I am having trouble understanding the Khan academy video of a hair dryer teardown here: https://www.youtube.com/watch?v=gFFvaLzhYew. In this video, the lecturer explains how the safety plug works (so you don't fry yourself if you drop the hair dryer in the sink). At 3:50, the lecturer says that the copper coil can sense when the neutral and hot wires are charged differently, and it then alerts the control chip to turn off the power. The explanation of the operation of the coil was very vague. The picture below points to the part I'm talking about:

Picture of the copper ring in the hair dryer safety plug

What is the copper coil that he shows in the video? How does it work? Is there a more specialized name for the coil when used like this ("safety coil" or "difference coil", etc.)? I ask for the name because I couldn't find an explanation of this via Google, probably because I didn't know what to search for.

  • 5
    \$\begingroup\$ This is called a ground fault detector, and it's also how GFCIs/RCDs work. \$\endgroup\$
    – Hearth
    Apr 29, 2018 at 15:01
  • 1
    \$\begingroup\$ Google "cropping" \$\endgroup\$
    – PlasmaHH
    Apr 29, 2018 at 17:21
  • \$\begingroup\$ Related question \$\endgroup\$
    – Andy aka
    Apr 29, 2018 at 18:21
  • \$\begingroup\$ I know "toroidal ring" is redundant but it's what the lecturer calls it... \$\endgroup\$
    – Nate Glenn
    Apr 30, 2018 at 13:24

1 Answer 1


enter image description here

The coil forms a transformer with the Hot and Neutral. When the currents in the H and N are the same there is no output from the coil. If the difference in current is a little as 5 ma the SSC will detect an output and open the switching contacts.

  • \$\begingroup\$ Why is the push-to-test leg passed through the sensor as a separate conductor and returned to neutral rather than actually inducing a small ground-fault current through the grounding connector? \$\endgroup\$
    – nanofarad
    Apr 29, 2018 at 22:47
  • 1
    \$\begingroup\$ @hexafraction: Among other things, leaking current from the hot conductor through the grounding conductor could trip a GCD in the panel or receptacle which is feeding the plug. \$\endgroup\$
    – supercat
    Apr 29, 2018 at 23:21

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.