Firstly, this applies to the UK - so 3 pronged sockets and 240V. Short and sweet question - should there be any voltage difference between the live and neutral prongs on a surge protector once it has been unplugged?

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    \$\begingroup\$ Use a voltmeter and see. \$\endgroup\$ Dec 17, 2011 at 13:05
  • \$\begingroup\$ Already done - however I'd like to know if this is characteristic of a surge protector. \$\endgroup\$
    – R4D4
    Dec 17, 2011 at 13:07
  • \$\begingroup\$ @Olin - Methinks "should" and "will there" may not correlate well in his universe. \$\endgroup\$
    – Russell McMahon
    Dec 17, 2011 at 13:08
  • \$\begingroup\$ A voltmeter may not give you the answer you expect, because the meter's impedance (10 megohms, usually) will discharge the capacitor. Also, the initial voltage on the caps depends on when in the AC cycle you unplugged the device. \$\endgroup\$ Dec 17, 2011 at 13:46

1 Answer 1


Transmogrifying your

  • "should there be any voltage?"


  • "will there be any voltage?"

the answer is "There might be, it all depends on circumstances and equipment involved".

I don't know if "a surge protector" tends to be a reasonably specific device in the UK but here in the antipodes at the dawn of time it could mean a wide range of things.


  • IF a surge protector consists of a device which conducts when line to line voltage exceeds normal max by a significant margin - such as back to back zeners, a MOV, a transzorb, a gas discharge tube, a neon or similar.

  • but which is of very high resistance when no voltage is applied

  • and IF the surge protector contained X and/or Y capacitors (line to line or line to ground)

then YES voltage would be very likely to be present, because the capacitor(s) would probably retain some charge if the mains was disconnected with the load switched off.

If mains is disconnected by opening a switch or pulling out a plug, then line to line voltage and this capacitor voltage could be anything from about +330 to about -330 V as the disconnection timing is not synchronised with mains zero crossing. A capacitor connected across the line or leg to ground capacitors will be left with the mains value at the time of disconnection. If there is no load present this voltage could remain "for some while".

  • \$\begingroup\$ Zeners and MOVs will have some leakage current, though, which will discharge the capacitor over time. Also, for safety purposes, the amount of energy stored by the capacitor is a factor; a 1 uF capacitor with 240 V on it has 28.8 mJ of energy, which isn't that much. And that's assuming the surge suppressor was disconnected at the full-voltage part of the AC cycle. \$\endgroup\$ Dec 17, 2011 at 13:44
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    \$\begingroup\$ @MikeDeSimone - I agree - BUT - If you've ever touched a capacitor with AC mains remnant stored on it (I've touched a few over the years) you would no doubt have found it exciting. Also, with 230 VAC, when starting at 0 volts at the zero crossing the voltage has reached 84+ volts by 15 degrees into the cycle. You'd feel that. So Voltage will be high enough to give you a "shock" over 80% of the time when randomly turning off mains which is feeding a capacitor. \$\endgroup\$
    – Russell McMahon
    Dec 17, 2011 at 15:12
  • \$\begingroup\$ Also depends on whether they put in a bleeder resistor to take out the charge soon or skipped that from the circuit. Some electrical codes specify that capacitors must have some means of discharging them in 1 min, but I'm not sure whether that applies to off the shelf surge protectors. \$\endgroup\$ Aug 20, 2021 at 3:51

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