John Wiles & 600 VDC switches [RE-wrenches]

[Jeff Oldham]

I always thought that you needed a dedicated disco per inverter so I asked John Wiles about it as I could not find a code referance to support my misimformation. It turns out it is OK to have up to 3 SB's on a single 3 pole disco as stated by John Berdner. There does seem to be a possible disagreement about combining strings on a disco pole. It seems that Berdner has been very closely involved in this and is probably right about the double conductors/single lug combining. Most importantly, it seems that this applies to SB inverters ONLY at this time anyway.

John Wiles asked that I post his message for him:

Jeff:


Here is your answer and please post the following on the Wrenches Web site for me.


The PV Industry (at least one PV engineer got the call/letter) has been formally notified by Square D (in December) that the HU361-R disconnect and other similar Square D fused and unfused disconnects rated at 600 volts DC are not suitable for use on grounded PV arrays even when the poles are connected in series and switch only the positive conductor of the PV array.  The use of these disconnects in this manner violates the UL Listing on the product and therefore also violates the provisions of the NEC.  Both positive and negative conductors must be switched and the power system must be ungrounded to meet the listing requirements.


Siemens and GE "say" that they have appropriately rated disconnects (600 volt dc, grounded systems), but the tech people have been unable to provide substantiation.  Furthermore these switches appear to be tested to the same UL Standard that is used to test the square D disconnects and that standard uses ungrounded power supplies and ungrounded loads.


For the techies. Having the switch enclosure connected to one of the circuit conductors (as it must in a grounded system) may create additional electromagnetic/electrostatic static forces on the arc causing it to strike the grounded enclosure or move out of the normal arc chute charring the insulation.  If the  enclosure is grounded, but neither of the circuit conductors is grounded or connected to the enclosure, then there is no electromagnetic/electrostatic affinity between the arc and the enclosure.


The good news is that UL, Square D, and SMA have run special listing tests on the HU361-R disconnect and when used only with the SMA SunnyBoy Inverters, the special listing says OK for 600 volts per pole, grounded systems, one pole per string. This means that up to one string may be connected to each pole of the switch (up to three strings) and combined in one Sunny Boy inverter or that the separate strings can be connected to three separate inverters.   The lack of backfeeding from the Sunny Boy facilitated the testing for this special listing. Installers/systems Integrators should contact SMA/Square D Applications Engineering to obtain a copy of the Special Listing on the HU361-R.



Jeff Oldham


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