stan2004 posted:I agree that explains how 12 relays could be deployed...but I don't see how it performs any different/better than your 8-relay method. The force-OFF switch in your method could be placed on the output relays (red X) and these switches would not need to be heavy-duty 10-Amp (or whatever) switches.
Plus, with the presumed 12-relay method, the nominal condition is all 12-relays are "active". And for heavy-duty relays which might draw 1 Watt per coil... that's 12 Watts of overhead. OK, 12 Watts does not break the bank. But all 12 relays carry the full load current per block...whereas with your 8-relay method only half of the relays carry full block power so lower capacity relays can be used for the sense function so less expensive relays could be used.
In the absence of further information, I vote for your 8-relay method which AFAIK meets the known requirements.
A few random thoughts-
Why couldn't you reverse the logic of the whole thing; use the NC contacts on the control relays and put the sense relay contacts in parallel? That way the control relays would normally be de-energized, no heating and no power consumption. Yes, if the control supply fails you would have no protection.
I'm guessing that the DC supplies were intended to permit the use of 12 VDC sense relays, but the filter caps in the supplies caused the sense relays to hold in for a while after the power was lost, so the 8.x volt setting was chosen to put the relays right on the edge of dropout to speed up the response to a power loss.
Because there are no inexpensive 18 VAC relays, the sense relay function is going to require a bit of fudge. I think the original design is a good attempt to use plain old stock parts.
I understand that the builder of the original unit may well have used what he had on hand. No criticism of the original design is intended.
Just talking........
