John Wiles & 600 VDC switches [RE-wrenches]

Tue Feb 11 14:58:59 PST 2003

Jeff:

My 2 wires per pole info came directly form the product engineer at
Square D.
In my experience the digest is frequently inaccurate when you get down
to the real nitty gritty. 
I don't know if the listing only applies to SB's but the test protocol
that we developed and UL approved did have some limitations on the ratio
between Voc and Vmp.
There were also some additional requirements concerning labeling.
Hopefully we can get some clarification from Square D in the next few
days.
No matter what I think it is still great news.

If you have any questions, or if I can be of any further assistance,
please do not hesitate to contact me.

Best Regards,

John Berdner

SMA America, Inc.
12438 Loma Rica Drive, Unit C
Grass Valley, CA  95945
530.273.4595 (voice)
530.274.7271 (fax) 

-----Original Message-----
From: Jeff Oldham [mailto:starpower4u at juno.com] 
Sent: Tuesday, February 11, 2003 11:27 AM
To: RE-wrenches at topica.com
Subject: John Wiles & 600 VDC switches [RE-wrenches]

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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