Safety and "Chinese whispers".

  Diemmess 18:06 02 Aug 2004

There is a thread in the Helproom now asking again whether to unplug the mains lead when fiddling with the bowels of a computer.
Most replies advise unplug, but the reasons are less sound. Where apart from any parts in the PSU at mains potential is the risk?

Granted that it is safer always to say NO, an intelligent appreciation of the risk should be part of the advice from a serious forum.

The PSU apart from its mains supply, may have a hefty charge on some capacitors, but not more than 12 volts surely capable of a fat spark but unlikely to shock anyone this isn’t an EHT affair like a CRT monitor. It’s the scientific basis I am arguing, not the morality or wisdom to protect everyone, assuming they have never looked inside an battery powered electric toy!

Those who don’t trust the polarity of the house wiring are very sensible. A friend survived (with bad burns to his hands) changing his cooker, and found out later the cooker had its neutral lead wired into the live at the consumer unit, so when he was handling the cables at the cooker switch……!

There must be an anthology of tales out there. Stories of the unlikely and the lucky or unlucky.

I had a bad one yonks ago when looking for why some fixed equipment was “dead.” It had shown a fluttering pilot light just previously.

I switched off fused spur in the room took a good hold on some alloy casting and touched a huge old fashioned cartridge fuse. The near death experience was caused by this bakelite DPDT spur box with amazing complications of silver plated contacts and fine springs. Its bad design flaw, was bakelite cams which levered open the circuit when toggled. Unfortunately, the cam breaking the live connection was broken off and floating about and though the neutral disconnected - the live was still just that.

  Dorsai 19:58 02 Aug 2004

the safety measures a person takes is down to their level of 'perceived risk'.

if they don't see the activity as dangerous, they won't take safety measures.

If they perceive it as dangerous, they will, in accordance as to how dangerous they see it.

This perceived risk may have no relation to the actual risk involved!

Is it safe to change the light bulb now? or do i need to turn it off at the wall first/pull the fuse/throw the mains switch/call an electrician?

On a road in dense fog/Heavy rain. Someone roars past at some ridiculous speed, despite the fact that they cant see past the end of their bonnet. they just don't perceive their actions as dangerous (to them self, or others), or don't care.

Or out on a sunny day, and get stuck behind a driver pootling along at 30 MPH on a dry road, with perfect visibility, and a 60MPH limit (ETC), who is scared to go faster, as they think it is too dangerous.

We all do this. We take into account many factors.


IS it safe to pull out into the traffic?

so we consider;

The perceived safety of the car we are in, in an accident.

The urgency of getting where we are going.

The size of the vehicle on-coming.

Road conditions (wet- he can't slow down well, we can't accelerate well) (dry, his brakes will work well, and we have lots of grip to accelerate with)

and so on.

And we do this mostly subconsciously. We wither pull out, or we don't. If asked why we say

'I thought it was not safe to go'


'i though it was safe'

So we end up with (for any situation):

Reckless (most say 'should not have done it')

Sensible (mostly agree with, would have done the same)

Over cautious. (could have done it OK, so why did they not?)

and all the shades of Grey between.

If we get it right, we are OK.


All we can do when giving advie to others is to try and err on the side of caution. It's up the person we advise as to wether they take the precautions we advicse or not.

  zootmo 20:00 02 Aug 2004

If you are burrowing in the bowels of your computer, unplugged of course, and you are connected via a wrist strap cable to the casing. Are you not both at the same electrical potential?

  oresome 20:11 02 Aug 2004


I understand what you are saying, but accidents are often caused by a series of errors.

The lethal voltages are still enclosed with the outer cover removed, but what if you drop a screw or worse, the door bell rings, you go to answer and a toddler wanders by and pokes something through the PSU ventilation slots.

What if the earth return is defective and there is an insulation breakdown within the PSU.

All unlikely events, but they DO happen.

  Diemmess 20:47 02 Aug 2004

crx1600- HSE suggests plugging into the mains with socket switched off. I cannot see danger there for ordinary (card swopping) reasons.

Dorsai - great stuff, sound philosophy and a neat bit of shoulder-shrugging to end.

zootmo - with a wrist strap I agree that I would be at the same potential as the computer to which it is clipped...... What would worry me is to be insulated on a nylon carpet and for the mains plug earth connection to be absent. Remember how hair crackles if combed in really dry weather, or clothing of man-made fibre builds up static.

oresome - the cautious approach does you credit but to me there is a sense of overkill with good advice. ........The toddler's finger is so much shorter than the Standard Finger that even if he were able to poke it through very small holes and slots he couldn't reach a live spot.

There's not much you can do with a PSU ......They are not user serviceable units so there is every reason to replace every time rather than consider opening.

One danger which is very real, is that of being an "expert" and "It will be quite OK"

  Diemmess 21:08 02 Aug 2004

But then I know better than HSE!

My heirs and successors will sue the outfit that wired my house.

  georgemac 21:25 02 Aug 2004

I live in a modern house, which was wired by professionals, with modern sockets, and is fitted with earth leakage circuit breakers.

I leave the plug in, switch off at the wall, and I reckon I am at extremely low risk. My choice, would not advise others what to do.

I would never dream of poking about inside a crt. Higher risk and much less knowledge of the dangers on my part.

Intersetingly, I had an electrician friend from work (fully qualified) fitting a new ring for an extension for our house. When it came to connect to the main fuse box with the circuit breakers, I asked him where he isolated the incoming supply. I don't he said, I won't make a mistake, insulated tools and wiring! I was very surprised, at work there is no way this could be done!

  oresome 22:37 02 Aug 2004

A good wrist strap should be fitted with a current limiting resistor to prevent the possibility of a lethal shock. A static charge on the body will be discharged in a controlled manner, providing an earth return is available. This won't be the case when the computer is unplugged and the wrist strap is attached to the chassis. It should be attached to an earth point and special plug tops are made for this purpose to utilise the mains earth.

  Chegs ® 03:11 03 Aug 2004

Interesting reading.I can only add a couple of my experiences when messing with "lecky"

I used to rent a 32Ft static caravan,it was equiped with insufficient plug sockets.Having always had a penchant for poking around electrics,I off'ed the power at the Main Switch(with coin meter attached)and quickly removed a socket with the intention of joining another socket to this ones.Bunged a screwdriver into the terminals to undo the screws,and my hair stood up,and I threw the socket+screwdriver away.On investigating why this socket was still live,I found the fusebox had been attached into the earth,which was why 50p had lasted an incredibly long time.

The second crap wiring job I found when I moved here(council house)The bathroom light didn't work,the reason...the on/off switch was connected from live to neutral,which was why the trip box always had a switch in the off position.

The PSU usually has a sticker proclaiming "No user servicable parts inside" which isn't strictly true.I'm not going to dispose of a PSU just because its fan has gotten clogged up with dust.

  Diemmess 09:41 03 Aug 2004

1952, I was a weekend guest of then Pye Chief TV engineer at home. He had an experimental telly and with his wife we were watching the Sunday Play. He muttered about a syncronising "bottle" needing replacement.

Not wishing to deprive his wife or me of the play he reached inside the wooden cabinet with its back removed to change the valve.........There was a hell of a crack, and "Mac" shot backwards onto the floor.

He claimed to be OK and his wife kept asking him.
I played the guest and kept quiet, but noticed apart from his deathly pale appearance, he was only breathing volutarily and in occasional deep breaths for about 5 long minutes.

It seems that the set had not been on for long, and moist ionised air had made it easy for home made lightning to strike him from the EHT circuit to his hand as he reached inside the set.

A few years later a local supply engineer told me of two cases he had investigated. In a new housing estate there had been an explosion and small fire in the bathroom which had a gas water heater. It seems that a short to earth at "Number 2" where the electician had used a gas pipe for an earth had caused mayhem at "Number 9" where there was a better circuit to earth via the heater.

His other tale was......... Called to a farmer whose grazing cattle were falling down after an 11K line had fallen some yards away onto the pasture. The weather was very dry and the unfortunate cattle (while standing)were making a better earth path through their bodies to ground than the fallen cable lying just on the surface.

  oresome 21:18 03 Aug 2004

I knew a few TV engineers who used to detatch the EHT anode connector from the tube and use the resultant arc to clean the other electrodes within the tube neck by passing the connector close to them. This was done with the TV switched on of course and generating several thousand volts!

The purpose was to put some new life in a fading tube.

I also know of a small child who was killed by a TV stand that was screwed into the base of a TV using overlong screws that touched a live circuit.

You don't always get a second chance.

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