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Clamp Meter

Discussion in 'Electronic Basics' started by Binnyrus, Jan 21, 2008.

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  1. Binnyrus

    Binnyrus Guest

    Is it possible to use a clamp meter (Fluke 322) to distinguish between
    current carrying wire and a switch leg? I have a space in my home
    that I'd like to add a pull chain light fixture to. There are two
    wires that run through the space. I am suspicious that one is a switch
    leg for a three-way switch . . . but have no idea how to determine
    this. Can anyone help me out with how to diagnose the problem?
     
  2. Bob

    Bob Guest

    Assuming it is mains wiring.

    You can put the clamp meter around one of the wires on the AC current
    range then switch all the switches in the area. If the wires are
    connected
    to a load you should see current/no-current on the meter depending
    on switch position. Check light bulbs are working if you can't find a
    load.

    You could also use the fluke 322 on it's voltage range with the probe
    leads.
    You will need to make contact with the copper inside the wires without
    getting your hands in contact with the probes.
    The results will depend on how the switch is wired and how your mains
    distribution is configured. If you measure between local mains earth
    and both of the wires you should see a change when you operate the
    connected
    switch.

    Bob
     
  3. ehsjr

    ehsjr Guest

    You can't find the "switched leg" (xxx in the diagram
    below) with a clamp on meter. The "switched leg" will
    always have current when the light is on, and never
    have it when the light is off. The same is true of the
    hot leg and the neutral leg - if the light is on, there
    is current in the wire. The clamp meter can be used to
    determine which wires are the "travellers".

    Regarding "switch leg for a three-way switch" :

    SW1 SW2
    o-------T-------o xxx
    AC -Hot-o o------+
    o-------T-------o |
    [Light]
    |
    AC -Neutral-------------------------+

    Above is typical 3-way wiring. The AC could also come in
    on the right, with slightly different wiring. Or the
    light could be between the switches. Regardless of how
    the wiring is run, the two wires marked T are "travellers"
    and are always accompanied by a third wire when properly
    installed.

    A clamp meter on a traveller will show no current
    with the light on with one of the switches in the
    up position, and current in that wire when the
    same switch is in the down position and the light
    is on. The same meter clamped on any wire other
    than a traveller, will always show current when the
    light is on, regardless of switch position.

    This does not apply to your wires - they are not
    travellers. Proper wiring would have a third
    wire accompanying the travellers. They could be
    the switched leg and the neutral or the hot leg
    and the neutral, but your clamp meter can't tell
    the difference between hot, switched or neutral
    leg.

    Ed
     
  4. To get a useful reading with a clamp-on ammeter, you must put it
    around a single conductor. If you put it around the two-conductor
    cable normally used in house wiring, you will read zero regardless of
    the current carried by the wires, as the magnetic fields produced by
    the supply and return currents will cancel. This will happen with a
    branch going to a switch, as well as on a simple feed to an outlet.

    Re-reading your post I see that you mention a cable to a three-way
    switch - such a cable would likely have three conductors, where a
    cable feeding an outlet would only have two (in both cases, plus the
    bare ground conductor)
     
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