Parallel String Brain Teaser [RE-wrenches]

Bill Brooks billb at endecon.com
Fri May 17 21:31:33 PDT 2002


Drake,

Got me thinking for a few seconds, but after reading it again, I don't think
it means what you want it to mean. I'm not sure how being supplementary is
relevant to the fact that standard fuses cannot be run at more than 80% of
their rating continuously. We could get into a long discussion about the
rightness and wrongness of this, but field experience says that if you try
to use a fuse rated at 1.25 x Isc it will blow occasionally, if you use 1.56
you almost never have a problem. Might be dumb luck, or it might be design.
Many folks historically just put the 60-amp breaker on the whole array and
called it good--does that make it right?

Calling something one source circuit under 50 volts is nomenclature and by
no means absolves us of the requirement to fuse modules.

Bill.


-----Original Message-----
From: Drake Chamberlin [mailto:solar at eagle-access.net]
Sent: Friday, May 17, 2002 3:32 PM
To: RE-wrenches at topica.com
Subject: Re: Parallel String Brain Teaser [RE-wrenches]


One thing to consider with series fuses is this interesting statement from
690-4 (C)

"Supplementary overcurrent devices used for the exclusive protection of the
photovoltaic modules are not considered as overcurrent devices for the
purpose of this section."

I think series fuses are therefore not required to use the same multipliers
as overcurrent devices classified as overcurrent devices.

Also, we can consider an arrangement of modules one source circuit from
690.4 (C), so long as the circuit is rated under 50 volts.

"sets of modules interconnected as systems rated at 50 volts or less, with
or without blocking diodes, and having a single overcurrent device shall be
considered as a single-source circuit.

Remember that series fuses are not considered as overcurrent devices.

You Said:

Why can't we use 9 amps
for the fuse size--because 2 (source circuits) times 6 amps (per circuit)
equals 12 amps which will blow the 9 amp fuse quite nicely on any sunny day.

Also, I am confused as to why we can't use 15 amps for the series fuse size
instead of 9.  I haven't been following this thread, so forgive me if it
has been covered.

Thanks,

Drake


At 12:59 PM 5/17/02 -0700, you wrote:
>Wrenches,
>
>Since we have been talking about parallel strings and max series fuse
>ratings--let's have a little more fun with this.
>
>The Siemens SP-75 has a max series fuse rating of 15 amps and a Isc of 4.75
>amps. The code in 690-8 says that the Isc of the series string will be 4.75
>x 1.25 or 6.0 amps.
>
>Question: What fuse size is allowable for two strings of modules in
>parallel?
>
>Answer: There Ain't None
>
>Why?: Total allowable input to a series string of modules is 15 amps. Take
>the output of one string plus the string fuse and you get 9 amps (fuse)
plus
>6 amps (parallel string of modules) equals 15 amps. Why can't we use 9 amps
>for the fuse size--because 2 (source circuits) times 6 amps (per circuit)
>equals 12 amps which will blow the 9 amp fuse quite nicely on any sunny
day.
>So who screwed up? I don't know, but some at Siemens claim that you can put
>two sets of modules to a 15-amp fuse and they are clearly not accounting
for
>the current from the other string of modules.
>
>Question: Is there a case where two sets of modules can be wired in
>parallel?
>
>Answer: YES, in fact you can do three in parallel!
>
>Why?: If the strings are going into a source that can provide no backfeed
>current (e.g. a SunnyBoy inverter that has been listed for zero backfeed
>current or a direct drive DC motor), we can parallel up to three sets of
>SP-75s in parallel since the maximum current is only 12 amps from the other
>two modules strings.
>
>
>
>See how much fun PV can be! Oh the wonderful things that Mr. PV can do.
>
>Bill.
>
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