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.

  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.

  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.

  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.

  Forum Editor 22:46 03 Aug 2004

"No user servicable parts inside" for good reason - there are no user serviceable parts inside.

Anyone who thinks they know better is risking injury at best and electrocution at worst, and not just for themselves - others may use the computer now, or later. Why try to be clever with other peoples' safety, all for the sake of saving a few pounds?

  Diemmess 18:18 07 Aug 2004

I started this thread really to see what bizarre incidents others might relate.
Of course total isolation is safest, but safety has a price sometimes of inconvenience, and there's the rub.

In these litigious days when sadly so many insist on it always being someone else’s faulty it gives me a smug feeling if “His Honour” explains to the plaintiff that he has a “Duty of Care”.

Yonks ago when a son (still at school) became interested in fiddling with transistors and capacitators as he once called them, and built a basic radio, I joined in for a year or two reviving my own school day memories of playing with valves and 300volts DC. This time, 9 volts was pushing it!

I remember being in awe of CMOS circuits which seemed to be ready to self destruct if I so much as laid a finger on them away from the circuit board (home etched of course). These were the days when integrated circuits were just beginning to take over.

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