported or anecdotal, thay are the facts in the situations I have encountered;

Three, I have in the past and again reiterate that since each of us works in different areas of electrical installations, that what works (or doesn't) for me will or won't work for others.



It's not a matter of scared, it's a matter of providing the best possible performance to my clients. And if it works for others, so be it. I have no arguement with it.


Nope, have to disagree here. Out of all the cases (50+ and counting) I've dealt with, only two were workmanship issues. One was a poorly made connection, another was overloading due to sloppy phase placement.


LK, well said.

Obsaleet also made good points.

We all have our hot button issues, and I think we all know what mine is.

Tony IMO you are mistaken, your equipment can not tell the difference between a properly wired two wire circuit and a properly wired multi-wire.

It is impossible.

Draw the circuits, from source to load and figure out how it can possibly make a difference.

You have put blinders on.

But to each their own, some people also believe in ghosts.

Bob

[This message has been edited by iwire (edited 01-04-2007).]

Mean to tell me you don't?????? Believe in ghosts....

I believe in only what can be explained with facts.

Of course that whole 'how did we get here' question is a tough one.

Where does (outer) space end? And whats after that? How infinate is infinate? Questions that have boggled me since I have been able to think. Also being fact based - but not without opinion - ghosts are hard to believe.

I agree with Bob, the loads wouldn't care how the power is delivered, two wire or MWBC, as a matter of fact the neutral conductor in a MWBC would even be cooler in operation if both circuits are being used (bar any additive harmonics)

Roger




[This message has been edited by Roger (edited 01-04-2007).]

Would a GFI receptacle care if it were wired on a multi-wire branch circuit...or a better question is would in trip when the shared neutral read an imbalance?

And another question: "is it darkest just before it turns completely black?"

No and no

Roger

It would be darkest when it turned completely black.

Roger

Roger...good answers and both correct. How about trying to protect downstream of a GFI receptacle when installed into a multi-wire circuit? Wouldn't the GFI trip when wired in this location?

Devices fed downstream from a GFCI receptacle would not change it.

Now on the other hand, a MWBC fed from two single pole GFCI breakers would cause tripping from unbalanced current, but we would need to recognize this would not be due to fault currents.

Roger

The GFCI Can be sourced from a shared neutral but you wouldn't be able to share the neutral downstream.
Joe

Here we go.
I was for some years, an Appliance Serviceman, repairing things like M/Wave ovens.
This was a required part of my Electricians Apprenticeship.
Now,
A microwave oven uses a high current and frequency to cook food.
It uses 2.4Ghz as a standard frequency.
The biggest thing to look out for in a microwave oven is not the Magnetron, but the Capacitor that drives the HV side of the magnetron.
Only 2uF, but these capacitors can store up to 1500V, I am not making this up.
If one of these capacitors becomes a dead short, it can have real ramifications for the connected supply.
Not only will it blow a fuse, but it will make a nasty mess of molten copper.
That is why M/Wave ovens are required to be protected by ceramic fuses, not glass type fuses.

To do so would be a violation......even if it would work (which it wouldn't).

"300.13(B) Device Removal
In multiwire branch circuits, the continuity of a grounded conductor shall not depend on device connections such as lampholders, receptacles, and so forth, where the removal of such devices would interrupt the continuity."

A pretty common failure becomes a nightmare on a MWBC when the neutral isn't pigtailed. A (often backstab) receptacle removes itself from the circuit, opening the grounded conductor.

As always, improper installation is the culprit.



[This message has been edited by electure (edited 01-05-2007).]

HLC, I think I misunderstood your last post. If a single neutral is used downstream of the GFCI recptacles then I agree with Joe and Electure.

Roger

Is this in a house?.
You guys should be ashamed of yourselves.
Shared Neutrals in a house?.
Get Real!.

Mike what the heck do you mean?

Multiwire branch circuits (common neutral) are common in all occupancy's.



[This message has been edited by iwire (edited 01-05-2007).]

This is intriguing, but frankly, I'm lost. Can someone explain how a tv can catch fire with loss of neutral? [ We only have 230v 2 wire here.]

Alan

Electure,
Thanks for clearing this up for me. I completely overlooked 300.13(B) I knew it wouldn't work, but thinking now of the wiring scheme there leaves no doubt!

Alan, consider 2 possibilities:

1.) 2, 20A single pole breakers are in adjacent slots 1 & 3 in in a 120/240 single phase panel. Each circuit has its own neutral conductor landed on the neutral bus bar. The circuits supply your living room. (You just moved here.) You happen to have your TV as the only load on one circuit and an 1800W space heater as the only load on the other. If one of those neutral conductors should come loose, you will either be watching your TV in the cold or reading a book in the warmth. You probably won't be letting the smoke out of anything.

2.) Since the two circuits above are on opposite legs, a common neutral is permitted. One white wire is landed on the neutral bus and it is carrying the difference current between your two loads. Today, it happens to be the return current from your space heater. Things get really interesting when that neutral comes loose. Basically, you're left reading a book in the cold wondering what you're doing in USA. Meanwhile, Your TV is climbing through 500 feet thinking it has just been plugged into an outlet in the North of France. The voltage to the TV would have increased as the heating element was cooling, dropping almost all of the available 240 Volts across the TV mains.

We always run the risk of an unequal split from a failed center-tap connection from a transformer. Shared neutrals, while working fine most of the time, do offer the potential for more exciting failures. Even with a proper panel Neutral, loads on breaker pairs with shared neutrals are at a higher risk of over voltage.

Consider a case where the shared neutral is intact but the space heater suffers a direct short. It would not be unreasonable to have half the voltage drop on the hot and half on the neutral until the breaker tripped. This could add 60 volts to the other leg's 120 for a short period of time. (But it doesn't take long to let the smoke out)

Joe

[This message has been edited by JoeTestingEngr (edited 01-06-2007).]

Thank you Joe, for a lucid, calm and impersonal explanation of the issues with MWBC's.

[This message has been edited by mxslick (edited 01-24-2007).]

For my $.02 worth, first I have no problem with MWBCs, I can see the problems with an open neutral or an overloaded neutral, which would usually come from improper installation. I personally like them for a few reasons; first, less wire to pull and second, a less crowded breaker panel (half the neutrals and grounds). I guess I like them since I have yet to see the horrors of a MWBC gone bad.

Second, I have seen something funky like that, but it wasn't because of a bad neutral. It was on a house (my grandmother's...I've been hounding her to get her service upgraded) with a 60A service with a cartridge fuse disconnect (I fused it to 55 A/leg). Anyways, there was alot of cooking, etc, and one of the fuses blew. The strange (at the time, I know what happened) part was the lights on that leg didn't go out, they just dimmed, and the microwave even made an attempt to work. Since the oven was on, the blown side was putting its hot thru the heating element of the stove (hence the dimming) I figured it out quickly after turning the oven off and the lights went completely out. Anyways, it was strange, and strange to see the current cross over a 240V load to get to the other leg.

Open or loose neutrals are the bread and butter of our trade; many are the problems that can be traced to the "less important" neutral wire!

Indeed, I think I became a "real electrician" the day the neutral went from "less" to "most" important wire in my mind.

There was once a splice going bad at the PoCo transformer. This resulted in voltage surges at the house, with much damage to the expensive stuff. Simply put, if two loads are in series, the one with less resistance sees more voltage, and dies.

A shop was experiencing failures of their "surge suppressor" power strips. It was also interesting that some lights got brighter whenever the table saw operated. That problem was traced to an broken nuetral lug at the service.

Seven wires under a wire nut is almost never a good idea. When used to combine three circuits, and run but one wire to the panel, it's a really bad idea. One wire with a poor connection led to $300 of repairs to assorted power supplies. (BTW, in this specific instance, this was NOT a proper 'multi-wire branch circuit. Someone had thought to combine three 20 amp neutrals (single phase) into a single #10 to the panel. The three circuits were so cross-connected that it was not possible to fix this without completely re-wiring the office )

One can almost see a time-line in an electricians' training by watching the attitude toward 'shared neutrals' change over time:
- When first introduced to it, apprentices are mystified;
- The mystery is replaced by enthusiasm as they realize that they don't need to pull as many wires;
- If the guy fails to complete training, he later adds to this 'short cut' by, say, combining neutrals that ought not be joined, putting neutral busses somewhere besides the panel, etc.;
- He begins to see problems that are traced to bad neutral connections, and starts fretting about 'unqualified handymen;' and, finally,
- He gets to buy $300 of smoked power supplies because of an "oops" moment, and he begins to re-think sharing neutrals. Especially when he knows some untrained folks will follow after him, and mess with his fine efforts

I think my attitude towards neutrals changed once I put 2+2 together after seeing sparks from a neutral that was in use. I mean I know that a complete circuit needs a live and a return (neutral) you just don't think too much about it since it usually won't shock you (unless there is something wrong). But what goes in must go back out so the neutral is just as important as the live wire, if not more so since a bad neutral poses even more danger than a bad live wire. Good discussion though.