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'[EE] Short circuited solar panel - fire hazard?'
2008\01\17@230327 by

PICers,

Is there a fire hazard when a bank of solar cells is short circuited?

I had to fix the cable on a solar panel battery-charger yesterday - the unit
was approx 30cm x 12cm with o/p volts ~20V in direct sunlight. The solar cells
sit inside a  plastic case. The o/p cable had been damaged leaving the copper
strands exposed and shorting together.

When I got home I got to wondering about how much internal heat a
short-circuited solar panel might generate. So I set up 3 solar cells in series
to give ~5.7V o/p when open circuited (direct sunlight). The short circuit
current for this test setup seems to be about 1A.

It's a hazy day in Melbourne so I couldn't get the voltages to sit still but,
just for general interest, these are the figs. Yeah, I know, they're all over
the place - will try again on a hot sunny day.

v     i
5.7V  0    (o/c)
4.3V  0.19A
3.6V  0.24A
3.2V  0.32A
1.4V  0.30A
0.5V  0.20A
0.12V 0.55A
0     1.2 A  (s/c)

There was no fuse on the solar panel in question. The unit was situated outside
and it's summer in Australia, right now. Say 40+ degC. Let's say when the
output is short circuited, it's pulling >1A. Which means you've got that amount
of ohmic heating on top of the heat input from the sun.

QUESTION: Could a fire start? Could the plastic case get hot enough to melt and
catch fire? Or maybe the sheath on the cable could melt? ie how hazardous is a
short-circuited solar panel (no fuse!) in an outdoor situation, bearing in mind
bush-fire danger?

Thanks for any thoughts/insights.

Debbie :)

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Debbie,
As I'm sure you realise from your measurements, the worst case is
where you don't quite have a short circuit & the resistive losses
(possibly concentrated in a small area) matches the output impedance
for the panel. (i.e maximum power transfer condition).

Then , if the temperature rises above the appropriate ignition
temperature for  the the materials exposed, you will get a fire.

I'd guess that even for a small panel like you were testing, a fire
would be possible under worst case conditions. For larger panels there
is certainly a risk unless  flammable materials are avoided.

A fuse would be difficult to use as the worst case situation is just
getting the optimal output from the panel - which is what would happen
if any sort of optomiser was used.

RP

On 18/01/2008, Debbie <cyberia429-piclistyahoo.com.au> wrote:
{Quote hidden}

> -
> Is there a fire hazard when a bank of solar cells is short circuited?

Still trying to destroy those CDs eh ? What does an amp do to the
foil on a CD ?

On 18/01/2008, Jinx <joecolquittclear.net.nz> wrote:

> Still trying to destroy those CDs eh ? What does an amp do to the
> foil on a CD ?

Interesting. This calls for a test in the lab!

--
- Rikard - http://bos.hack.org/cv/
Den 2008-01-18 10:09:20 skrev Rikard Bosnjakovic <rikard.bosnjakovicgmail.com>:

> On 18/01/2008, Jinx <joecolquittclear.net.nz> wrote:
>
>> Still trying to destroy those CDs eh ? What does an amp do to the
>> foil on a CD ?
>
> Interesting. This calls for a test in the lab!
>

I would try a microwave oven...

--
Morgan Olsson
On Jan 17, 2008 11:03 PM, Debbie <cyberia429-piclistyahoo.com.au> wrote:
> Is there a fire hazard when a bank of solar cells is short circuited?

Yes.

> v     i                 W
> 5.7V  0    (o/c)   0
> 4.3V  0.19A       0.8
> 3.6V  0.24A       0.9
> 3.2V  0.32A       1.0
> 1.4V  0.30A       0.4
> 0.5V  0.20A       0.1
> 0.12V 0.55A      0.07
> 0     1.2 A  (s/c)  0

The panel is probably rated higher than 1W, so you can use the panel's
maximum output for further worst case scenarios, but for this I'll
assume it'll never go above 1W.

> QUESTION: Could a fire start? Could the plastic case get hot enough to melt and
> catch fire? Or maybe the sheath on the cable could melt? ie how hazardous is a
> short-circuited solar panel (no fuse!) in an outdoor situation, bearing in mind
> bush-fire danger?

There is practically only one way to start a fire - you reach the
ignition temperature of an available fuel source.

The arcing wires at even low power will start a fire with the correct
fuel, and require almost no power the generate the arcs.  Hopefully
you don't have to worry about fuel vapors in the area, but if you do
I'd reconsider placing anything electric in the area that is not
intrinsically safe.  Arcing rarely causes fires with solid fuels, as
the arc would have to touch the fuel, and the solid fuel is probably
insulative enough that it would stop the arc.  In this case you
probably don't need to worry about arcing causing the fire unless the
heated plastic vaporizes into a gaseous fuel (not likely).

For other fuel loads where sparking is not an issue you need to worry

1W of heat is very little, all things considered, but in a proper
insulated environment it is possible to reach thousands of degrees
with only one watt of power input.

So you need to determine how fast the heat leaves the area.  If the
wires are enclosed then the heat will go up a bit depending on how
insulated they are from ambient.  If they are in the open then they
will rarely get warmer than a hot 1W resistor in the open - too hot to
keep your fingers on, but certainly nowhere near ignition temperature.
If the wires are running through paper fiber insulation in the attic,
then the risk is greater (especially since the dust could be hit by an
arc).

All of this boils down to:

Given the low power panels, and typical installation, it's unlikely to
cause a fire.

If you set up a set of panels that output hundreds or thousands of
watts, then there is a very real possibility of fire (just as with
household wiring) if the wires are not adequately fused, protected and
terminated.

If you are worried, you might consider putting a fuse in line, but
since the panel is a power source, and the wiring leads away from it,
you'd need to put the fuse next to the panel the relieve the problems
you're encountering.

--
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - - - - - - - - - - -
Moving in southeast Michigan? Buy my house: http://ubasics.com/house/

Interested in electronics? Check out the projects at http://ubasics.com

Building your own house? Check out http://ubasics.com/home/

--- "M. Adam Davis" <stienmangmail.com> wrote:

{Quote hidden}

Thanks Adam & Rich - worst fears confirmed, eh?

Those figures I gave were for my own xptal set up, with a compeletely
different, unrelated set of cells: I just wanted to get a handle on what the
issues are. I make the o/p current for my test setup to be, very roughly ...

i = 1.2 - 0.21v
Leading to Power = vi = 1.2v - 0.21v^2 and Pmax = 1.7W when output resistance =
load resistance, just as Richard reminds us.

The actual solar panel out in the field I was not able to test but it would
certainly be greater than 1.7W capacity.

The problem is that the installlation is outdoors and the output cable drapes
across dry leaf litter on the ground. The problem came about because rats
gnawed thru the plastic sheath. So, yep, there is a tinder-dry fuel source in
close proximity on a hot day. This is starting to look very bad.

Ideas?

Looks to me like the cable better go in some kind of conduit, at the very
least. A fuse? Well, the current at Pmax is less than short-circuit current, so
a fuse should be able to protect against that without messing up normal power
delivery. The big danger, as Rich points out, is what happens if you get a
partial short thru a few strands of wire - the temp would rise above copper's
melting point and then you definitely would get melting/arcing/ignition.

How to make it safe?

How about if I tell them to strap the solar panel onto the battery so there's
only a short section of cable from panel to battery, & maybe enclose the whole
thing in some kind of container? Then run the battery's cable to the
installation - th battery (load) cable is fused anyway, it's only the solar
panel's one that is not. That way, you'd keep the solar panel + battery
together as a single integrated unit, instead of having the installation +
battery lumped together and the solar panel as a remote add-on, like it is now.

The installation is a security video camera + RF transmitter for an alarm. They
have problems with illegal shooters on the property. The camera tapes car
registration plates as they go thru a back gate. Hence the need for a high
current capacity supply - the video cam draws a pretty high current.

Good points, guys.

Debbie  :)

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--- Jinx <joecolquittclear.net.nz> wrote:

> > Is there a fire hazard when a bank of solar cells is short circuited?
>
> Still trying to destroy those CDs eh ?

Nah, they've been and gone. Trouble with electronics is, as soon as anyone
finds out you can join two wires together, they trot out stupid odd jobs,
generally of the sort that have nothing to do with what you're supposed to be
doing. Like your time's not worth a cracker but theirs is >:( Usually it's
their clapped out CD players and TV sets. At least this job was out in the
country and more or less on target, electronics-wise.

Debbie

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I'd put it in a metal conduit (flexible or rigid - doesn't matter) and
be done with it.  A fuse won't stop a fire from starting, it simply
protects an undamaged wire from heating up if too much current is
drawn.

The current is already limited by the solar panel - it can't put out
more than its rated wattage.  The battery, however, can put out a
great deal more power.  You might consider keeping the solar on the
long wire rather than the battery and solar.

A plastic conduit might keep the vermin out, but a metal conduit will
last much longer, and even if it tasted as good as the plastic it
would still take some time to eat through.

On 1/18/08, Debbie <cyberia429-piclistyahoo.com.au> wrote:
{Quote hidden}

> -
> Trouble with electronics is, as soon as anyone finds out you can
> join two wires together, they trot out stupid odd jobs

Since waaaaay before the beginning of December I've had probably
one day that I could truly call my own. Even Christmas Day I ended
up at relatives, fixing their computers and deciphering gadget manuals
for them

An enquiring and (hopefully) expanding mind is a real burden sometimes

> there is a tinder-dry fuel source in close proximity on a hot
> day. This is starting to look very bad.
>
> Ideas?
>
> Looks to me like the cable better go in some kind of conduit

Grounded metal might be best

What is it with rodents and cables ?

--- Jinx <joecolquittclear.net.nz> wrote:
> > Looks to me like the cable better go in some kind of conduit

> Grounded metal might be best
>
> What is it with rodents and cables ?

I heard someplace that parrots are just as big a pain. They chomp thru the
cables on u-wave telemetry dishes, apparently.

> > Trouble with electronics is, as soon as anyone finds out you can
> > join two wires together, they trot out stupid odd jobs
>
> Since waaaaay before the beginning of December I've had probably
> one day that I could truly call my own. Even Christmas Day I ended
> up at relatives, fixing their computers and deciphering gadget manuals
> for them

I got smart: I treat my electronics knowledge like a "state secret" ;) What
little knowledge I have, anyway :)

Debbie :)

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> > What is it with rodents and cables ?
>
> I heard someplace that parrots are just as big a pain. They chomp
> thru the cables on u-wave telemetry dishes, apparently

NZ keas (alpine parrots) are very inquisitive and playful. They'll pull
anything apart. Quite happily strip all the rubber seals and detailing
off a car just for laughs. Which is so gosh-darned cute if it's not your
vehicle

www.newzealandatoz.com/index.php?pageid=163&A+Kea+Story&PHPSESSID=864
9d3e156061610383feee504d9b01e

Also they're the bette noir of any cabling in the area too of course

Can't say I've ever had to do a repair due to that sort of damage
or infestation

> What is it with rodents and cables ?
>
>

It has been well-known that some rats and other members
of that family are partial to certain plastics, and like to  gnaw
on them.

The solution is a bitter chemical that when embedded in the plastic,
makes the rats stop chewing. It seems to work well for a while, until
the chemical has dried up.

--Bob
On 19/01/2008, Bob Axtell <engineercotse.net> wrote:
>
> > What is it with rodents and cables ?
> >
> >
>
> It has been well-known that some rats and other members
> of that family are partial to certain plastics, and like to  gnaw
> on them.
>
> The solution is a bitter chemical that when embedded in the plastic,
> makes the rats stop chewing. It seems to work well for a while, until
> the chemical has dried up.
>
> --Bob
> --

I'm told that early trials of this method used tabasco sauce mixed
into the plastic!

It sounds to me like a metal conduit (earthed) would be the best
protection.  What about vandal (etc.) protection? For the planned use,
rodents might be the least of your worries. In that event you might
want to keep the batteries (& the camera / electronics) separate from
the panel so the system won't be disabled just when you need it most.
As noted above, a fuse at the panel won't protect much, but a fuse at
the battery end of things might be advisable to prevent a backfeed
problem. I guess you could add electronic protection to ensure that
the current leaving the panel actually matches that arriving at the
battery but it introdues a whole new level of complexity &
instrumentation.

RP

--- Richard Prosser <rhprossergmail.com> wrote:

> On 19/01/2008, Bob Axtell <engineercotse.net> wrote:

...
> > The solution is a bitter chemical that when embedded in the plastic,
> > makes the rats stop chewing. It seems to work well for a while, until
> > the chemical has dried up.

> I'm told that early trials of this method used tabasco sauce mixed
> into the plastic!

Errm .... would that be the spice model? :) :)

>
> It sounds to me like a metal conduit (earthed) would be the best
> protection.

I'm thinking that. Maybe a metal plate (earthed)on back of the (plastic) solar
panel to act as a heat sink. The current is self limiting from the solar cells,
so can't go as dangerously high as the Pb-Acid battery current, therefore with
a little bit of heat sinking there'd be no way it could get anywhere near
melting point.

? What about vandal (etc.) protection? For the planned use,
> rodents might be the least of your worries. In that event you might
> want to keep the batteries (& the camera / electronics) separate from
> the panel so the system won't be disabled just when you need it most.
> As noted above, a fuse at the panel won't protect much, but a fuse at
> the battery end of things might be advisable to prevent a backfeed
> problem. I guess you could add electronic protection to ensure that
> the current leaving the panel actually matches that arriving at the
> battery but it introdues a whole new level of complexity &
> instrumentation.

Too complex imho. They have the camera fair distance back from the gate and the
vid cam has enough res to make out the reg numbers. They seem happy with its
situation so ... not my problem. :)

Thanks for the insights, guys

Debbie

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> Is there a fire hazard when a bank of solar cells is short
> circuited?

Notwithstanding all the good advice, probably not.

A solar panel is typically specified for Isc, Voc and Pmax.
Pmax is less than Voc x Isc BUT is surprisingly close -
usually well more than half the maximum values product.

So, the operating current will be a large percentage of the
be vastly overstressed by operating continually at Isc. As
the voltage drop in the cable at Imax_operating should be
low for efficiency reasons, even doubling current under
short circuit should produce only slightly warm wiring. If
your wiring gets hot on Isc then it gets warm on
Imax_operating and you are wasting substantial solar energy.

Russell

Debbie,

On Sat, 19 Jan 2008 08:59:00 +1100 (EST), Debbie wrote:

{Quote hidden}

Howcome nobody has thought of mixing in with the plastic used for the sheathing something that tastes awful (perhaps that stuff they paint on childrens' fingernails
to stop them biting them - Bitter Allows?) so that anything trying it decides it's not nice to eat after all?

Remember, you saw it here first!  (Hopefully that should stop anyone pinching my idea and trying to patent it! :-)

Cheers,

Howard Winter
St.Albans, England

Bob,

On Fri, 18 Jan 2008 16:21:04 -0700, Bob Axtell wrote:

{Quote hidden}

Dammit!  Another of my ideas that someone else has had before me...

Cheers,

Howard Winter
St.Albans, England

Jinx,

On Sat, 19 Jan 2008 09:58:09 +1300, Jinx wrote:

{Quote hidden}

I don't know, but it's a common problem.  When they changed the laws here about what you can do to the electrics in your own house without having to pay
someone who can prove that they know what they're doing, one of the things that you are allowed to do is replace a cable that has been damaged by "fire, impact
or rodent attack".  So if a burning rat runs into a cable, you're well covered!  :-)

I don't think they'd get through the flexible metal conduit, but that stuff isn't cheap, and as it's steel it won't last forever.

Cheers,

Howard Winter
St.Albans, England

Never short out a solar panel, you might make the sun go out!
:)
Howard Winter wrote:
> or rodent attack".  So if a burning rat runs into a cable, you're well covered!  :-)

Wasn't there a rock band called the "Burning Rat Cable Chewers"???
Ok, now I see. That's why we have that global warming thing! Too many shorted
solar panels. :-)

Global warming is really nothing to joke about - but I couldn't resist.

/Ruben

> Never short out a solar panel, you might make the sun go out!
> :)

==============================
Ruben Jönsson
AB Liros Electronic
Box 9124, 200 39 Malmö, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
rubenpp.sbbs.se
==============================
>>> What is it with rodents and cables ?

> Howcome nobody has thought of mixing in with the plastic used for the
> sheathing something that tastes awful (perhaps that stuff they paint
> on childrens' fingernails to stop them biting them - Bitter Allows?)
> so that anything trying it decides it's not nice to eat after all?

I seriously doubt that it would work.  Rats don't seem to have
any taste buds.

I had an antenna installation where rats chewed up some 9913 cable.
They ate (or chewed off) the outer PVC insulation, the copper braid
sheild, the mylar foil shield, and the nylon like dielectric.  The
_only_ part they didn't eat was the 0.11" copper center conductor.
Probably because they couldn't bite through it.

It was a digital link, so the throughput declined but the link was
up until they chewed through the last bit of ground connection...
then it dropped out entirely, all of a sudden.  Of course, the rats
chewed it up in a section that wasn't visible from the ground so it
took a couple days before I could find the source of the problem.

Solution, that has worked so far, is to install the new 9913 cable
partly through copper water pipe (1/2", I think; smallest diameter
through which the cable would fit).  Copper water pipe runs through
area where cable is hidden from view and extends about a foot past
the wall (up which rats can & will climb).  Makes cable inspection
much easier since rats can't eat the part that you can't easily see.

Lee Jones

>
> A solar panel is typically specified for Isc, Voc and Pmax.
> Pmax is less than Voc x Isc BUT is surprisingly close -
> usually well more than half the maximum values product.

That would make a fuse somewhat problematical. Not sure what the tolerances
are. Could a "1A" fuse blow at, say, 0.9A or 1.1A?

ie if Pmax = 0.7Psc, you might have problems finding a suitable fuse?

>
> So, the operating current will be a large percentage of the
> be vastly overstressed by operating continually at Isc. As

That's reassuring! :)

> the voltage drop in the cable at Imax_operating should be
> low for efficiency reasons, even doubling current under
> short circuit should produce only slightly warm wiring. If
> your wiring gets hot on Isc then it gets warm on
> Imax_operating and you are wasting substantial solar energy.

One of the guys suggested that a partial short, ie where the cable has been
chomped down to a couple of strands of Cu wire could be a hazard when a short
cct current melts the thinned-out wires at that point. You could get tiny blobs
of molten copper splattering a short distance in that case. If the cable runs
thru dry leaf litter/dry grass, that would be bad.

I was mainly wondering if it might be possible for temperature in the solar
panel itself to rise high enough to melt the panel's plastic case. ie Could you
get melted plastic dripping out onto a fuel source? Could plastic inside the
panel get to a high enough temperature to catch fire? Remember the panel is
outdoors in summer at high temperature anyway. Say 40 deg celsius. It's
coloured black and, on short circuit, you'd have the extra resistive heating
thrown in.

But then, considering your point above (Pmax ~ Psc), that means the panel
*should* be rated to handle a long term short cct load @ hight ambient
temperature without self destructing? One hopes.

The scenario is all somewhat unlikely, for sure. But on the other hand, you're
looking at the risk of starting a \$\$\$\$million(s) bushfire. You'd wanta be 100%
sure .....

Debbie

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