Exractor control

[cropwell]

This circuit is drawn for 6v but 240VAC is the same principle. It uses two normally open switches to turn on power and complete the loop that holds the relay on. and two normally closed switches as a hold loop which when you press one, lets the relay drop out. In my workshop, to switch single phase power, I use a 100A contactor with two poles used to switch live and neutral and the other to make the loop. It means I can put emergency switches round the place, but I only have one switch on point, near the door. It also means that I can switch off everything in one go, when I leave. It also acts as an NVR switch if there is a power failure.

NVR switches generally have a simpler design, where the ON button pushes the contacts and the Off button breaks contact or pushes the contacts apart, like the manual operation on a contactor.

Hope this helps.

Rob