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:

  1. Use a reversing mechanically/electrically interlocked contactor to switch between the two power sources.

  2. 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.

  3. 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.

  • \$\begingroup\$ First Question... What are your two power sources? \$\endgroup\$
    – R Drast
    Apr 15, 2016 at 17:05
  • \$\begingroup\$ Is this a one time installation or a product you want to sell? \$\endgroup\$
    – Voltage Spike
    Apr 15, 2016 at 17:16
  • \$\begingroup\$ The two power sources come from the city...each are 480VAC 3 phase...This design will go into an OEM product. \$\endgroup\$
    – Greg
    Apr 15, 2016 at 19:24

2 Answers 2


If you are building this for sale probably needs to have the label of an nationally recognized independent testing laboratory such as UL. If you don't have a relationship with such an organization that allows you to label control units that you build, you can't meet that requirement.

If you can get it approved, you still have the question as to whether or not it will work. Directly connected motors will have an unacceptable level of stress from an unsynchronized transfer. A VFD should be able to withstand the transfer, but will likely shut down. Many VFDs have features that may be able to prevent a shutdown, have only a speed variation, or shut down and immediately restart automatically.

  • \$\begingroup\$ Great thanks for the response. The panel is and will be UL labeled to include this. We are building the prototype to see how it goes. I will call the VFD manufacturer to see if they have anything to say about switch power sources. \$\endgroup\$
    – Greg
    Apr 18, 2016 at 22:28

An ATS is most commonly used to switch a load from two power sources but not a different feed from the same or another utility. This is not an allowed practice (at least by FPL). The most common use for an ATS is to switch from the utility to an auxiliary power source like a Generator, this will provide relief during a lengthy power outage. My concern by using a reversing contactor as an ATS is that the utility is supposed to be UP at least 90% of the time and the contactor that connects the load with the Utility must be highly reliable (and expensive). The ATS also should have to take decisions in different times during the transfer process in order to protect the load and avoid potential accidents caused by undesired contact between the utility and the auxiliary source.


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