I have the following situation:

  • Living in Europe (220V sockets, 50 HZ)
  • My wife came to me and said she tried to start up our sons PC and she heard a short zap, then nothing happened; my son confirmed he heard the zap also (also he said he also heard it earlier for a longer time - that's when he called my wife to see what is wrong, when she came there was no sound but zapped when she pressed the button)
  • When I pushed the button nothing happened (there was no power)
  • When I checked I noticed that the socket cable was not fully pushed into the PSU
  • When fully inserted everything was working fine
  • Before leaving the kids play again I am trying to understand what happened: clearly the cable was incorrectly fit (probably got loose when vacuuming around or something). However... I don't understand the zap
  • There was no smell, or any kind of visible damage to the cable
  • There was no smell (or maybe a super faint one) over the PSU... only sense it when literally pressing my nose against it
  • The fuse didn't pop
  • The differential protection didn't pop
  • There is no voltage on surface of pc (since I have grounding that's normal but thought worth mentioning)
  • The PSU didn't enter protection mode
  • No real damage to internal components seem to have occurred
  • The differential protection pops fine when the tester is being pushed
  • I am sure that neither my wife or my kids is lieing to me regarding the sound (and both described it as an electrical zap quite consistently)

Initially I thought maybe it was an electric arc "in" the live wire connection but... how come no damage to the cable then? Or if it was only an imperfect connection that was kind of working why fully stop with no reason? Also an electric arc should have losses so why didn't the differential pop?

What I am ultimately trying to asses is if the socket and PC are safe to be used by my family. I am inclined to change the PSU (but it is a seasonic prime... which I consider actually being a good PSU... I am nost sure the problem is there).

Any idea what might have happened?

  • \$\begingroup\$ Not just PSU but any cable socket and plug of any type attached to a high load (eg a PC, an oven, hair dryer etc.) can arc and produce zapping sound if the contact is intermittent. \$\endgroup\$
    – slebetman
    Jun 7 at 3:58

3 Answers 3


Probably a small arc contained entirely in the affected contact cavities. No damage to the cable since it would just be in the mating surfaces. Remove the cable from the power entry port of the power supply and closely examine the three prongs for any discoloration or evidence of sparking. It might have also been wiped completely off by the fingers on the cable side.

Edit: to prevent a repeat performance, I would purchase a new IEC cable as some of the cables shipped with equipment from even reputable manufacturers can be sad stuff. I assume it's an attractive target to save a few cents.

  • \$\begingroup\$ Tyvm for the answer. I assumed that and when I examined the cable I did look inside the connection holes. The gold seems untouched by any kind of heat. Same for the father prongs, they look pristine. Additional question: shouldn't have that trigger the differential protection due to energy lost through heat? \$\endgroup\$
    – zozo
    Jun 6 at 17:30
  • 1
    \$\begingroup\$ @zozo differential current systems cannot detect energy lost to heat. They can only detect current leaking away from the circuit (e.g. if the arc went to ground rather than being contained within the prongs/plug which seems to be the case) \$\endgroup\$
    – nanofarad
    Jun 6 at 18:03

When I plug the mains cord into the power supply for either of my laptops I hear an arcing zap sound (or a crackle) and sometimes nothing (if I push the connector home quickly enough). It makes a very faint sound occasionally when I pull out the plug. I expect the sound that was heard was an intermittent making/breaking connection.

I've literally done this hundreds of times and things are still working just fine. I expect there will be a little scorched area on the mating pins but, I'm not bothered.


It's pretty normal. I don't think I got anything at home with a switching power supply that won't arc when the connection to mains is made. And that's at 120V. Would be much worse at 240V.

What I am ultimately trying to asses is if the socket and PC are safe to be used by my family

Arcing in the PSU circuitry is typically catastrophic, and a fuse inside of the PSU would protect the PSU from fire due to continued heating.

Most PSUs don't work at all if they have arced anywhere inside of the PSU, so if yours still works, it's very likely that it will keep on working, and the arc wasn't inside of the PSU. Having to push in the cable to get the computer back into operation confirms this.

In other words: We're glad you asked, but there's thankfully nothing to worry about. The suggestion to replace the cable so it'd be a tighter fit is a good one.

Initially I thought maybe it was an electric arc "in" the live wire connection but... how come no damage to the cable then?

Not all arcs are catastrophic. When the arc completes an otherwise normal circuit, it's just like a piece of resistive wire in series with the mains supply. Not all arcs have to short Live to Neutral or Earth. Direct live to N/PE arcs are loud and leave visible damage. They have enough energy to vaporize several cubic millimeters of steel before the circuit breaker kicks in. I have a couple of screwdrivers that shorted the 220V/50Hz mains. They have nice gouges melted on them. They all occurred with a loud bang, the circuit breaker tripping, and a flash that would be hard not to notice.


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