I have a GFCI socket in a recently-build bathroom. My hair trimmer trips it often when stopping under some conditions and while I have a fix I'd like to better understand what exactly is going on here.

I need an extension cord to use my hair trimmer inside the bath (not to use it with the shower, just to collect the hair...), and the issue was when using the extension. I could fix it with an AC capacitor... so let's start with the setup:

The trimmer is an Andis PM-1 pivot-motor marked with the following specs: 12W 120V 60Hz. It has a ~220cm cord of unknown gauge (outer shell is 5.8mm, round, flexible so I presume stranded copper).

The extension cord is ~305cm long, hand-made from fairly thick lamp wire (the kind that has two visible wires that can be teared apart). It's likely 14AWG; I measured single strands at 0.28mm and counted 26 of them. Plugs on both ends are rated 15A 125V (peak? I hardly think this is sustained rating but it doesn't say).

The cap used is a film capacitor marked .47µF 275V, type MKP (I can provide full markings and even a photo if needed), salvaged from a dead laptop adapter.

The issue

I actually didn't realize the extension was the issue until I started testing with the capacitor... So:

With no ext cord, the GFCI never trips.

With the extension cord, the GFCI trips often on stop, likely depending on motor position. I presume the current feedback is higher if the motor's rotation isn't stopped early by the blade reversing.

With the cap at the trimmer cord / extension junction the GFCI never trips.

The cap has no effect if connected directly on the GFCI socket.

The questions

I understand AC motors cause a shift between current and load and that can be problematic... and also that they can have a very high power draw on start and feed it back on stop causing a surge (correct me if I'm inexact or even plain wrong here...)

Based on that I understand the GFCI could be seeing this as an earth leak, though since it's only on stop and that should be feeding current back in, that's kind of the opposite of a "leak".

Therefore I'm wondering about the following:

  1. Why is the GFCI actually tripping? Are my assumption correct? Is is only detecting differential (meaning leak or feeding current back both cause it to trip the same way)?

  2. What makes it only happen when using the power cord?

  3. I tried using the cap because that's what used on large-scale motors to correct power factor, though I'm not even sure it's the right cap for the job (after looking it seems to be used for EMI filtering...). It is actually correcting PF / absorbing the off surge, or something else is happening here?

4. (if not answered previously) Why the cap has no effect on the GFCI end?


The original issue no longer appears after is almost impossible to reproduce since I rewired the socket-end of the extension (male plug), despites that the connection looked pretty strong before (5-6 strand were broken, new connections have no more than 1-2 broken strand and does a longer loop around the screws). It also work (doesn't trip) with the cap on the socket end but I'm unsure if I somehow misconnected the alligator clips in my earlier tests or if it was just helping a bit (I didn't analize precisely fail rate, just noted whenever it failed at all).

I can still trip the GFCI by pulling any plug directly with the trimmer turned on as long as the cap isn't connected somewhere, although this may be an entirely different issue; the extension isn't even needed for that. If the cap is connected, it doesn't trip no matter which plug I pull or whenever the cap is before or after the plug I pull. It even works (doesn't trip) if I connect the capacitor on the other socket of that GFCI (I remember that was the first way I tried to connect the cap and it didn't work, then I tried between the ext. cord and trimmer as it was easier than connecting on the socket side).

If that may be meaningful, my earlier tests were with the cap connected using very cheap alligator wires (always the same wires though, moving the whole setup) connected directly on the plug prongs with the plugs halfway in. I have since then soldered it inside a ground plug adaptor with the ground pin removed & hole filled with epoxy (no wires or risk of a bad manual clip connection and plugs fully inserted when testing now).

  • 1
    \$\begingroup\$ if you use the same extension cord with other appliances is there a problem? \$\endgroup\$
    – danmcb
    Jan 10, 2022 at 7:26
  • 1
    \$\begingroup\$ @danmcb, It works well, I even used it with a 1500W kettle recently. The wire is in good condition, no signs of damage (looked it all over multiple times trying to find a gauge inscription) and both plug connections were good too (I opened both ends to measure the gauge while writing this, first the one easiest to open, then realized it would be a PITA to reconnect back the wires inside so I opened the other end). \$\endgroup\$ Jan 10, 2022 at 7:36
  • 1
    \$\begingroup\$ @danmcb Oddly after reassembling the socket end of the extension (male plug) I no longer have the issue. The connection looked pretty strong before (with maybe 5-6 strands loose/broken) but I did a much better job this time anyway. I'm also wondering if I really had the issue with the cap on the socket end as I can still reproduce by unplugging the socket at any ends with the trimmer powered on, but it never trips with the cap connected anywhere (I have built/soldered it in a ground plug adaptor, before I was using cheap alligator wires to connect it with the sockets halfway in) \$\endgroup\$ Jan 10, 2022 at 8:09
  • \$\begingroup\$ sounds like it should be OK, but also check with a meter? should be too high for the meter to read between any pair \$\endgroup\$
    – danmcb
    Jan 10, 2022 at 8:09
  • \$\begingroup\$ Those loose/broken strands may well have been the problem. the whole cap thing sounds like a blind alley to me. If you had something intermittently arcing or shorting inside, that could lead you to all kinds of weird confusing ideas. Intermittent issues are horrible like this - X happens when Y happens, you think Y causes X but in fact it is just random crap caused by something much simpler. It happens to the best of us. \$\endgroup\$
    – danmcb
    Jan 10, 2022 at 8:14

1 Answer 1


You may know INA's have CMRR of 120 db and that GFCI's operate on the small difference current between line and neutral, not by sensing ground current. That's the assumption, that the difference is a safety hazard from gnd leakage, but not in your case.

This INA CMRR is often reduced as low as 40 to 60 dB by the wire pair because it's hard to make stranded wires identical (common) for RLC parameters to 10 ppm or more ? and the stray leakage imbalance is what RCD's and GFCI's detect to trigger a latch.

Your extension cord is doing this by THE GFCI detecting x mA IMBALANCE out of xxxx mA.

BTW , use For INA'S use STP CABLE, it is balanced better with a shield.

  • \$\begingroup\$ Why do 3 incompetent silent people think this is wrong admins? (-3) e.g. @Andyaka pls answer \$\endgroup\$ Jan 11, 2022 at 7:16
  • 3
    \$\begingroup\$ I didn't downvote, and I don't want to. But honestly, I'm having a very hard time figuring out what you're trying to say. Your answer looks more like a terse private note that needs to be fleshed out than like an answer that can be understood by other people. Some specifics: lots of unexplained acronyms, some of which I don't grok. What do stranded wires have to do with it? Where does your conclusion "Your extension cord is doing this" come from? What does STP cable have to do with the question? Is INA an instrumentation amplifier? If so, what is the relation with OPs question? \$\endgroup\$
    – marcelm
    Jan 11, 2022 at 11:54
  • \$\begingroup\$ It's very simple as I explained. @marcelm CMRR is the ratio of rejection of Common Mode to differential mode. INA's like GFCI's depend on good CMRR and stranded cable denies this balance. That's how GFCI's work by tripping on the mA difference from one wire leaking current to the atmosphere more than the other (s) as a false alarm. Or by a true alarm by leaking current from hot to ground via a human, What's so hard about looking up unfamiliar abbreviations? If cables were well balanced, we would never get hum in microphones or guitars or any other interference \$\endgroup\$ Jan 11, 2022 at 23:35
  • \$\begingroup\$ A Common mode choke at line frequency would be rather large in inductance but work to raise the CM impedance and thus reduce unbalanced leakage. In this case, don't use stranded extension cord. I wonder why no one else has responded with an affirmation or a better answer. doh Mods \$\endgroup\$ Jan 11, 2022 at 23:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.