Form of MCC Construction (IEC 60349) - Definition of Functional Unit

[123MB]

Hi All

I have a noob question regarding the definition of 'functional unit' with respect to IEC 60439 (and the new IEC 61439) and there are other equivalent standards over the world.

The standards define a functional unit generically as whatever is between the busbar and the outgoing terminals... does this include VFDs? Soft starters? Contactors? Or is it a generally acceptable interpretation that it just applies to switchgear?

Considering the reason for MCC segregation, i.e. limiting the propagating affects of arcing faults, ease of maintenance and continuity of supply and/or production, I don't see why a VFD or soft starter should not be included in the functional unit. But most of the panels I have seen that claim to be form 3 or form 4 provide segregation between the feeder circuit breakers but do not provide the same for the drives (if present)

Thanks

 

[jraef]

We don't use that spec in North America, so "all over the world" is relative!

My company however makes IEC MCCs and as I interpret that terminology, they are referring to functional units as being anything, including VFDs or Soft Starters. The issue as it relates to Arc Resistant design however is ventilation, because power electronics generate heat and are sensitive to heat buildup. So if you are going for arc flash energy and pressure containment, you need very complex and expensive ventilation designs with baffles etc., yet even then there is a limit as to how much air you can move and still maintain the design specs for personnel safety when standing in front of the MCC during an arc flash event. That ends up with only small VFDs and SS units being included in an Arc Resistant design spec. It will vary between mfrs because of VFD sizes and temperature requirements, but I have not seen anyone offer it in Arc Resistant NEMA MCCs above 125HP.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[123MB]

Thanks mate, so there is no equivalent means of defining MCC levels of segregation for you guys?

 

[jraef]

Yes, it's just different. The term "Arc Resistant" is a specifically defined term for Switchgear and Medium Voltage MCCs that requires certification, but not yet for LV MCCs (too much infighting still on design specs). So for the 3 MCC mfrs who offer Arc Resistant LV MCCs, we just use the MV MCC design and test criteria, ANSI C37.20 (IIRC).


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[123MB]

I see... We use these forms of segregation for other measures also, for instance, you know that if an MCC is constructed to form 3 then one starter can be isolated and worked on safely while the busbar and other starters are energised. If your board is form 4 then you can access outgoing terminals while the other starter outgoing terminals are energised.

However I'm sure you guys have your own means of controlling risk when working on live MCCs.