Me wrote:

In article ,
"Pete C." wrote:

What happens if a coil burns out on one of the contactors? One side
switches and the other does not, you could end up with a situation where
you have lost your neutral, i.e. hots from one source and neutral from
the other source. Could be ugly.

If the coil fails, most contactors will be unenergized and that makes
the contacts OPEN in most cases. This is standard for MOST electrical
systems design. You design the system so that if therer is a failure,
It is in the unenergized way and all contactors are OPEN.....

Me

Right and that was my point and also the way I designed my electrically
interlocked transfer switch using only the NO contacts on the contactors
and one contactor per source.

In the RV style auto transfer switches I've seen that use DPDT relays,
they use a single DPDT relay and switch either a 120V source with
neutral, or a 240V source with a solid neutral. A relay coil failure
will leave you stuck on one source but will not create a hazardous
situation.

The transfer switch as described by Steve appears to be using two DPDT
relays to emulate a 4PDT relay with one source on the NO contacts and
the other on the NC contacts, the common feeding the load. With one
relay you are safe, but with two relays a failure of one will put you in
a half switched state which could be hazardous.

Steve stated that the relay on the left was switching the neutral and
the relay on the right was switching the two hot legs of the 240V feeds.
If one of these relays were to fail you would get the two hot legs from
one source and the neutral for the other source which could certainly
cause significant problems.

My recommendation is that Steve review carefully the "what if" scenarios
for the cases of the failure of either relay. I think a safer route
would be to locate a suitable 3PDT or 4PDT contactor to replace the two
relays or to go with a solid neutral.

Pete C.