AC Power Switching (was Electronic components sources)

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed Mar 24 22:40:11 2004

>
> To all that have posted....
>
> My original comment was made specifically in reference to 2-wire (therefore
> single phase) power sources WITHOUT an independent ground. Note: This is

And everybody else has been talking about machines with a built-in PSU
with a separate safety earth (ground) wire. The Rainbow tht started this
discussion is certainly like that.

Nobody is suggesting that the switch should open the safety earth -- in
fact I think that is prohibited by every set of regulations I've seen.

> ALWAYS the case with a device that uses an external power supply wich just
> supplys a low voltage to the actual device (e.g. Wall Warts).

I have worked on devices that have wall-wart type PSUs (that is to say an
oversized plug containing a transformer) or with external mains
transformers and with a safety earth in the low-voltage cable that's tied
to the eart hpin of the mains plug. This earth was not connected to any
of the low-voltage current-carrying wires in the cable.

>
> The comment is totally in-applicable (IMHO) when there is a dedicated
> (non-switched) ground connection OR the power is multi-phase.

Ah... Well the 'dedicated ground connection' is common in
properly-designed equipment IMHO.

>
> With this as the configuration there are a number of issues that can (and
> undoubtably will) arise if both of the legs are disconnected in a power off
> condition. It is necessary to realize that this condition is EXACTLY
> equivilant to the device being unplugged!
>
> The first condition is discharge. This can be the result of either static or
> an external application of charge. [Oversimplifing things a wee bit] Charge
> will always seek to evenly distribute itself. It will do this through the
> path of least resistance. If the Ground of the circuit is available, this
> will typically be the most commong route, and is usuaslly designed to handle
> larger currents than paths between signals [which typically also have higher
> resistance].

If you'ew going to allow a relatively low resistance path (low enough to
disipate static charges) between one of the current-carrying mains
conductors and the low voltage side then all bets are off!. Maybe then it
would be safer for the device if both wires are not switched (although
this is not certain!), but the device is possibly lethal to the user
anyway! I'd rather have an unswitched ground connection (also used to
disipate static) and double pole swtiching of the current-carrying
conductors.


-tony
Received on Wed Mar 24 2004 - 22:40:11 GMT