I'm so stupid... Was: Head Cleaners

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sat Jul 3 18:05:08 2004

>
> > I used to work in a factory where the electrical installation was
> > basically undocumented. I was tought that to make sure to "power
> > off" the right section, I had to make a short circuit. [...]
>
> > I've seen many people looking surprised when I did this, but I feel
> > it's indeed is a very secure way to make sure to switch off the right
> > section.
>
> Yes, it is that. But I still don't like it, and I suspect it's hard on
> the breaker.

It is. The fault current could easily exceed the maximum the breaker can
handle (for a lot of UK MCBs, that might only be 16kA), at which point
the breaker is not certain to close again, and if it does, then it's not
certain it will trip properly in the future.

It's like the crowbar thyristor in a PSU. If the regulator has failed and
the crowbar has tripped and protected the rest of the machine, then I
always replace the thyristor as part of the repair. I am not certain it's
not been overcurrented (particularly if the overcurrent trip didn't work
either, and I'd rather replace it now than have it not work next time the
regulator falls.

Incidentally, UK microwave ovens have a second interlock switch on the
door that shoirt-circuits the mains connections after the normal interlock
when the door is open. The idea is that if the normal interlock fails
(contacts welded shut, or whatever), the second interlock will short
circuit the mains nad blow the fuse. There's nearly always a resistor of
about 0.25 -- 0.5 Ohms in series with it, which limits the fault current
to something sane, preventing damage to breakers, etc.

-tony
Received on Sat Jul 03 2004 - 18:05:08 BST