how to control power within a triangle automatically
I think that what you are asking is this.


I have two mains powered extract fans with sensor operated controllers.
I need to add a third extract fan that will run if either or both of the other two are on.
How can I wire this to operate without back feeding the existing fans?








simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. RLY1 and RLY2 monitor the operation of FAN1 and FAN2. The relay contacts are wired in parallel so that either will turn on FAN3.


You need to check that the fans are not variable speed type but just ON/OFF and are mains powered rather than low-voltage DC.
The relay coils should be rated at 230 V AC for UK mains.
The contacts need to be rated to carry the expected FAN3 motor current.