Adrian Brentnall wrote:

HI Bill
I'm always a bit nervous of the 'commoned-contacts' approach, as
Murphy's Law suggests that one set of contracts will operate slightly in
advance of the other set, and will end up making or breaking the lion's
share of the current.

I did wonder about that, but it doesn't seem to have happened.
Since I wrote before I've remembered that I made a unit that switches in
two sets of batteries on different time delays, the idea being that the
alternator wouldn't find itself supplying 24V to the vehicle battery
plus two 110Ah batteries both of which might theoretically be flat. That
device used 20A commoned contacts relays and it's had some right hammer
over the years, and it's been fine.


I'm deliberately over-engineering this project! (Partly because I want
it to be reliable, and partly for safety reasons - as it's a box of
heat, operating unsupervised, in a timber workshop.

How's your insurance?

For this reason I'm forgoing the usual 'let's see what's in the junk
box' approach, and speccing / buying new components. Call me paranoid!

You need to be paranoid by the sounds of it.

Bill