Control Circuits - Dry contacts

Thread Starter


Joined Aug 29, 2010
G'day mates ...

Please find attached schematics and see if you can read it and write it down in plain English - Reason for that is that I want to know if I am correct for the application I wanted ...

Maj - Sydney



Joined Jul 16, 2011
No-one else has given this a go so I'll try!

The R/1 will become high when either of the start switches are closed and the stop switch is closed, or when both the start switches are closed and the stop switch is closed.

If the stop switch is open, then R/1 will not go high, even if both or one of the start switches are closed.

As for the rest of the circuit, if the switch for each component is closed, then that respect component will 'go high', independent of each other.

For example, you could have the fan and the contractor on, but not the BMS relay.

For each component to work, each respect switch must be closed.

I hope that's been helpful,:)



Joined Jun 7, 2009
funny that you'd want us to guess at the application, to see if we're right or wrong.

Your stop / start circuit is not typical. You'd normally have a NC momentary stop switch. Your start would be NO momentary, paralleled with R1 NO contacts. That would give you a holding start/stop circuit. That's assuming that your BMS input must be maintained.


Joined Jul 17, 2007
START seems to be sort of a normally-open latching switch; once you turn it on, it stays on until power is cut. It could also be a momentary switch, and the contacts below it would be labeled R1-2; when the coil R/1 is energized, the contacts R1-1 through R1-2 are closed.

STOP is a normally closed pushbutton. Pressing STOP breaks the path of current to R/1, wich turns off START and stops current through R/1, causing its' contacts to open as well.

When power is applied to R/1, contacts R1-1 close, supplying power to the coil of R-2. I do not know what "BMS relay" stands for offhand. R2-1 closes, supplying power to the coil of contactor K/1. Contacts K1-1 close, supplying power for the fan.

If at any point there is a power "glitch" or failure, the start circuit should de-latch so that power to the fan is disconnected.

I don't know why it was drawn so complicated; there must be some other significance to this "BMS relay".

Thread Starter


Joined Aug 29, 2010

The application is that when lights get "On" through smart Automation, that is C-Bus, then the AC fan turns on too ..

Now the C-Bus Automation provide 24V every time the lights turn on, so with 24V I need to activate the relay from Honeywell Building Management system (BMS) and then the BMS activate 3phase contactor that turns on the AC unit ...

See it the diagram is good