Protecting Variable Frequency Drives?

[monkeyhead]

I've seen VFDs protected with all of the following... Is there any standard dictating what type of device a a VFD should be protected with in the US?

• UL 508 Type E Circuit Breaker (motor overload)
• UL 1077 Supplementary Circuit Breaker (possibly dual rated w/ UL 508)
• UL 512 Fuse Blocks with type CC fuses

On the little systems I've done and where I've added VFD controlled motors to existing systems, I've always stuck to fuse blocks, but on our OEM and Integrator supplied control systems, it seems to be a free for all in what gets used to provide circuit protection to the inverters.

I haven't looked super hard, but I poked around quickly and couldn't find any specific guidelines in the NEC or NFPA 79.

 

[DickDV]

I think you see the variations due to different manufacturers specifying different devices.

And, beyond that, it seems a waste to protect with semiconductor fuses a drive so small as to not be repairable. Often you will see standard fuses used on these where semiconductor fuses as recommended.