On Sun, 31 Aug 2008 15:31:35 -0700 (PDT), Jonathan wrote:

Is there much difference between a "transfer switch" such as this:
http://www.mowers-online.co.uk/itm01417.htm

and a suitably rated contactor such as this:

http://cpc.farnell.com/MC00148/elect...y/product.us0?
sku=imo-k2-30a10230ac

Well as I read the description of that it is a simple 3 pole on/off switch
with an extra aux on/off pole. It has no "change over" function and as all
contacts are NO you can't do "something clever" with the wiring.

Obviously one comes in a nice box, with accessories - but as long as
the contacts break before they make, surely these will both do the
same job?

If the contactor you linked to was at least double pole change over break
before make yes but as I see it it isn't... There may also be regulations
about contact separation within the switch, a contactor might not have
enough.

The B&S transfer switch does have everything in the one box, the RCD is
essential IMHO.

I'm also unsure what extra complexity an "automatic" transfer switch
has,

What are you backing up with a genset, ordinary domestic stuff or
work/income related? TBH unless you *really* need 100% up time an
autostart genset, UPS to bridge the gap, and all the associated control
and switch gear is not worth it. If you have to drag the genny out of the
garage, run a cable etc, is operating a switch that much more?

beyond perhaps a short time delay circuit that will activate the
contactor once the secondary supply starts up..

Don't forget what happens if the genset fails. Do you want to drop back
onto the failed, over/under voltage mains supply? Give much more thought
to failure modes and what happens as a result.

--
Cheers
Dave.