small spark (not 110v) when circuit not energized

I was helping a friend of mine by replacing a defective outlet in her 1920's era home (all two prong ungrounded circuits). I turned off the breaker to the outlet in question (and verified that the outlet was dead via a lamp plugged into the outlet).

When I removed the wire from one side of the outlet, there was a small spark between the wire and the outlet screw. It was definitely much smaller than a 110v arc (I've seen plenty of those...). By turning off another breaker in the panel box (in addition to the one controlling the circuit in question) I could eliminate the spark.

What's going on here? What would cause a lower voltage "leak" from one circuit to the next? Is this a serious issue?

 

You were working on a multi-wire branch circuit. The other hot leg and the neutral both had current on them. You were breaking the circuit when you loosened the screw.

 

Monkeying around with this could have easily gotten yourself hurt or caused serious damage in that home.

Best advice, hire an electrician instead of guessing.

Out of curiosity, is your outlet now grounded? if so, how?

 

Why is the only answer you give people on a do it yourself forum is to hire a professional? Just doesn't make sense to me. Many situations can be rectified by homeowners with caution and good guidance.

 

Yes you can figure out things for yourself and do work yourself that is allowed by law.

But you should not guess anything and should instead stop there until you or someone with additional expertise figures it out.

The spark you saw was small because a relatively low current (amperes-wise) was flowing at that moment.

Unhooking a multiwire branch circuit that still had the other leg live could result in damage to electronics plugged in on the other leg. You could get voltages of up to 240 flowing in what should be a 120 volt circuit even though you might get only a tiny spark when you finally unhook something that breaks the circuit.

Separate steps are needed to check whether you are working on a MWBC and to check whether the entire MWBC is shut off.

 

Unless it is improperly connected, a MWBC is attached to a double breaker with the handles tied. Turn one off, you turn them both off.

 

Well Chandler when someone asks for advice that might be over the level of their expertise and that could potentially get them electrocuted or damage the electronics in their home it might be better suited for a professional.

We do a lot of assuming of ability here... that could get someone hurt (I would bet it has!).

And besides buddy, aren't you a professional? Don't you want the work! hahahaha

 

as stated, the home is circa 1920... something tells me there aint no handlties my friend.

 

Even if the house was much more recent the code only required a common disconnect if both hots landed on the same device yoke in a residential setting.

 

I guess it is interpretation. NEC 210.4 (B) indicates there must be a common disconnect means for breakers in a mwbc. There is a difference between "disconnect" and "overcurrent". The handles may not provide common trip on overload, but will on disconnect.

 

The pros on this board are well able to determine if someone can do the work themselves or not. If they deem the work too dangerous for a homeowner, they will be warned and the thread will be closed.

 

The disconnect just means that both are turned off together to avoid having a repeat of the problem the OP saw.

The important thing to remember before doing any electrical work is to verify the power is off with a known working meter.

 

That code of a common trip for MWBC did not go into effect until 2008. IIRC, the common trip of two circuits on the same device yoke did not come into effect until 2002.

The OP's friends home is from 1920.

 

Eeew. Slightly before our time. May even have fuses, huh? Wonder when the upgrade was made?

 

I believe you meant the common disconnect requirement did not go into effect till 2008, common trip is not required on a MWBC, but it is allowed if the common disconnect is accomplished with a 2 pole breaker rather than handle ties.

 

Well, I guess I poked a bit of a hornet's nest, didn't I?

Thanks to all for their helpful responses.

I think of myself as a fairly experienced amateur...I've done a lot of work over 35 years in my own homes, and I keep an updated copy of "Wiring Simplified" around and pay attention to things like wire sizes, # of conductors permitted under a given size wire connector, capacity (size) of box, pig tailing devices, and so forth. I do my best to try to make sure that my work would pass muster with the NEC. But I also know I am NOT a pro and that therefore I need to try to be careful not to get over my head and deal with things I know too little about. I may have violated that principle here...

Of course being a pro is no guarantee of good work. Years ago I had a heavy up done on a 1940s house by a "reputable" larger company, and noticed after the work was done that the guy in the field had installed several 20amp breakers on circuits with #14 wire.

Another time in a different house I needed to have a section of wire feeding my heat pump replaced and the original install was aluminum. The pro who came out to do the job used copper for the new section and must have had on hand only one of those special AL to CU connectors and spliced the other leg using a standard wire nut.

In both cases a quick call to the company resulted in a no charge visit to fix the issue.

For the sake of my education, I'd like to ask a followup question about multi-wire branch circuits. If I understand that concept correctly, it's when two hots, each connected to a different breaker in the box, share a neutral. And I know that when this is done, it's important to ensure that the two hots are connected to breakers on opposite poles. A simple example of this is running 12/3 to feed two outlets, say for a dishwasher and disposer, that are often right next to each other.

In the box I was working on, there were 4 wires, two coming from below (basement) and two leaving the box at the top. I know that an outlet on the second floor was on the same circuit as the outlet I was working on. The wiring was too old to be color coded, so I didn't know which side was hot and which was the neutral. Two of the four wires were connected to screws on the left side, and two of the four were connected on the right side (no pigtails or wire nuts anywhere in the box). The tab between the two receptacles was NOT broken, suggesting that this wasn't wired to have each of the two receptacles on separate circuits.

Q1: Is this kind of wiring consistent with this being a MWBC?

Q2: Given what I've described, should I recommend to the homeowner (my friend) that she should have this checked out by a pro? If so, how should I suggest she communicate what's going on to the pro so he has an idea of what she's asking to be done?

@bigboypete: I replaced the 2 wire receptacle with a 2 wire receptacle. I didn't have the equipment with me to see if the box was grounded. And even if it were grounded, it would have been through the BX cable, and I wouldn't ever think of trusting a 90 year old armored cable to be reliable enough to provide proper grounding.

Thanks again for the help.

 

No it is not. However, sometimes it can be tricky to tell just buy what is in a device box. It would be best to look at where the circuit(s) originate. Your box has two wires in, two wires out.

Q2: I don't think so. You disconnected the wires a got a spark on, what we think is a neutral. If you wire it back up, or better yet splice the wires with a nut and use a pigtail to the device, it should be just fine.

@CasualJoe - You are correct, Common disconnect. However, with most MWBC the circuit breaker is the common disconnect.

 

Thanks for your reply and your help.