drmanny3 said:LTLFTcomposite posted the following comments back in 2011:
This might be overkill but if you had extra relays could you protect against hot-neutral reversed wiring on the receptacles? If the relay is double throw it could open the neutral wired across the normally closed terminals if it detects current between (what should be) the neutral and ground.
Then
garygid wrote:
If Hot-Neutral is reversed on one input, you typically just get 120v out, not 240v.
If the phases of the two inputs is the same, you get HOT zero-voltage out.
This suggest that we do not need to add that third relay? I don't believe the box I used will accommodate the third relay. I will take it apart and get another box if this is the consensus.
Thanks
Manny
The third relay is there to insure you get 208/240 or you get nothing at all. If you have a hot-neutral reversal, that means that one hot has a 120 volt potential to ground, the other has 0 to ground. This means that the downstream equipment won't work, which means you may think it is malfunctioning and start poking at it, neglecting the fact that it's plugged in and one of the hots has 120v to ground on it.
The third relay is not supposed to be able to be "brought in" with 120v, so it will prevent hot-neutral reversal activation.
The first two relays are slightly overkill, perhaps, but they're there to insure that if anything goes hinky with one of the inputs that both are disconnected. This prevents any potential for a disconnected plug from becoming a virtual neutral conductor just waiting to find a damp meat path to ground. Yes, the third relay would likely prevent this as well, but on the off chance it was stuck closed...