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Thread: SCCR Ratings VFD

  1. #1
    Join Date
    Aug 2004

    SCCR Ratings VFD

    I am an Industrial Controls Engineer and I am up to my neck in a project where I am being forced to use circuit breakers per a customer requirement which is a pain when trying to maintain high SCCR rating. My issue is that I have to maintain a min 40kA SCCR for my panel. I have a 480V 3ph VFD that is rated for 100kA SCCR which is great, no problem. Connected to the line terminals, R S and T I have a 65kA rated circuit breaker contactor combination for power control and overcurrent protection, again no problems there. DIRECTLY after the VFD connected to terminals U V and W (motor terminals) I have some safety contactors that are only rated 5Ka. These contactors have to remain in that location due to my category 3 safety circuit requirements. The question is, does the VFD act like a current limiting device since the connections between the line terminals and the motor terminals passes through electronic circuitry (rectifiers etc) and there is no real direct connection? Or am I in trouble here, and my "weakest link" now becomes the safety contactors after the drive at 5kA SCCR??

  2. #2
    Join Date
    Jan 2007
    Columbus, Oh
    I have a 480V 3ph VFD that is rated for 100kA SCCR which is great
    Better check with the manufacturer about that 100KA rating. It usually is with some form of current limitation ahead of it, for a combinational rating and not a stand alone.
    The VFD does not offer any type of current limitation, per UL508A, only a fuse or current limiting circuit breaker can do that.
    What amperage is the safety contact. Put a Class CC fuse in front of it, to use table SB4.2 as a combinational rating up to 200KA.
    Just my $.02
    I found my easter eggs & my car keys, life is good!

  3. #3
    Join Date
    Feb 2003
    While the VFD does not act as a current limiter it does act like a new power supply. Ask the VFD manufacturer for the available output short circuit current.
    Just because you can, doesn't mean you should.

  4. #4
    Join Date
    Feb 2003
    New York, 40.7514,-73.9925
    Does the VFD have a bypass that goes "around" the inverter output?
    That might put you back into the requirements of fully rating the safety contactors.

  5. #5
    Join Date
    Jul 2006
    San Francisco Bay Area, CA, USA
    Wow, when you think about it, this is a very interesting and unique issue as it pertains to the SCCR requirements. IS the VFD a "new" power source in this situation? I think it is, because a short circuit on the output side of the VFD cannot exceed the power rating of the VFD, it will current limit into the short, or the transistors will fry.

    But if they fry, then what is the failure mode? If they open, no problem but if they short, then the available fault current becomes DC at the capacity of the DC bus, not AC. So what is that current level then?

    Now what happens if the fault then also shorts the rectifier, in which case now the VFD is no longer the power source at all, it is potentially just a conductor!

    But I am not an AHJ. This is one that might need official clarification.

  6. #6
    Join Date
    Mar 2004

    There is a lot of info on the Bussman Fuse web site on how to increase

    the SCCR of assemblies.
    Frank Arizona,USA

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