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  • Load Disconnects 




    DC Disconnect

    The National Electric Code, section 690.15, requires PV arrays to have a disconnecting means to isolate the inverter from the PV power source. DC Disconnects have to be able to quickly and safely extinguish the DC arc caused by disconnecting an array under load. DC arc it is more thermal damaging as an AC based arc. Disconnect switches are available in fusible and non fusible models with 30 A, 60 A and 100 A ratings in indoor or outdoor enclosures. Those are used for residential and small commercial plants installations.

    Utility grid-tie inverters that utilize PV arrays with voltages above 250V DC require a 600V DC rated disconnect to perform this function. The most common disconnects used are Square-D switches on 600V DC with 30A, 60A, and 100A. 3-pole safety switches are rated by the factory to handle one 600V DC circuit per pole. The 30 A switch is rated for 15 A Isc for non-fused and 12.8 A Isc for fused. The 60 A switch is rated for 48 A Isc for non-fused and 38 A Isc for fused. The 100 A switch is rated for 80 A Isc for non-fused and 64 A Isc for fused. They can be used for disconnecting up to 3 PV arrays for 3 grid-tie inverters.

    Switch Nameplate 600 V

    Switch DC Rating per Pole 1

    Photovoltaic Maximum Circuit Current 2

    Photovoltaic Short-Circuit Current (lsc)

    30 A

    20 A

    16 A dc per pole

    12.8 A (20/1.56)

    60 A

    60 A

    48 A dc per pole

    38 A (60/1.56)

    100 A

    100 A

    80 A dc per pole

    64 A (100/1.56)


    For the DC disconnect the fault current becomes the max current the disconnect must break in order to clear the fault and separate the PV power source from the inverter and the AC utility grid circuit.

    three-strings-fault-protection.jpg


    If three PV source circuits are combined at a common point and a fault occurs in any of the strings, then the fault current from the PV modules on the faulted string at Point A is the maximum circuit current IMAX. In this case, however, the fault current traveling in reverse from the common connection point is two times the maximum circuit current plus the inverter back feed current—up to 15 A, at which point the fuse would open.

     If there were no fuses, the fault current could it will be bigger!

     IFAULT = (2 x IMAX) + IBACKFEED = (2 x 10.2 A) + 0 A = 20.4 A;
     As more strings are added, the difference between fused and unfused currents will increase;
     The fault current through the DC disconnect it is: IFAULT DISCONNECT = IBACKFEED

     [Tech Support]  Read from Knowledge Base and download a pdf file @ Article:  DC Discconect Short & Fault Current Evaluation 

    The information posted herein has been compiled by Clean Energy Brands from OEM product data and reputable publications. All rights reserved!



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