Motor Circuit Protector with a VFD

[CharlieSW]

A VFD manufacturer is proposing to utilize a motor circuit protector for the incoming disconnect switch in lieu of a circuit breaker. Is this kosher? I thought you could only use a motor circuit protector if it was part of a UL listed combination starter assembly. If so, what does UL listed combination need to include? Does the actual breaker needed to be tested as part of the assembly? So if they are proposing a cutler hammer MCP with a AB VFD does this actual combination need to be tested?

Any informative thoughts would be appreciated.

 

[petersonra]

A VFD manufacturer is proposing to utilize a motor circuit protector for the incoming disconnect switch in lieu of a circuit breaker. Is this kosher? I thought you could only use a motor circuit protector if it was part of a UL listed combination starter assembly. If so, what does UL listed combination need to include? Does the actual breaker needed to be tested as part of the assembly? So if they are proposing a cutler hammer MCP with a AB VFD does this actual combination need to be tested?

Any informative thoughts would be appreciated.

Good question. I think the answer is that the MCP can only be used for short circuit protection as it only has the magnetic trip, and not the thermal trip.

VFDs typically provide overload protection much like a starter does, so I am not sure there is any real difference in the application.

I don't recall MCPs being limited to use in a listed starter assembly.

 

[don_resqcapt19]

Bob,

I don't recall MCPs being limited to use in a listed starter assembly.

Take a look at the following code section. I think this would prohibit the use of an MCP on the line side of a VFD.

430.52(C)(3) Instantaneous Trip Circuit Breaker An instantaneous trip circuit breaker shall be used only if adjustable and if part of a listed combination motor controller having coordinated motor overload and short-circuit and ground-fault protection in each conductor, and the setting is adjusted to no more than the value specified in Table 430.52.

Don

 

[tony_psuee]

Don,

If the VFD manufacture has submitted the MCP&VFD to UL and received the appropriate listing would that not make it allowable?

tony

 

[don_resqcapt19]

Tony,
Only if it is listed as a combination starter. I'm not sure you can get a VFD listed as a starter.
Don