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101 renewable - dc disconnect short and fault current evaluation

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Last Updated
1st of October, 2018

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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 infusible and nonfusible 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 to 15 A, at which point the fuse would open.

If there were no fuses, the fault current could be larger still:~

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


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