Electronic components sources

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Tue Mar 23 17:53:24 2004

> > > NO,NO,NO!!!!!!!!!!!!!!
> > >
> > > This can potentially leave a circuit floating, wich can be
> > much more
> > > dangerous. Try to HI_POT test a device in this
> > configuration, and you

If a HI-POT test is what I call a megger test (that's a high voltage
earth leakage test) or flash test (high voltage breakdown test) then it
must be done with the switch _on_ (and in certain cases with internal
relays shorted out or otherwise operated). Otherwise you're just testing
the bit of wire from the mains input to the switch.

> > > may very well let the "magic-smoke" out. If you are
> > switching on the
> > > raw power side, the neutral should always remain connected. In fact
> > > this is code for many (most/all) CE/UL approval stamps!

Odd. The UK regulations specifically suggest breaking both live and
neutral (although live only is acceptable in many cases). And for 3 phase
star, breaking all 3 phases and neutral.

You can't have independant protective devices (like fuses) in both wires,
in case the one in the neutral wire blows first and leaves the whole
thing connected to the live side of the mains. But you can certainly have
a double-pole circuit breaker that breaks both wires together. You cna
even have trip coils in each wire, provided that overcurrent in either
wire causes the breaker to break both (If you only have a single trip
coil, it has to be in the live side. For 3 phase, it's recommended to
have a trip coil in each phaae wire, but for delta connected 3 phase,
you can have one in each of 2 of the phases, I believe).

It's also strange that _every_ piece of quality electronic equipment I
have here, from the likes of Tektronix, HP (that's _old_ HP, not the
modern trash with that name on it!), DEC, etc, has double pole mains
switching. It's also recomented in _every_ electornics construciton book
I've looked at recently.

> >
> > Certainly something could be designed so a hot and neutral break
> > ("disconnect") is quite safe, but you are right, most
> > applications are safer just breaking the hot. Plus, breaking

Can you give me a good phusical explanation for this. The only one I can
think of is if the switch fails so that the live side doesn't open any
more then you'd have the equivalent of a single pole switch in the
neutral side. You;d not notice that (the switch would still turn the
device on and off), but it could be dangerous. But I have never seen that
failure mode.

> > both really is just a waste of money for a more complex switch.
> I'm lost as to how a device with a switch that breaks both
> Live and Neutral can be *more* dangerous that a device with
> a switch that breaks only Live (or just breaks one line,
> for those of you who have to work with people who wire
> things up wrong ... "consumers" is the technical term,
> I believe).

No, the techincal term is 'electricians', at least over here ;-).
Seriously, I've seen some terrible wiring done by so-called professional
electricians, including a distribution panel on a workbench where
alternate socket outlets had live/neutral swapped (!).

Received on Tue Mar 23 2004 - 17:53:24 GMT

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