We have a an industrial control system that utilizes a 480VAC 3 phase 10 Amp service. For redundancy reasons, we wanted to add an automatic transfer switch to the system to switch between two redundant power sources (each are 480VAC 3 phase). Automatic Transfer Switches (ATS) are expensive and usually pretty large. We would like to use the following scenario instead of the ATS:
Use a reversing mechanically/electrically interlocked contactor to switch between the two power sources.
Use two voltage sensing/monitoring relays (3 phase) to sense if the phases and power is good on either power source. If the primary power source fails, the voltage sensing relay would detect this and energize the coil on the contactor to switch to the redundant power source.
Add circuit protection (most likely breakers) before and after the reversing contactor.
We do this type of setup on a 208VAC single phase system and it works great. Here is a diagram of that setup:
Other concerns I have is arcflash requirements, phase sync issues, and if UL would be ok with it.
The 480VAC in our system supplies power to a VFD and a 480VAC to 120VAC transfomer. These would be the piece directly affected from switching from one power source to the other.
Any insights would be greatly appreciated.