I have an application where I need a relay to control a generator. The specifics are that this relay needs to have 240vac input (120 on each terminal) and if one or both of these legs of power are lost, it closes a normally open contact that will activate a device. None of the normally open contacts can have any power on them in either state, just complete a connection. All help is much appreciated and thank you very much for your time!

 

Sounds like a simple relay or contactor with a 240v coil and keeps a N.C. contact open, it closes on loss of 240v.
Max.

 

If you are looking to detect if only one leg loses power than you likely need two 120V relays (assuming you have a neutral line available), since one 240V relay may not open with 120V across the coil after being activated with 240V.

Wire your alarm circuit in parallel to a NC contact on each relay.
That way when either relay loses power it will release and make alarm continuity through the respective NC contact.

 

You need a 240Ac relay across both phases, or two relays across each phase,with a set of NC contacts, they will open with power on, and they will close without power applied...

 

If each of the 240v lines are referenced to each other I would think the single relay would work.
A little diagram may be of help.
Max.

 

I have an application where I need a relay to control a generator. The specifics are that this relay needs to have 240vac input (120 on each terminal) and if one or both of these legs of power are lost, it closes a normally open contact that will activate a device. None of the normally open contacts can have any power on them in either state, just complete a connection. All help is much appreciated and thank you very much for your time!

If you are monitoring a US 240 VAC Split Phase I would do as mentioned using two independent 120 VAC coil relays and run one off each line. You can use SPDT (Single Pole Double Throw) relays or even DPDT (Double Pole Double Throw) relays depending on what you want the loss of a line, either line, to do. Each relay coil is across a 120 VAC to Neutral line.

Ron

 

It all depends if you wish to monitor all three conditions separately or use the same result for all three.
L1 and-not L2
L2 and-not L1
Not-L1 and-not L2.
If any of the above is all that is required for a single result.
For a N.A. type service a single 240v relay is all that is needed.
Max.

 

If the normally supplied power is from a national power grid. I would purchase a rated generator disconnect. The prices have come down greatly over the last few years. Just to be on the safe side and still covered by insurance.