# Measuring impedance of wall socket

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

2. ### Rene TschaggelarGuest

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.

Rene

3. ### cyrille perronGuest

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 WalpertGuest

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. ### Rene TschaggelarGuest

The impedance always belongs to a frequency. You're
measuring at line frequency here.

Rene

6. ### Paul BurkeGuest

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

Paul Burke

7. ### John LarkinGuest

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.

John

8. ### John LarkinGuest

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

John

9. ### Rich GriseGuest

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

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

Cheers!
Rich

10. ### The PhantomGuest

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. ### Richard HenryGuest

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. ### Keith WilliamsGuest

Refrigerators are not supposed to be on GFCIs (there is an exemption
for them).

13. ### Jim ThompsonGuest

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

...Jim Thompson

14. ### Spehro PefhanyGuest

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

Do you happen to know why not?

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. ### Aubrey McIntosh, Ph.D.Guest

I performed "Tickling the dragon" by dropping a K2 neon bulb with the

18. ### Michael A. TerrellGuest

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 WalpertGuest

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
RCD.

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.

Jim