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Thread: Knife switch disconnect for high voltage DC?

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  1. #1
    Join Date
    Mar 2016
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    Washington State
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    Knife switch disconnect for high voltage DC?

    Hi All,

    We just installed a 3-pull, knife switch with handle style disco for a PV array. Its rated AC/DC and to 600 volt. The PV array is something like 450 VDC.

    My question is......how do these not arc when actuated under load? Fuses holders, MC4 connecters when disconnected under load will arc at that voltage, so why won't these do the same? Or maybe they do arc but it is considered safe?

    Just want to make sure this is an appropriate use for this, and that it is in fact safe.

    thanks

    Brett

  2. #2
    Join Date
    Dec 2012
    Location
    Placerville, CA, USA
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    17,991
    The switch is rated for 600V, but you have not specified whether or not it is rated for load interruption, or at what current.
    It may or may not actually be able to properly quench the DC arc caused when the contacts open. And it may or may not be damaged in the process. Not all "disconnects" are rated for load braking. Do you see arc chutes or other provisions around the switch contacts?
    If it is an unenclosed switch the arc may be a hazard to you as you open it.

    If you have a manufacturer and part number we can be more specific.

  3. #3
    Join Date
    Mar 2016
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    Quote Originally Posted by GoldDigger View Post
    The switch is rated for 600V, but you have not specified whether or not it is rated for load interruption, or at what current.
    It may or may not actually be able to properly quench the DC arc caused when the contacts open. And it may or may not be damaged in the process. Not all "disconnects" are rated for load braking. Do you see arc chutes or other provisions around the switch contacts?
    If it is an unenclosed switch the arc may be a hazard to you as you open it.

    If you have a manufacturer and part number we can be more specific.

    SQUARED HU361RB 600V DC 30 AMP 3-POLE DISCONNECT SWITCH

    This is the part number. Thanks.

  4. #4
    Join Date
    Dec 2012
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    Placerville, CA, USA
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    Quote Originally Posted by bbartmasse r View Post
    SQUARED HU361RB 600V DC 30 AMP 3-POLE DISCONNECT SWITCH

    This is the part number. Thanks.
    Looks like it is rated for load breaking specifically in current limited PV applications.
    https://d1819pwkf4ncw.cloudfront.net...tems-65277.pdf
    But note carefully that to handle 600V (or more than 300?) you must wire two poles of the switch in series with each panel circuit, not just one.

    The switch contacts open far enough fast enough to extinguish the arc without damage. But it is possible that at 600V a single contact does not open far enough to quench an arc.

  5. #5
    Join Date
    May 2011
    Posts
    3,704
    And with those switches you have to use the outside poles and not the middle, presumeably because the arc might jump to the middle contact if you don't.

    Interestingly, if you are disconnecting both the positive and negative of the same circuit, that's considered series and you can just use one outside pole for each conductor. But if you are only disconnecting one conductor in a grounded circuit, you need to jumper from the line side of pole1 to the load side of pole 3 (or vice versa) to use the switch properly.

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