is it safe - light switch question

Discussion in 'Electrical Forum discussion & Blog' started by beekerc, Oct 27, 2008.

  1. beekerc

    beekerc IT Consultant / Network Engineer

    Messages:
    94
    Location:
    Seattle
    to an experienced electrician, this is probably a "DUH" question. and since most of my experience is with networking and low-voltage, i need to ask. if it's a stupid question, please don't hesitate to say so, you won't hurt my feelings.

    a light fixture where power comes to the switch, the neutrals are tied together, a regular SPST toggle sits in the flow of the hot wire, and the light/load is connected white-to-white and black-to-black.

    is safe to work on the light (repair/replace/touch-wires/etc.) if the circuit is still live but the light switch is turned off?

    if the switch were a DPST where both the hot and neutral lines were broken (but ground was still intact - would that be safe?

    the DIY-homeowner part of me says i should probably kill the circuit at the panel. however, after hearing and reading about pro's working with "hot panels" and "live circuits" I can't help but wonder if there are situations where it could be safe.
  2. leejosepho

    leejosepho DIY scratch-pad engineer

    Turning off the breaker adds a little assurance beyond simply turning off the switch, but the commom (white) would still be connected ... and that leaves you with no absolute guarantee of not getting shocked even though that is unlikely.
  3. Southern Man

    Southern Man DIY Hillbilly

    Messages:
    530
    Location:
    North Carolina
    The most common issue here is that someone turns on the switch while you're working. "Honey, let me turn on the light for you so you don't have to work in the dark..." That's why I put a piece of masking tape over the switch in the "off" position.
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,270
    Location:
    Cave Creek, Arizona
    switch

    The neutral would normally never be broken by a switch. A DPST switch would normally connect a 220/240 volt line.
  5. beekerc

    beekerc IT Consultant / Network Engineer

    Messages:
    94
    Location:
    Seattle
    agreed, an SPST only breaks the hot wire. with the neutral wire still connected, if there was any leakage, the load at the end could still be hot. my aim is to totally isoate (with the exception ground) the load from the circuit.

    when the switch is thrown and hot and neutral are cut, isn't that basically the same as unplugging a device from a wall outlet? the device is cold (which is what i want) and the hot and neutral in the outlet are still live, just not connected to anything.

    just wanted to make sure i could do this without violating code or the laws of physics.

    thanks.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,816
    Location:
    New England
    Switches fail, both common light switches and CB. The only safe thing is to use your meter at the fixture before you stick you hands in there to verify it is safe. Personally, if I've checked there's no phantom voltage there, I'd work on the fixture if the switch was off and it wasn't fed through the light fixture's box. I wouldn't do this if there were other people around or there was a lightning storm going on. While very uncommon in civilian residential boxes, they do make lockable CB and panels, where you can then ensure nobody turns it on while you are working on it, unless they also have a key.
  7. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    You are correct in your understanding of the switches function using standard wiring methods.. The switch mechanically disconnects the hot wires 100%.


    There ARE cases where you could have voltage on that white wire. There ARE cases where you could have voltage on the ground wire. People miswire and cobble things every single day.


    Anything can malfunction, including circuit breakers, so test voltage first then short out the wires to double check. I call it the flash test.

    When is doubt, short it out.


    Yes, if it is a DPST switch. I can't see any issue with doing this, it's just unorthodox.
    Last edited by a moderator: Oct 27, 2008
  8. beekerc

    beekerc IT Consultant / Network Engineer

    Messages:
    94
    Location:
    Seattle
    DPST switches

    i'm not too worried about someone accidentally throwing the switch. for one, they'll be 6+ feet off the ground and two, i would put red e-tape over it while working on it. i thought about a keyed switch (like they have in schools) but i'm not sure those come in DPST.

    Here's what i'm trying to do. i want to hard wire a pair of garage door openers instead of using a the regular outlet plugs. occassionally i like to kill power to the openers (at night or when i go on vacation). when I was growing up, my parents woudl accomplish this by pulling the plug. i know, simple, cheap, but i want to use a switch. the circuit i want to use is the same one that feeds all the lights in the garage, so if i needed to access/service the openers, and I flipped the breaker off, the i lose all my lights in the garage. i know, a plug in work-light with an extension cord - if i was happy with that workaround, i wouldn't be asking. as i said, i am looking to completely isolate the opener from power when i flip the switch. just looking to see if there's anything wrong with my plan or the rationale behind it.

    slapping a multimeter or line voltage detector on a device before working on it, even after you've just flipped the breaker, is good practice.
  9. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    988
    Location:
    NY State, USA
    DO NOT switch the neutral. There is NO reason to.
    You are putting way more thought into this than is necessary.
  10. Southern Man

    Southern Man DIY Hillbilly

    Messages:
    530
    Location:
    North Carolina
    I think you're making this too complicated. But then again, I am The Southern Man.

    The neutral wire goes to the same bus as the ground in the main panel. There should be zero resistance between the two. Just put in a $1.19 switch on the hot wire and be done with it.
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