Transfer switch, or not transfer switch?

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MFS-PDX-313

MFS-PDX-313

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Location
Tualatin, Oregon, USA
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Electrical design engineer, Industrial equipment
Our equipment is fairly simple, but the customer wants it upgraded to allow two separate AC feeds. The main breaker is a 15A, that branches to a 10A breaker feeding a 24VDC power supply and 5A breaker feeding PLC power supply. Can I just use a 15A relay and run feed one through the NO contacts and feed two through the NC contacts, or would I need a transfer switch?

At what amperage do you need a transfer switch?
 
Besides the two feeders are all the components and other wiring inside the equipment? If so is the equipment listed?

If the answer is yes to both just have your engineers have it listed to the simplest fix .
What's inside a listed piece of equipment is outside of the NEC's control.

Roger
 
I still have to meet NFPA-79, which does deal with everything inside the equipment.

But yes, all the components are internal. What do you mean by listed? It is UL 508 listed.
 
MFS-PDX-313 said:
I still have to meet NFPA-79, which does deal with everything inside the equipment.

But yes, all the components are internal. What do you mean by listed? It is UL 508 listed.
Click to expand...

If it's already listed as it is I would say it will need to be relisted if you modify the design, meeting NFPA 79 would be part of the listing.

Roger
 
MFS-PDX-313 said:
Our equipment is fairly simple, but the customer wants it upgraded to allow two separate AC feeds. The main breaker is a 15A, that branches to a 10A breaker feeding a 24VDC power supply and 5A breaker feeding PLC power supply. Can I just use a 15A relay and run feed one through the NO contacts and feed two through the NC contacts, or would I need a transfer switch?

At what amperage do you need a transfer switch?
Click to expand...
there is no code requirement to use a listed transfer switch unless the code says so for the application. I don't recall there is such a requirement found in article 670.
 
Thanks for the info folks!
Now I just have to see if the PLC power supply can survive a power switchover without resetting the PLC, or if I need to switch to a DC fed power supply.
 
Good luck.
Way back when, I had a problem with switching the AC power supplies feeding the PLCs. In one direction the PLC would ride through the power glitch, in the other direction it would begin to power down and then immediately restart resulting in frequent crashes. We ended up going to a single power supply that had an adequate ride through.
 
If you are only worried about dc, there are plenty of solutions that basically amount to multiple dc power supplies feeding the same point via diode isolation.
 
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