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House wiring question for the pros

Discussion in 'Electrical Engineering' started by Peter, Nov 30, 2005.

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  1. Peter

    Peter Guest

    Last night I was fixing an electrical box connection and I touched the
    wires that came off the switch for the lights downstairs. The switch was
    off, but for some reason I got a small shock when I touched the box (which
    is Earth grounded) and the wire.

    I measured the wire to the box (Earth ground) and read approx. 16VAC.
    Someone told me this is normal for switches to "leak" a little bit of juice
    when in the off position.

    It seems a bit strange to me, but can someone tell me if this is true
    and/or what this would be called so I can look up the explaination myself?



    Thanks in advance!
     
  2. Beachcomber

    Beachcomber Guest

    Switches don't leak and even if they did, certainly not enough to
    produce 16 volts. Without more info, it's difficult to diagnose...

    But, I would check for an open or high-resistance neutral (Which could
    very well put out 16 VAC between certain points).

    The neutral wires (any of the white wires) should read close to zero
    volts between neutral and ground. Your grounding system could also
    be bad (high resistance at any number of points, or the fault could
    line in the transformer connections that serve your house.

    See if you can get a utility professional or a licensed electrician to
    take a look at it. This could be a dangerous situation.

    Beachcomber
     
  3. Bud--

    Bud-- Guest

    If you are measuring the voltage with a modern digital meter another
    possibility is that you are measuring a voltage derived from capacitive
    currents between wires or possibly across the switch. If the wire was
    connected to a good light bulb you probably have a problem, as in other
    posts. If the wire came from the switch with bulb not connected the high
    resistance of the meter may have allowed you to measure capacitive
    currents. Old analog meters are a lot more reliable for this kind of
    measurement because they have a lower resistance.

    You could also make sure a good light bulb is in the circuit and measure
    from the switched off hot side the bulb to ground which should measure
    the neutral to ground voltage which could be up to a couple volts if the
    circuit is loaded.

    bud--
     
  4. Peter

    Peter Guest


    Thanks for everyones input. I enjoy chatting about electronics and
    different things with people because I learn so much. Usually you can learn
    a few things from school or books, but when it comes to the real world, you
    can't beat it because you come across every day problems.

    As far as the house wiring goes, I've asked numerous people and got
    numerous answers. Most answers were that something is wrong. Another person
    said 'old switches' sometimes develop corrosion and cause voltage to
    'leak'.

    In either case, this suggestion about the meter is interesting and I may
    try it. I wasn't sure if this occurance has a name associated with it or if
    it's a problem.
     
  5. Beachcomber

    Beachcomber Guest

    Please keep in mind that if, in fact, you do have a leak, that is the
    same as an electrical resistance in the circuit. The power developed
    across this resistor will depend on the resistance in ohms of the
    material, and also on the SQUARE of the current. Too much current
    over a certain threshold and the device will just keep heating up and
    eventually explode.

    I have seen this happen to cheap receptacle outlets that deteriorated
    over a period of hours to days. It is easy for this to start a fire.
    That's why U.L. standards exist for plugs, cords, switches, etc.

    Beachcomber
     
  6. ehsjr

    ehsjr Guest

    It (the erroneous voltage reading) is a very common occurrance.
    People think they can use a digital meter on their household
    circuits, and trust the readings taken. It may be the most common
    mistake made by those unfamiliar with house wiring who use a meter.

    When measuring voltage on 120 VAC branch circuits, use a 60 or
    100 watt test lamp in parallel with the meter leads and you won't
    get erroneous readings. Until you have a known correct measurement,
    any answers based on that 16 volt reading are speculation.

    Ed
     
  7. Kirk Johnson

    Kirk Johnson Guest

    What Country?

    Makes a big difference..

    KJ.
     
  8. w_tom

    w_tom Guest

    You did not specifically state where you measured from to
    measure 16 volts on the box. Yes leakage always exists. But
    if that leakage created 16 VAC, then you have dangerous
    (excessive) leakage. Two volts with something else running on
    the same circuit. little to worry about. 16 volts in most
    cases - excessive - in any country. But again, what
    specifically and exactly were the two meter leads connected
    to?

    How to weed out wrong answers. If that person said leakage
    is acceptable and did not specifically cite numbers, then
    assume junk science reasoning. The reason why and why not is
    defined by specific numbers. Those who don't temper their
    reasoning with speicific numbers - assume the worst from their
    response.
     
  9. Peter

    Peter Guest

    It all started because my father and I were in the attic doing some re-
    wiring and he warned me that he has seen voltage leak through the switch
    and not to touch the wire. During the gathering of wires, he accidently
    touched the wire coming off the switch that is suppose to be dead because
    the switch is off and the metal box (which was Earth grounded). After
    touching both points, he got a shock and jumped back.

    I thought this was strange, so I measured the wire that should not have
    anything on it to the box (which is Earth grounded) and saw 16VAC.


    Tonight we tried removing bulbs in random rooms and found that when the
    bulb is in, we get mV AC (basically 0 volts), and when the bulb is out we
    get about 12VAC.

    I'll accept that my digital meter is not meant for this purpose, but why
    the zap in the attic when accidently touching the wire to the Earth
    grounded box? For those who asked, I live in Massachusetes.


    I was informed that the switch could be leaking and have a resistance of
    10Mohms, the meter has a resistance of 1Mohms which we create a voltage
    divider thus reading 1/10 of 120VAC or 12VAC and when you add the bulb, it
    reduces the bottom half of the voltage divider to approx. 0 volts AC.
     
  10. ehsjr

    ehsjr Guest

    But that's wrong. The only way the circuit is dead is if the circuit
    breaker is turned off. One wire to the switch is live, regardless
    of whether the switch is on or off, unless the breaker is off (or
    the fuse removed.) The metal box being grounded has absolutely
    nothing to do with whether a circuit is on or off. However, because
    the metal box is grounded, you can get a shock by touching it and
    a live wire at the same time.


    After
    As mentioned in an earlier post, in the absence of a known, valid
    measurement, speculation is all we can offer about the 16 volts
    you saw.
    "removing bulbs in random rooms" and "when the bulb is in" --- um,
    which bulb is "the bulb"? Is it the bulb that is controlled by the
    switch in question, or does it have something to do with the bulbs
    removed in the random rooms?
    The zap occurred because the circuit was not dead, as you
    erroneously thought.

    For those who asked, I live in Massachusetes.
    The numbers are wrong. A typical digital meter has an impedance of
    10 megohms, not 1 megohm. To have your voltage divider ratio of
    1/10, the leakage would have to be 100 megohms.

    Lets assume there was a leakage of 100 megohms. How much current
    would flow? 120 volts divided by 100 megohms, or 120/100000000,
    which equals .0000012 amps - *way* too low to be felt as even the
    slightest tingle.

    Forget the idea of "switch leakage". It is a non technical
    "fuzzy answer" and a meaningless phrase that tells you nothing.
    One of three things (or some combination of them) could put
    16 volts on the wire you measured: a high resistance from
    a live wire, capacitive coupling or inductive pickup.

    Your entire problem is based on incorrect procedure (circuit
    was not dead) and incorrect measurement (using a DMM on the
    circuit without ensuring it was loaded with a low impedance)
    and your problem understanding what is going on is exacerbated
    by incorrect data (the input impedance of the meter) and lack
    of familiarity with capacitive coupling and induced voltage.

    The good that might come out of this in the future depends
    on you: *shut off the breaker* when working on your wiring.

    Ed
     
  11. Beachcomber wrote :
    It is perfectly normal for there to be a few volts difference between
    your earth and nuetral connections - 16v is normal if that is what you
    were measuring?

    The nuetral would be connected to the switch via the lamp or fitting
    and the 16v would still be there despite your having turned off (what I
    assume) is the mains supply to the lighting circuit.
     
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