Fried Combinor Box [RE-wrenches]

[jberdner at sma-america.com]

Don:

The problem may have been the undersized fuses or possibly high ambient
temperature.
When I ran the original UL tests on the PCB-10 (later changed to TCB-10 for
Trace)  it got VERY hot when we pushed the fuses beyond 80% (divide by
1.25)of their rating.
At 100% of rated current we discontinued the test when the holder
temperature got above 150 C and it was still rising fast.

This is one of the reasons for the 1.25 derate is in the Code for continuous
duty.
Just to be complete - The second 1.25 is for high irradiance (1.25*1.25 =
1.56).
You should always use a MINIMUM of the 1.56*Isc derate from STC when sizing
fuses.
The basic rule of thumb is use the biggest fuse you can get away with while
meeting wire ampacity and UL series fuse requirements for the module.

Fuses are thermal devices and, at the fundamental level, they work by
getting a wire hot enough to melt.
If you run above the 80% level then the whole box just sits there and cooks.

We ran the box with (10)10 Amp fuses at 8 amps per fuse until we reached
thermal stability.
This is where the 80 Amps continuous output rating comes from.
We then ran the tests again with (6) 15 Amp fuses in every other holder at
12 Amps (15/1.25).
As I recall, the hot spot temperatures were about the same for each test.
This should be outlined in the manual.

This testing was with an ambient room temperature of circa 25 C.
UL allows you to extrapolate the data from 25 C up to 40 C by simply adding
15 degrees to the test data.
As long as you are within the temperature rating of the materials in the box
after you add the 15 degrees you can use the 40 C ambient.
If you want an ambient temperature rating above 40 C then you have to run
the test at that temperature.
This adds significantly to the cost for UL Listing and is why we have a 40 C
rating.

If you are going to use the PCB-10 in higher ambient temperature then you
need to add additional derates.
The fuse manufacturer (Bussman in this case)publsihes some guidelines for
additional temperature derating for operation at higher temperatures.
Then there is the whole issue of solar gain contributing additional heating.
UL doesn't even have a procedure to deal with this yet.

While I know this doesn't help with the problem I hope it helps to
understand why it might have occurred.
If you want to discuss it some more please feel free to contact me directly.

Best regards,

John Berdner

SMA-America, Inc.
20830 Red Dog Road
Grass Valley, CA  95945
Tel: 530.273.4895
Fax: 530.274.7271












-----Original Message-----
From: Don Loweburg, Offline [mailto:i2p at aol.com]
Sent: Friday, March 23, 2001 12:11 PM
To: RE-wrenches at topica.com
Subject: Re: Fried Combinor Box [RE-wrenches]


In a message dated 03/23/2001 11:31:55 AM Pacific Standard Time,
mmangelsdorf at hei.com writes:



That installation had Solarex MSX120s, four to a string,
using slightly undersized 10a. fuses when they should have been 15a.  Bill,
chime on this one if you want.  Since the TCB10 is one of only a few


Marco, I was using 4 kc120 in series with a 15 fuse (according to
kyocera-slightly big-they want 11 a).  I feel like saving the box and giving
it to Bill for his collection of "things that suck".

Don
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To check out the other RE-Wrench participants:
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Hosted by Home Power magazine: 
 www.homepower.com

For info contact list moderator by email:
 michael.welch at homepower.com

____________________________________________________________
T O P I C A  -- Learn More. Surf Less. 
Newsletters, Tips and Discussions on Topics You Choose.
http://www.topica.com/partner/tag01