Pulling out mains plug at back of PC

  keewaa 11:42 23 Nov 2005

Can a knowledgeable person (someone who understands electricity) explain why pulling out or pushing in the mains plug into the back of the PC puts the motherboard at risk of a fry up.

I know even static electricity on fingers can fry things, but is there a genuine risk from the above, and if so why. Would the same risk exist if it was the plug in being pulled in and out of the socket while on, or does the fuse in the plug eliminate the risk.


  John B 11:59 23 Nov 2005

"Voltage spikes or surges are typically caused by switching heavy loads... on or off. At a point in its sinusoidal cycle when the mains power-supply voltage nears its peak (a nominal 230Vrms), the actual peak instantaneous voltage is 325V. Now consider a (not uncommon) brief voltage spike of several hundred volts on top of that sine-wave crest and you can imagine how damaging it could be. The spike could also be in the opposite polarity to the mains cycle, reducing the instantaneous voltage instead of boosting it, which could cause just as many problems. Voltage spikes of up to 650V (twice the normal mains voltage) are not that uncommon"

From here..click here

  keewaa 12:08 23 Nov 2005

That seems to be more to do with voltage spikes coming down the supply rather than me pulling a plug out of or into the back of a computer ... I don't think computers are classed as "heavy loads"

I'm just wondering if it's true or just a myth, and if it's true why, what causes the risk?

  John B 12:26 23 Nov 2005

I think the same thing applies..pulling a socket out of a plug may cause the supply to arc or jump across the contacts causing a voltage increase or spike. I agree computers aren't heavy loads but I think the principal remains the same. If you start feeding a higher mains voltage into your pc's power supply there is a chance the wrong voltage is going to get out at the other end and into your motherboard.

Try it a few times?

  John B 12:27 23 Nov 2005

only joking!

  TonyAA 12:44 23 Nov 2005

With any electrical device you are advised to use the switch as this shuts off the power almost instantaneously.When you pull a power plug out,especially if it's stiff,you tend to waggle it about and as John B says this causes arcing as you can be going from zero to the full voltage many times when you are doing it.

  Stuartli 13:13 23 Nov 2005

Remember too that you are plugging and unplugging the mains lead into a transformer - you should use the PSU's on/off switch first before pulling out/replacing the mains lead.

  keewaa 13:21 23 Nov 2005

So it's a bit like this in miniture? click here

The PSU doesn't have an on/off switch on some machines.

If I go to the other end of the lead and pull the plug in or out of the mains wall socket, does this have the same risk, or is there something in the plug (fuse?) that would protect?

  Stuartli 13:27 23 Nov 2005

Exactly the same as with the PSU - turn off at the mains before taking out the plug.

  Gongoozler 13:32 23 Nov 2005

Although I haven't studied the workings of a PC power supply, I think I can offer a possible explanation. When the computer is switched off at the front panel, there is no physical isolation of the mains from the psu. 5V standby is permanently supplied to enable the power switch and various wake-up options to operate. I believe that the mains input is still rectified onto the large storage capacitors that supply the switched mode circuits. If I'm right, then when the mains plug is inserted, or mains power switched on, there will be a large surge current as the capacitors are charged. The psu components will be specified to withstand this surge current, but it will still be the time when a sub-standard component will be most likely to pop, just as a light bulb is most likely to fail at switch-on.

  woodchip 13:36 23 Nov 2005

Only thing that you should not do is pull the plug with the computer switched on. Other than that no problems

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