Irrelevant Aside: Household Electrical Problem

Update: I fixed the problem, solution is below!

This topic is not really relevant to this blog, but it’s interesting to me and I had to share.

The other day I experienced a weird electrical problem in our house. The master bedroom lights started to flicker periodically, and eventually went out completely. A power bar spontaneously blew up (ok, made a popping sound and then smelled of ozone and burnt plastic).  In another bedroom, the ceiling CFL lights would pulsate dimly, not able to fully light. The rest of the house was fine, it was only the back two bedrooms and hall/bathroom lights that were affected (two different circuits, one breaker for each bedroom).

I stuck my multimeter into a master bedroom outlet and an outlet in the second bedroom and was…surprised.

So, how is this even possible? I have a hypothesis, and as soon as I get a chance to open up the electrical panel, I’ll be able to confirm. For now, the breakers to these two circuits are shut off, and we’re camping with candles in our own bedroom. OK, that’s not actually true; I ran an extension cord from the bathroom to power a lamp and my laptop charger. Priorities…

If you have any ideas about the cause of this problem, please comment!

Update: It’s fixed! My hypothesis was correct, I had a multiwire branch circuit with an open neutral. When my house was built, it was a common practice for electricians to save money on wiring by putting two circuits on a single 3-conductor wire, with a common neutral. This was allowed if the two hot legs were connected to opposite phases; in this way, the current on the neutral return would be the difference of the amperage draw of the two circuits, not the sum of the current draw of the two circuits (which could overload the 14ga neutral.) It looks something like this:

These multiwire branch circuits suffer from a serious weakness – they have a dangerous failure mode. If the neutral conductor is disconnected (fails open), the branch circuit changes from two 120V circuits in parallel, to a single 240V series circuit. This can and will damage devices connected to the circuit! There’s a full explanation of this dangerous failure mode over at EC&MWeb, but here’s a simplified diagram:

So, where was my actual failure? In the panel, at the neutral/ground buss bar, the neutral for these two shared circuits had not been tightly clamped and had arc-faulted. Here’s a pic of the damage.

 Upon investigation, a number of connections in this buss bar were loose, and there were some instances of neutral and ground wires for the same circuit being bundled under the same terminal. This latter situation is very dangerous – if a loose connection arc-faults both the neutral and ground conductors, there is no path to ground for the hot leg of that circuit. This means that if you touch it, YOU become the path to ground.

I remedied all the faults that I found in the panel, and trimmed and reconnected the neutral for my two broken circuits. All is well now, the extension cords have been removed from my bedroom, and I learned all about the dangers of multiwire branch circuits.


About Matt Greensmith
Sysadmin, robot-builder, linux geek, etc.

One Response to Irrelevant Aside: Household Electrical Problem

  1. tktim says:

    I remove an outlet (for upgrade) on circuit #5 (bedroom) and burned out a surge protector on circuit #12 (living room). Later I found out they shared a neutral white wire. Surge protector (melted) saved the TV but burned out DTV box and DVR. Circuit #5 & #12 are not near each other in circuit box. Their breakers are not tied together as required. Circuit breakers did not trip. Circuit #5 was turned off. I quickly shut everything off, but too late. Local shop repaired DTV box and DVR. Put warning note in circuit box.

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