I have been asked to assist in trying to come up with a method to control an exhaust fan. Currently the exhaust fan is controlled with a typical HOA except it is interlocked with another HOA controller for a supply fan. They want to keep this system intact with the option to control the exhaust fan separately when needed (supply fan would not be running). I'm envisioning a break-before-make rated switch at the supply side that would feed a separate controller for the exhaust fan. As well, I would need a similar switch at the load side to avoid back feeding. Does this sound reasonable? Anyone put together something like this before?

Use two relays. The first isolates the supply fan interlock and energizes the second relay. The second relay starts the exhaust fan. The second relay should give the necessary delay.

Put a DPDT relay at the fan so the N/C contacts from the existing controller are isolated when the optional controller takes over on the N/O contacts.

Are they really sure that's what they want to do?
The air being exhausted has to come from somewhere.
When you reduce the air supply the exhaust fan won't work quite as well.

I appreciate the feedback.

Have another idea... What about a simple disconnect on the load side of the supply fan's contactor? The exhaust and supply control circuit's are interlocked which will continue to operate as designed with exception of the supply fan not being energized while the disconnect is open.

Ghost307, I did not provide much information on this matter. The room is at a hospital and is used for isolation. I believe they need the option to run only the exhaust fan while testing the room or something of that nature.

I would work within the existing ladder but don't have enough info as to what's really needed. I take it that both of the switches are normally in the "AUTO" position. You would want to keep any LOTO capability. Does supply AUTO have priority over exhaust AUTO now? Should both AUTOs be able to control the exhaust fan at the same time or do they want a LOCAL/REMOTE AUTO switch? You should ask them for a flow chart. I find ladder design to be fairly easy once folks decide what they actually want. They usually think that they know until you show them that the devil is in the details.
Joe

Joe, under normal operating conditions both fans are in "auto". There is a small mechanical room located behind the isolation room where the HOA controls are mounted side-by-side on a wall. On occasion there will be a need to run the exhaust fan in "hand" without the supply fan running. This is why I think a simple disconnect to prevent the supply fan from running would suffice. With both fan controls switched to "off" open the disconnect to break the circuit on the load side to the supply fan and then switch both controls to "hand". Once they're done running the exhaust fan switch both controls back to "off", close the disconnect, and then switch back to "auto".

It would seem that you could just have the Exhaust "HAND" position pick a relay with an NC contact, in series with the Supply Fan interlocks. Or you could add a section to the Exhaust selector switch. If they're interlocked, there should already be a warning label about multiple control voltages being present in the cabinet.
Joe