Electric problem?

Discussion in 'Electrical Forum discussion & Blog' started by rayh78, Jun 25, 2012.

  1. rayh78

    rayh78 Member

    Messages:
    81
    Location:
    Virginia
    Could not find a problem. This is in a rental I own. Has a 3 light fixture above sink. Switch and light maybe 12 years old.
    Tenant turned on light all 3 bulbs blew with black spots on the bulb, and there was a pop at the switch.
    What could cause this. When I got there I installed 3 new bulbs to try and duplicate but everything worked fine.
    But went ahead and inspected the switch, removed and inspected the fixture. Everything looked great. No loose connections or black/burnt spots on any wiring, voltage good.
    Went ahead and replaced the switch but really just to do something. Had the switch out and had a spare with me so figured why not and they are cheap.
    So everything works fine now. But I would have preferred to actually see a problem and know I corrected it.
    I can understand if it was just a bad bulb going out. And bulbs were about 5 years old. But three at the same time.

    Thanks for any suggestions.
  2. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    Is this one half of a multi-wire circuit?

    That is: is it sharing a neutral between two hots?

    And if you have no idea of what I write, you probably don't want to learn on this job.
  3. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    1,010
    Location:
    NY State, USA
    My first thoughts as well.
    Intermittent problem with a multi-wire circuit.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,348
    Location:
    New England
    That could happen if you actually applied 240vac to the lamps instead of 120vac...it's possible if there's an intermittent wiring problem on a multi-wire circuit.
  5. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,569
    Location:
    North Carolina
    A power surge could cause this also. Were there any storms in the area at the time?
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,348
    Location:
    New England
    A surge is possible, but unlikely to occur simultaneously with turning the switch on. One of the missile control vans I used to work on had 28vdc backup lights and 120-vac circuits as well...boy, those 28-v lamps did NOT like to be installed in the 120-vac circuits! They did get bright for less than a second, though! It' would have been better if they used a different base, but they didn't, and different bases would have probably made them cost even more money, so I guess it was a tossup.
  7. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    "boy, those 28-v lamps did NOT like to be installed in the 120-vac circuits!"

    How very small minded.....
  8. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    DO you know what a multi-wire circuit is, and if this is part of one?
  9. rayh78

    rayh78 Member

    Messages:
    81
    Location:
    Virginia
    Does not look like it from what I could tell. And it was what I would call regular romex cable 14G.
    The box behind the light was also a junction box with a few other cables cramed in there. What I needed was on top so did not pull them all out to inspect carefully.
  10. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    That is not enough to establish the question.

    The two circuits that share a neutral could split some distance from that box.

    If you are comfortable taking the cover off the breaker panel, identify what breaker serves the circuit in question.

    Then trace the hot wire from that breaker to where it exits the panel. If it is a multiwire pair of circuits served by NM, it will have a black, red, white and bare conductor going into a sheath. Any situation where two hots and one neutral (white) are matched is almost certainly a multi wire circuit.

    Do be careful. This business of tracing a hot in a crowded panel is not for the faint of heart.

    If it is arranged as I describe, then the white (neutral) is used by two circuits to return the current from both. This works if one understands to have the two hots on opposite ends of the phase. That may not be an issue, then again, more than a few arrangements have been compromised over time by people swapping breakers around and arranging both hots using the same neutral to be on the same end of the phase. That is pretty bad. It COULD be your problem.

    More likely, the point where the neutral in this pair of circuits splits is falling apart. It was common practice until a few years back for the neutral to just run to the back of a receptacle and come back off of it twice. Such that if the continuity of the splitting neutral (because now the two hots are diverging, and each needs to take the white with them) and should that happen, it is completely possible for the voltage on the two circuits to change suddenly and dramatically. At an extreme, one circuit could drop to zero and the other to 240 volts. Which would frost your bulbs but quick. Actually, they would likely explode.

    But it is readily imaginable that one circuit jumps to 160 volts and the other drops to 80 volts. If the light bulbs were on the 160 volt end of this party, they would fail just as you describe.

    And if, as I can readily imagine, having seen it happen many, many times, a cheap receptacle (outlet) in your apartment has three 14ga white wires back-stabbed into it, and those damned back-stab receptacles are worth less than a warm bucket of spit. It could EASILY be that one of these, in a critical location, loosened briefly (plug in a damned vacuum cleaner) and then tightened back up.

    And blew your bulbs.

    If you can figure out which outlets are on the two circuits, and think a bit logically about where the breaker panel is, you MIGHT work out which one it is, first time out.

    The current standard in this sort of wiring is to have a "pig tail". All the whites are brought together and a short pig tail is added. It is that which takes the neutral to the plug. That way the plug can be removed without disrupting the continuity of the neutral.

    Obviously you turn off both circuits before proceeding. Or seriously consider having an electrician in to do it for you.

    And it is way bad for the TVs and what have you should the current leap as I described.
  11. rayh78

    rayh78 Member

    Messages:
    81
    Location:
    Virginia
    Thanks for such a good explantion.
    Will have to do some more checking.

    Thanks again
  12. kreemoweet

    kreemoweet Member

    Messages:
    378
    Location:
    Seattle. WA
    Y'know, I've noticed that, once in a while, you do not get an Exactly Accurate description of a problem from a client/tenant/whoever, and that
    sometimes they do not relate events Exactly As They Occurred. Personally, I'd lay good money on a bet that two of the bulbs were already
    burnt out, and the tenant only noticed that fact after the third one popped, and just assumed they all went at once. A bulb burn-out can
    draw an arc with a fair amount of current, causing an arc at the switch contacts as well.
  13. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    Naaaaaaaaaaaaaaaaaaaaaa.

    Some of my faves are the ones that are so vague that you wonder if they are talking about the plumbing or the electrics....
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