LSIG CB supplying VFD 1

[trat1208]

Hi all,

Probably a stupid question, but . . .

I'm spec'ing out a 480V MCC lineup to replace an existing one that is at end-of-life. Getting a Rockwell Centerline MCC with all the bells and whistles, E300 overloads etc. Going thru the approval dwgs now.

3000A Main CB has an LSIG trip unit. Majority of loads on MCC are small motors, 30hp and below (there's a lot of them), whose MCP's should trip on instantaneous prior to causing an issue coordinating with main GF settings. There's a couple feeders to 480V panels with larger CB's - on these I asked for LSIG CB's as well so their GF protection will coordinate with the main CB. The only large feeder CB left on my MCC that would cause a potential issue with main GF coordination is a 250A thermal-mag CB feeding a VFD bucket for a 100hp motor. I'm guessing(?) this was an omission on their part because I had it spec'd as needing a LSIG CB on the proposal.

Just curious . . . Is there a problem with having an LSIG CB on a VFD motor feeder? Will the GF protection be prone to nuisance tripping? Would suppose to set at generic guideline of 20% of CB rating for pickup and 0.2sec delay, which will coordinate with my main CB GF settings, can bump up the pickup if needed. Just wondering if the calc done by the CB trip unit for GF current (vector sum of IA, IB, IC) gets fooled by non-linear loads in some way and makes it prone to misops. Think this is a non-issue, but just wanted to get input because I couldn't find anything written on the subject.

Also, bonus question . . . NEC says that where GF protection is required for CB's greater than 150V ph-G and no more than 600V ph-ph and >1000A it must pickup at max of 1200A and trip in max of 1sec for a 3000A GF. Why is it that majority of trip units I have come across with GF protection have a max pickup setting of 1200A and 0.5sec delay (AC PRO, I think all Eaton 480V power CB's, feel like this applies to other CB's out there as well)? These settings limitations seem to be more stringent that the NEC requirement, wondering why manf's don't just build their product to meet the code requirement to allow for cases where people might need higher settings upstream for coordination purposes.

Thanks

 

[waross]

Not what you are looking for but this may help.
EATON White Paper on LSIG CBs with UPSs
Does your VFD regenerate? That may be an issue.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jraef]

In an MCC the VFD unit is UL listed with a specific CB, they really can’t change that without voiding the UL listing of that unit, then if even one unit in a lineup (from the factory) is not listed, the entire MCC is not listed.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[trat1208]

Sorry for not checking back sooner, been on the road lately and my thoughts have been elsewhere . . .

waross - Yeah, I had come across that Eaton UPS paper previously. Read through it more closely and didn't see the applicability to my question, kind of just mentioned briefly the need to coordinate GF prot and then went through quirks that might cause false trips of LSIG's related to UPS operation and maintenance switching. No, my VFD will not be a regenerative type.

jraef - I'm inferring from your response that there's nothing inherently problematic or prone to nuisance tripping with putting a LSIG CB on a feeder to VFD? Totally understand the possible listing issue for the manf when swapping in components that aren't part of their standard MCC bucket offering.

 

[jraef]

Personally, I see no point in having an LSIG breaker feeding a VFD. The VFD will protect the motor circuit from faults, including ground faults. The LSIG breaker will not even see any faults on the load side of the drive. So the only thing the breaker is doing is protecting the circuit feeding the line side of the VFD. There is not much to go wrong there that would warrant the added cost if the LSIG type trip over a standard Thermal-Mag trip.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden