Multi-Contact parallel connectors [RE-wrenches]

[Bill Brooks]

Ray,

The need for fusing is very real. Just because some combiner boxes have had
problems in the past does not mean that you can justify not following the
code and the UL listing of the module. We ARE talking about reality here. I
know there is a tremendous amount of experience on the wrenches list at 12
and 24 volts, but you need to trust people who have been working with 48 and
400 volt systems for just as long who have seen the problems that happen
under fault conditions.

It is very possible to drive current back through a PV module. That is why
UL tests modules with a series fuse connected to it driving current in the
reverse direction--this is a common case with a wiring fault.

Bill.

-----Original Message-----
From: Ray Walters [mailto:remotech at taosnm.com]
Sent: Thursday, May 23, 2002 9:58 AM
To: RE-wrenches at topica.com
Subject: RE: Multi-Contact parallel connectors [RE-wrenches]



>>Hi Bill;
>>
>>The Bp2150 is a 24 vdc nom. module, so 2 modules in series make 48
vdcnom..
>>Short Circuit current for the series string is 4.75 amps. I parallel 2
>>strings for 9.5 amps. With the NEC 1.56 multiplier that comes to just over
>>15 amps, which is the series string fuse rating of the modules and the MC
>>connectors.
>
>Ray
>
>It seems that you are not including the back feed from the parallel
>string in the current you will see in the fault string.  Check Bill's
>origninal posting
>Hugh
>
Hi All;

I am aware of the theoretical possibility  of the batteries backfeeding and
the other string adding up to more than 15 amps.(Ithink the worst case of a
triple fault condition allowing 19.75 amps to backfeed some 12 awg wire and
array for a few minutes until the breaker removed 15 of those amps is not a
danger)
 Has this really ever even happened? How would the SolarBoost 50 let
current backfeed through it, while the sun was out and the adjacent string
was putting out power?  Would the other subarrays backfeed this fault?
Don't modules have enough resistance(as Steve just pointed out)that they
would never develop high current backfeeds at any of the available fault
voltages? (array open circuit voltage < 100vdc, inverter AC voltage = 120
v) Am I to trade a method of wiring I have never seen or heard of having
serious problems, for a method documented on this list (and found in
personal experience) to have ended up in smoke and flame? I'm reffering to
the infamous fused combiner boxes.
Most of the protection devices and wiring methods in the NEC came about
because houses where burning down, kids were getting killed: there were
real problems that were being addressed. Are we talking about reality here?
Instead, my safety concerns lie in lightning damage and direct shorts at
the battery bank. These are real dangers that really happen: inverters
catching on fire from lightning surges, batteries blowing up and spilling
sulfuric acid while the terminals turn to molten lead that melts through
the plastic and drops into the acid below. I am an engineer, I rate all the
safety issues and allocate budget and resources according to the threat.I
am totally committed to safety, but if the system cost is too high, my
customers will go to an inherently more dangerous system: the grid.

We have two responsibilities as Wrenches:
1. Maintain the reputation of realibility and safety that solar has earned.
2. Make solar more accessible to more people.

Ray

PS. I love this list.

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