In article ,
Adrian Brentnall writes:
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.

It's not just the make or break order, but inevitable difference in
contact resistances means that the current will be badly balanced
between them (albeit, picking the better one at any one time;-)
You can mitigate this to some extent by making sure there's a length
of parallel wires connected to the contacts rather than paralleling up
as close to the contacts as possible - the resistance of that wire
will help reduce the ratio of differential contact resistances.

Relays are not usually rated for paralleling up contacts.

Another common mistake in this area is using a 3-phase contactor on
single phase. A 3-phase 4-wire contactor rated at, say, 100A, will
have 4 x 100A contacts. People will sometimes use this on a single
phase (where the load doesn't require phase rotation), and overlook
that the 100A neutral contact is now carrying 300A. Even when the
contactor is only switching the 3 phases and the neutral is
permanently connected, the connections and neutral conductors through
the device are still only rated 100A.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]