Question Can a power surge damage a PSU even when it's switched off at the rocker-switch but left plugged in to a mains socket ?

[woopxie]

I often switch off my PSU from the back. however, i don't usually unplug the PSU from the mains socket as much as it wear the prongs when repeated hundreds of times. would it still be protected from the power surge even when plugged into the socket but switched off in the back?

i currently use a Silverstone Strider Essential 700w plugged in to a regular thick-wired extension cable (seems durable so far).

 

[Murissokah]

Shrot answer is yes, It would. Long answer is the switch is rated to isolate the terminals up to a certain voltage when open. Overvoltage exceeding that rating may create a spark on the button and bypass it, likely destroying it in the process and blowing a fuse on the PSU.

The thing is in order for this to happen you'd have bigger issues, since you'd need very high voltage to reach the outlet in the first place. We're not talking about the usual spikes, more like lightning hitting your house and going through that particular circuit.

 

[Misgar]

Switching your ATX PSU on and off at the back of the power supply is not a good idea. A friend of mine does this each time he uses his computers and eventually the switches fail.

When I fitted new PSU switches for him, I opened up the old switches and wasn't surprised to find the metal contacts were blackened with soot and badly pitted. N.B. As an electronics design engineer working on industrial comms systems, I'm well aware of the potentially lethal voltages that can exist inside a PSU.

The reason for the switch damage is simple and it's mentioned in each test appearing in Tom's PSU reviews. The screen shot below shows the mains supply inrush currents at switch on for an Asus 550W supply. For a 115V 60Hz AC supply, the peak inrush current is 78.580A. For a 230V 50Hz AC supply, the inrush current is even higher at 93.990A.

https://www.tomshardware.com/reviews/asus-tuf-gaming-550w-bronze-power-supply-review/2

To quote from the review:

Inrush current, or switch-on surge, refers to the maximum, instantaneous input current drawn by an electrical device when it is first turned on. A large enough inrush current can cause circuit breakers and fuses to trip. It can also damage switches, relays, and bridge rectifiers. As a result, the lower the inrush current of a PSU, right as it is turned on, the better.

The very short duration inrush current occurs when the 400V DC bulk electrolytic in the PSU charges at switch on. It should be noted these currents would be even higher, were it not for the inclusion of a series varistor in the line input of most PSUs (with an optional bypass relay).

The current flowing into the PSU drops rapidly to "normal" levels, but repeated current surges burn away small amounts of metal each time from the switch contacts, as an arc forms at switch on.

You might think the on/off switch is rated to cope with repeated surge currents of 100A+, but if you check the text printed on the switch, they typically read 6A, 10A or 15A continuous current rating. No mention is made of the surge current rating. For that you have to delve deep into the switch manufacturers' data sheets, if you can find them.

If you plug an ATX PSU into a mains outlet at night with the room lights off, you'll often see a flash of light when an arc forms between the tips of the plug and the socket contacts. You'll also see a flash when you plug in a laptop power supply in the dark.

I suspect power supply manufacturers assume most people will not reach behind their computers to operate the on/off switch every day like my friend, so they fit cheaper switches which can withstand a "reasonable" number of power cycles, before they eventually burn out.

TLDR. Don't use the switch on the back of your ATX PSU any more than necessary. Instead use the switch on a "sacrificial" mains extension block. Better to destroy an easily replaceable and cheap extension block, than risk early death of the PSU switch or the wall outlet switch, which are more difficult to replace.

Similarly, an IEC mains lead for a PSU only costs a few dollars. Who cares if it gets damaged? Your PSU is far more expensive and should be treated with care.