More String fusing: Fronius 5100 [RE-wrenches]

Wed Aug 3 18:06:34 PDT 2005

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William / Phil/ Wrenches:

I believe the inverter must be considered to determine if fuses are
needed or not.

With an Isc of 3.75 and a UL series fuse rating of 15 Amps on the
module we have to meet 2 different criteria.

First : The UL series fuse rating that says we must protect the module
from backfeed currents greater than 15 Amps.

As an aside - UL guy once told me it is helpful to think of a string of
modules as a piece of wire with an ampacity equal to the UL series fuse
rating when thinking about where the fault currents flow and how big
they are.
I have found this to be a very useful way of thinking about it.
So, pretend the faulted string is a piece of wire that must be
protected for fault currents greater than 15 Amps.
The fault current can come from other strings of modules, batteries, or
back feed from the inverter. 
The mythical wire itself does not generate fault currents.

With an Isc of 3.75 at first glance it looks like 4 parallel strings
would be ok.
The faulted string is wired in parallel with 3 other strings each with
an Isc of 3.75 Amps 

- Article 690.8 (A) (1) says the maximum current shall be the sum of
the parallel module rated Isc multiplied by 125%

- Article 690.8 (B) (1) says the over current devices have to be sized
at 125% of the currents computed in 690.8 (A)(1)
There is an exception the above but it requires that the over current
device must be listed for continuous duty BUT fuses are not continuous
devices.

With and Isc of 3.75 the maximum current per parallel string, according
to 690.8 (A)(1), is 3.75 * 1.25 = 4.68 per parallel string.
If the array was configured with 4 strings only three strings an
contribute fault current to a fault in one of the strings.
The faulted string does not contribute fault current to itself.
Therefore, In the more general case of N strings the fault current is
equal to (N-1)*1.25*Isc

This leads to the 2 exceptions in Article 690.9 (A) 
- Exception (a) says there are no external sources such as parallel
connected source circuits, batteries, or backfeed from the inverter.
We do have parallel strings so we can not use exception (a).

- Exception (b) says the short circuit current from all sources does
not exceed the ampacity of the conductors.
The FPN following these 2 exceptions specifically includes a reference
to supply through an inverter into the PV output circuit. 

Assuming the inverter will not produce any backfeed current >> may not
<< be a valid assumption for all inverters. 
Check with the manufacturer and ask how they verified this.
If the inverter can contribute fault current then fuses would
definitely be required and likely regardless of the number of parallel
strings.

Although we can show this all should be ok without fuses the AHJ may
not buy off on it anyway.
The UL Listing of the module is probably worded such that it implies
that UL series fuses are required on every series string.
We have had some inspectors insist on fuses regardless of the
calculations shown or valid arguments made.

Saga part 2...

So,  we put fuses in the system and now the question becomes how many
strings os 190's per fuse.

Article 690.8 (B) (1) says the fuse must be sized for a minimum of 1.25
* 4.68 = 5.86 Amps per string.(1.25*1.25*Isc)
This says we could use either a 12 Amp fuse or a 15 Amp fuse (the
maximum allowed per the UL fuse rating)
In either case, we would be limited, by Code, to a maximum 2 strings of
Sanyo 190's per fuse.
For 4 strings, this means 2 parallel strings per 12 or 15  amp fuse and
2 12 or 15 amp fuses in parallel to the inverter.
Assuming the inverter does not backfeed the array fault everything is
still ok with 4 strings.

If, however, the array has more than 4 strings we have a problem
again.
Consider an array with 6 strings configured as 2 parallel strings each
feeding a 12 or 15 amp fuse and 3 12 or 15 Amp fuses in parallel to the
inverter.
With a fault in string 1 the fault current is 4.68 Amps from String 2
(the one in parallel with string 1 on the array side of the fuse) PLUS
the rating of the fuse.
Article 690.8 (B) (1) says the minimum allowable fuse size if 12 Amps
so the total available fault current is 4.68 + 12 = 16.68 which is
larger than the UL series fuse rating of the module. 

Essentially an array with more than 4 strings of Sanyo 190's requires a
fuse on every string no matter what.
In my experience fuses will be required in most arrays of more than 3
strings no matter what.

Sorry about the length of the explanation I hope it helps add some
clarity to a confusing issue.

Best Regards,

John Berdner

>>>

William, 

The driving factors in series fuse/no series fuse is the number of
strings
and module choice, not inverter.

For instance if using Sanyo 190 modules, which have a 3.75A Isc output
and
15 amp series fuse rating, you can run 4 strings of modules without
requiring series fusing. However, use BP 3125S modules (7.54A Isc and
15A
fuse rating) and you can only run 2 strings before triggering the
series
fuse requirement.

This is allowed by NEC 690.9(A), Exception B.

John Wiles wrote a short paper on PV modules and series overcurrent
devices
that explains more, although it can make your eyes cross. Anyone that
wants
it, request off list and I'd be glad to send a copy. Also, Brian Farhi
at
Fronius wrote a very handy fuse sizing Excel reference tool that I use
almost daily. 

Phil Undercuffler
Technical Support, ext. 238
p.undercuffler at conergy.us 

Conergy Inc.

888-396-6611 toll free
505-473-3800 phone
505-473-3830 fax
www.conergy.us 

-----Original Message-----
From: William Miller [mailto:wrmiller at slonet.org] 
Sent: Wednesday, August 03, 2005 2:21 AM
To: RE-wrenches at topica.com 
Subject: More String fusing: Fronius 5100 [RE-wrenches]

Friends:

New project: Bidding on a Fronius 5100 install. It says right in the
manual that you can connect up to 3 strings to the 5100 (and other
models,
and up to 4 strings in even other models). I see no fuses in the
pictures.
Isn't this an NEC violation?????

William Miller

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