CRT handling Was: Re: It's ALIVE!

From: Philip.Belben_at_PowerTech.co.uk <(Philip.Belben_at_PowerTech.co.uk)>
Date: Fri Feb 26 06:56:17 1999

[Earth Leakage Circuit Breakers]

> I am not sure what the UK IEE regulations state about them (Philip?), but
> they are not that common in domestic installations. I would always use
> one on a socket for something like a lawnmower or other outdoor device.


The IEE regulations require that any socket likely to be used for equipment
operated out of doors be protected by a residual current circuit breaker
with an instantaneous tripping current of not more than 30mA.


> 1) A lot of classic computers - particularly minicomputers - have proper
> mains filtering. The earth leakage current of the mains filter, when
> operating correctly, is enough to trip most RCDs. Useful, that.


Think of it as a protection grading problem. But here you have to choose
the protection settings upstream to grade with the equipment downstream. I
haven't heard of such filters drawing 30mA, though.

> 2) They are not a substitute for an isolating transformer. If you want to
> make measurements on the primary side of an SMPSU with an earthed 'scope
> then you need an isolating transformer. Put the PSU on a RCD and said RCD
> will trip (correctly) as soon as you connect the earth lead of the probe.
> As doing things like that is the main reason why I'd be working on a live
> machine, I'd rather have an isolating transformer. And of course an RCD
> won't protect you if you manage to connect yourself across the output of
> the isolating transformer.


True. Any protective device is useful in a specific situation, and
protects against specific faults. Anything can be used where it's
inappropriate. But that doesn't make it per se a bad thing.

> 3) They are only of use if you touch the mains side of a supply. They are
> no use at all if you manage to get connected across the output of an


See above.

> isolated HV supply. But IMHO having a safety device in line makes you
> careless. You may not realise it, but you will depend on the RCD to save
> you if you make a mistake. And then you make a mistake where it can't
help.


I disagree. I once said, joking, "I used to recommend steel toe-caps when
moving large computers, but now I generally prefer bare feet - so you make
damned well sure you don't drop it!"

My view is: A protective device - any protective device - is there to get
you out of trouble, not to get you into it. One should never take risks
(because one knows there is protection there) that one wouldn't take
without it. I disagree that the knowledge that there is an RCCB in the
circuit will make me complacent.

> I'd rather just work carefully, knowing any mistake I make could be
> fatal.


I'd rather work carefully, knowing that any mistake I make could be fatal,
but also knowing that I have taken all reasonable steps to ensure that it
won't.

Our protection specialist at Power Tech often talks about prortection in
terms of "the fence at the top of the cliff" and "the ambulance at the
bottom of the cliff". When working with live high voltages, one should
always try and put in place as many "fence" systems as possible - rubber
gloves, isolating transformers, insulated screwdrivers, hands in pockets
etc., but "ambulance" systems such as residual current devices have their
place.

Philip.
Received on Fri Feb 26 1999 - 06:56:17 GMT