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Measuring impedance of wall socket

Discussion in 'Electronic Design' started by JeffM, Jul 29, 2005.

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  1. JeffM

    JeffM Guest

  2. The impedance means the ratio between voltage and current
    plus the phase between them. So first you have to get rid
    of the DC stuff, also the 50/60Hz with a set of capacitors.
    Then take a sweepable or fixed frequency source and measure
    the voltage, the current, calculate the ratio and the phase.

  3. Hi, I'm not sure this is the best forum to post this question in but I was
    wondering how one would go about measuring the impedance of a wall socket?
    (Strange question, I know) I have some general ideas but would like to
    confirm my thinking. Any websites or other resources explaining the
    general concept would be greatly appreciated. Much thanks in advance.
  4. Glen Walpert

    Glen Walpert Guest

    Not a strange question at all, the only thing I find strange about
    line impedance testing is how few testers are on the market and how
    few electrical inspectors use them, considering how well they uncover
    the most common and dangerous wiring faults (high impedance
    connections, a common cause of fires) which are not otherwise easy to
    detect. A google search on line impedance tester will however turn up
    a few commercial testers.

    If I were to build one I would use a bank of motor run capacitors for
    a load, sized for about 10 A at 120 V for measuring typical US 15 Amp
    wall socket impedance. By applying the load to Line-Neutral and then
    Line-Ground voltage drop can be measured separately for all 3
    conductors. Since the change in neutral to ground voltage provides
    the voltage drop on the neutral or ground (whichever is carrying
    current), the rest of the total voltage drop is due to the Line (Hot)
    conductor impedance. Any other means of applying a load and measuring
    voltage drop at a known current can acomplish the same thing.
  5. The impedance always belongs to a frequency. You're
    measuring at line frequency here.

  6. Paul Burke

    Paul Burke Guest

    Most modern UK installations have an RCD on the socket rings, and this
    test would send it flying.

    Paul Burke
  7. John Larkin

    John Larkin Guest

    Poke your dvm probes into one side of an outlet, and plug a toaster or
    similar big resistive load into the other. Measure how much the
    voltage droops when you fire up the toaster.

    Impedance is pretty much droop_volts/load_amps.

  8. John  Larkin

    John Larkin Guest

    Hey, everybody do this measurement and report what you get. The
    toaster thing, not the bobby pin.

  9. Rich Grise

    Rich Grise Guest

    He's a coward! I did it with my thumb! ;-P

    Gum wrappers, however, do make a cool spark. ;-P

  10. The Phantom

    The Phantom Guest

    I've done this at my last two jobs, but the measurement was made at the service entrance,
    and not at some wall socket. Both times, the result was about 1/10 ohm (in the US, at 120
    volts). This is at 60 Hz. Rene suggests that by asking for "impedance", the OP wanted to
    know the result over a band of frequencies. I didn't have the equipment for such a
    measurement, but I did find a paper where someone had done it. The result was a wildly
    varying impedance vs frequency.
  11. My son tested the outdoor socket on the back of the house with a weedpuller,
    and found it full of sparks, until the GFI breaker popped . Since I was not
    using any electric devices in the backyard, I didn't know about it for
    several days. Then the backup refrigerator in the garage, which is on the
    same circuit, started to smell funny.
  12. Refrigerators are not supposed to be on GFCIs (there is an exemption
    for them).
  13. Jim Thompson

    Jim Thompson Guest

    The booby pin went off like a flashbulb. Fortunately just a mild burn
    to his palm. But lesson learned in true Thompson tradition... the
    hard way ;-)
    Don't you just love those occasions? I had a freezer in my carport
    storage room... no cooling... exposure to 120°F+.

    So the connector failed where the compressor plugged into the
    thermostat assembly.

    I don't know how many weeks later the wife comes screaming into the
    house... "You have to clean it up." Gag.

    So I replaced the connector and moved the freezer inside to a hallway
    adjacent to my office.

    ...Jim Thompson
  14. I did it with a butter knife across the blades of a plug. My son found
    a brass ball at the end of a pull chain fit nicely into an empty light
    socket (and made a nice sizzling and sparking, which left some welts
    on his leg). Do girls do stuff like that?

    Best regards,
    Spehro Pefhany
  15. Joel Kolstad

    Joel Kolstad Guest

    Do you happen to know why not?
  16. Ben Bradley

    Ben Bradley Guest

    I worked with X-10-type controls a while back. and I recall hearing
    that "the" line impedance at 120kHz was 6 ohms. I never tested this
    myself, but in retrospect, I (actually the guy who was designing the
    hardware) should have.
    You may find more info on power line impedance by researching power
    line carrier (PLC).

  17. I performed "Tickling the dragon" by dropping a K2 neon bulb with the
    conductors slightly spread apart through the blades of a plug.

  18. I'll bet you didn't have to tell him not to do that again! ;-)

    Link to my "Computers for disabled Veterans" project website deleted
    after threats were telephoned to my church.

    Michael A. Terrell
    Central Florida
  19. Glen Walpert

    Glen Walpert Guest

    Both good points. If you build a uP controlled tester with a couple
    of fast simultaneous sampling A/D converters for voltage and current,
    and apply the load close to midpoint between voltage zero crossings
    (at a voltage peak) with a fast solid state switch, you could probably
    get enough samples to estimate impedance vs freq over a reasonable
    range before the RCD (GFI in the US) trips. But the short sample
    period would defeat part of the purpose of the high current test load,
    which is to heat up any high resistance connections which will often
    change resistance noticably in under a minute of high current testing,
    at least from what I remember of an article on line impedance testing
    I read in EC&M a decade or two ago, probably written by an impedance
    tester salesman of course. You could still do the sustained high load
    test on Line-Neutral of course. And while you were at it you could
    put a separately switched low current load to ground in to test the

    This may be more than the OP wanted to know; the toaster with
    multimeter approach should work fine unless you want to do a lot of
    testing or investigate power line comms or lightning surge control.
  20. Guest

    I knew a girl once who sizzled when she sparked. She never left welts
    on my leg but I did get some scratches on my back.

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