I take it these are used when you're happy when the lag is just set by a master and you need no 2 way switching?
If so how are they wired, is there just a 2 core (+E?) switched live from the master to the slave which, when made, just activates the master just as switching it does?
If so how are they wired, is there just a 2 core (+E?) switched live from the master to the slave which, when made, just activates the master just as switching it does?