Series fuses/combiner boxes [RE-wrenches]

[John Berdner]

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Daryl / Wrenches:
 
UL considers Diodes (and other solid state devices such as Mosfets and
IGBT's) to be "inherently unreliable devices".
As such they can not be used to provide over current protection in
arrays.
The basic problem is that they can fail in different ways depending on
the type of failure.
A diode, for example, can fail shorted or open depending on the type of
failure.
If it failed shorted then you would lose the protection but the system
would still operate.
 
Best Regards,
 
John Berdner


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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