Series fuses/combiner boxes [RE-wrenches]

[Bill Brooks]

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Gary and Darryl,

Without going into a long and confusing explanation, any crystalline-silicon
module on the market today requires series fusing when 3 or more strings are
connected in parallel. Use the max series fuse rating on the back of the
module for simplicity and fuse longevity.

The assumption about 1 or 2 strings is that the inverter is listed to
provide no backfeed current. I believe UL 1741 is being adjusted to test for
this, but I have not seen much in the way of bulletproof documentation from
any manufacturer on this subject of no backfeed current. Maybe some of the
manufacturer reps can chime in here. When I say "bulletproof", I mean that
it is undeniably clear that UL has evaluated and tested this function of the
inverter so that the pickiest inspector is satisfied with the documentation.

Blocking diodes are not required by the code unless the manufacturer
requires their use with their modules. Blocking diodes can not be used in
lieu of fusing in PV arrays. They should not be used unless you know what
you are doing.

Bill.


-----Original Message-----
From: Darryl Thayer [mailto:daryl_solar at yahoo.com]
Sent: Sunday, October 31, 2004 5:38 AM
To: RE-wrenches at topica.com
Subject: Re: Series fuses/combiner boxes [RE-wrenches]

HI Gary
I went through th e very same question last summer.
For series strings, you can place in series as many
panels your charge control/inverter or module voltage
will allow.  This calculation is the Voc/module x
number of modules x 1.25 (code safety factor) x Temp
correction factor (the temp diff from 25C to the 99%
probability not this cold local temp=DT i.e 1+.05*DT)
This is the voltage calculation and it must be less
than 600 volts or the limit of the
controller/inverter.

For Parallel strings John Wiles has a white paper
which says the max current through the module from all
sources must not exceed the max series fuse limiting
that current.  That is the max of the Isc of all
parallel collectors or other equipment must not exceed
the max series fuse as labeled on the module.

As an example if you have two modules in parallel each
of 3 amps Isc, and the fuse protection is 2 X 3 x
1.25= 7.5 amps  And if one modules fails and the
second module supplies Isc the first  module could see
3 amps plus 7.5 amps reverse current through the fuse
or the failed module would have 10.5 amps.  To comply
with the code the module must have 10.5 max fuse size.


This is very limiting of course.  And I have wondered
the use here of reverse diodes.  Either at the breaker
or on each module.  The second solution is to fuse
each module at the junction box.

Now it is my turn.  What is the ruling on Blocking
Diodes?   What is the accepted fuse holder that can be
installed in module junction boxes?

Thanks Daryl

--- Gary Higbee <gary at windstreamsolar.com> wrote:
> Wrenches,
>
> I'd appreciate some help in determining when we need
> fuses or breakers for
> parallel strings. For some reason I thought that two
> strings in parallel are
> OK, but that for three and above we need to use the
> appropriate combiner box
> with breakers or fuses. This feels too
> vague--especially as I am now being
> asked to review and approve systems (for the Energy
> Trust of Oregon) with
> more than two parallel circuits.
>
> Please shed some photons on this--when do we not
> need and when do we need
> parallel string fuses or breakers, and where does
> this correspond to in the
> code--or make common sense?
>
> Thanks so much!
>
> Gary

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