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70 Posts
Discussion Starter #1
Hello all,



I've searched the archive for this topic but didn't find any that really
answered my question.



I'm about to wire up the engine compartment of my 914 and am acquiring the
odds and ends for it, one item being the inter-pack fuse. From what I've
read, I should use a fast-acting type fuse such as a Ferraz Shawmut
A30QA500-4 which is 300V 500A. These are fairly expensive at ~$50 per fuse.




I've read a little about fuse sizing but still it seems like an art form.
The fuse should carry the rated current for indefinite periods without
opening and should open at the interrupt rating. Is the short-circuit
current of a battery the max discharge current, and if so, the Thundersky
spec for its 200AH says it is <= 20CA or 4000A. If the short-circuit
current, say 4000A, is less than the fuse's interrupt rating, say 100KA for
the A30QS500, will the fuse open up? I'm just trying to figure out if I can
go with a lower rated voltage like 150V which is closer to my 120V and a
lower rated current like 400A. These are still Ferraz Shawmut fuses and a
A15QS400-4 goes (150V 400A) for about ~$30.



Both of the above fuses are rated AC and DC.



Here's the spec url for the 150V/400A fuse:
http://www.discountfuse.com/v/vspfiles/downloadables/Ferraz/PDF/a15qs.pdf

And the spec url for the 300V/500A fuse:
http://www.discountfuse.com/v/vspfiles/downloadables/Ferraz/PDF/a30qs.pdf



I don't mind spending the $$$ for the correct overcurrent device but want
to make sure I purchase what I need. Any advice you can give on this topic
is, as usual, much appreciated.



Thanks,

Hart Penn

http://evalbum.com/3402

http://hartpenn.blogspot.com









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70 Posts
Discussion Starter #2
Hi Hart,
In the teaching I have done in Industrial Electricity we include the
general determination of fuse ratings for a definable load. After
determining the design point for maximum current and voltage, We specify
wiring or conductor sizing and Insulation ratings. The fuses are chosen to
protect the wiring. You need to state max Current at *Normal* operation, not
at short circuit or malfunctioning current level, that is what the fuse is
protecting our wiring from experiencing. so if we are going to draw a max
amps of 300 in operation, we might fuse at 150% of that or 450 Amps, so that
a momentary load surge i.e. at starting out from a traffic light heading
uphill,the fuse does not blow. If we reduce current in a fraction of a
minute everything operates safely and the fuse "Holds" However a
catastrophic failure resulting in a "Short Circuit' draws maximum possible
battery current (4,000 Amps.??)and the fuse "blows" in less than a second
before the wires melt down. Also the fuse should be rated for about twice
the normally sourced voltage , in an EV situation that is the "Traction Pack
total fully charged voltage times two as a DC rating" (Even if the fuse is
located in the center of the pack and so it seems to only have only1/2 the
voltage, that is an error in semantics. the full voltage is applied in case
of a "short".) so we don't have a plasma arc form and maintain within the
fuse or in the fuse-holder. As that might set fire to your EV. Yes you can
use a somewhat higher voltage rating. (In low voltage situations like 12
volts, 32 volt fuses are used, not 440 volt fuses because they might drop a
volt or so in internal resistance especially if rated for a fraction of an
amp.) Keeping near specified calculated values at current at 150% and
voltage at 200% usually work well and note that most fuses (And circuit
breakers, Relays, and Contactors) have different ratings for DC versus AC
and somewhat different contact construction also. so a contactor rated at AC
100 [email protected] DC at 25 [email protected] 100VDC is not the same and cannot be
replaced with another contactor only rated for AC if being used in a DC
circuit. I know this is a bit long but I hope it helps.
--
Regards,
*Dennis Lee Miles* (Director) *E.V.T.I. inc*.
*www.E-V-T-I-Inc.COM <http://www.e-v-t-i-inc.com/> *(Adviser)*
EVTI-EVAEducation Chapter
*
Phone (863) 944 - 9913
Initial demand (computed by extrapolating the reservations for GM Volt and
Nissan Leaf,) shall exceed 200,000 vehicles in 2010 and 2011. However only
50,000 vehicles will be marketed, so a LARGE demand for Nice Newer
Conversions is predicted!
==============================================================

Hart Penn <[email protected]> wrote:

> Hello all,
>
>
>
> I've searched the archive for this topic but didn't find any that really
> answered my question.
>
>
>
> I'm about to wire up the engine compartment of my 914 and am acquiring the
> odds and ends for it, one item being the inter-pack fuse. From what I've
> read, I should use a fast-acting type fuse such as a Ferraz Shawmut
> A30QA500-4 which is 300V 500A. These are fairly expensive at ~$50 per
> fuse.
>
>
>
>
> I've read a little about fuse sizing but still it seems like an art form.
> The fuse should carry the rated current for indefinite periods without
> opening and should open at the interrupt rating. Is the short-circuit
> current of a battery the max discharge current, and if so, the Thundersky
> spec for its 200AH says it is <= 20CA or 4000A. If the short-circuit
> current, say 4000A, is less than the fuse's interrupt rating, say 100KA for
> the A30QS500, will the fuse open up? I'm just trying to figure out if I can
> go with a lower rated voltage like 150V which is closer to my 120V and a
> lower rated current like 400A. These are still Ferraz Shawmut fuses and a
> A15QS400-4 goes (150V 400A) for about ~$30.
>
>
>
> Both of the above fuses are rated AC and DC.
>
>
>
> Here's the spec url for the 150V/400A fuse:
> http://www.discountfuse.com/v/vspfiles/downloadables/Ferraz/PDF/a15qs.pdf
>
> And the spec url for the 300V/500A fuse:
> http://www.discountfuse.com/v/vspfiles/downloadables/Ferraz/PDF/a30qs.pdf
>
>
>
> I don't mind spending the $$$ for the correct overcurrent device but want
> to make sure I purchase what I need. Any advice you can give on this topic
> is, as usual, much appreciated.
>
>
>
> Thanks,
>
> Hart Penn
>
> http://evalbum.com/3402
>
> http://hartpenn.blogspot.com
>
>
>
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Registered
Joined
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Discussion Starter #3
Hart Penn wrote:
> I've read a little about fuse sizing but still it seems like an art
> form.

It's science; but any sufficiently well-developed science looks like
art. :)

> the inter-pack fuse... From what I've read, I should use a
> fast-acting type fuse such as a Ferraz Shawmut A30QA500-4 which is
> 300V 500A. These are fairly expensive at ~$50 per fuse.

Fuses are chosen to protect whatever wiring and parts come after them.

The fuse you put in a battery pack is there in case there's a short
inside the pack, in the wiring coming out of the pack shorts, or the
load connected to it. In general, the batteries and their wiring are
pretty tough, and can stand very high currents for at least several
seconds. Therefore, you don't need an expensive fast-blow fuse.

For example, you said your batteries were rated for 4000a short circuit.
If the wires coming from the pack shorted, that means something like
4000a would flow for a few seconds. Then, a battery would melt or catch
fire or explode! So, you need a fuse that will open *before* the
batteries fail.

Or, it could be that your wiring is sized so it would melt before a
battery fails. Copper is expensive, so let's say you're using #0 wire
which melts at something like 2500 amps.

> The fuse should carry the rated current for indefinite periods
> without opening and should open at the interrupt rating... Here's the
> spec url for the 150V/400A fuse:
> http://www.discountfuse.com/v/vspfiles/downloadables/Ferraz/PDF/a15qs.pdf

Sadly,
>
it isn't that easy. The current at which a fuse blows is a
function of both current *and* time. Look at the "Melt Time" chart in
the data sheet you referenced above: A 150 amp fuse opens at:

300 seconds at 250 amps
30 seconds at 275 amps
3 seconds at 300 amps
0.3 sec at 350 amps
0.01 sec at 475 amps

There is a *huge* tolerance in these times; they can vary 3:1 in either
direction. So at 300 amps, this "150 amp" fuse might take from 1 to 10
seconds to blow.

Also, a "150 amp" fuse *can* blow at 150 amps. It may take hours or days
to do it, though. To make sure it never blows due to normal currents,
size the fuse at about double the normal current.

> Is the short-circuit current of a battery the max discharge current

No; it is the current that *will flow* if you short it. The battery is
also virtually guaranteed to fail at this current!

The reason for giving you this current is so you can pick a fuse that
can safely interrupt a 4000 amp short (without itself exploding). The
fuses you referenced here have a 100,000 amp interrupt rating ("100kA
I.R.) on the first page of the data sheet, so they can *easily* stop a
4000 amp current without fail.

> If the short-circuit current, say 4000A, is less than the
> fuse's interrupt rating, say 100KA for the A30QS500, will the fuse
> open up?

Yes. The above chart tells you how long the fuse takes to open at
various currents above its rated current.

> I'm just trying to figure out if I can go with a lower rated
> voltage like 150V which is closer to my 120V and a lower rated
> current like 400A.

You *definitely* want a fuse with a current rating far lower than the
short circuit current of the pack and its wiring! Otherwise, it won't
blow when it needs to.

For these Thunderskys, I don't think I'd use a fuse over 300a. Even less
might be OK, depending on your controller.

The fuse has to be DC rated, or at least rated for an AC voltage about 4
times higher than your pack voltage (i.e. a 150vdc fuse is fine, and a
600vac fuse can be used in a pinch). A lower voltage fuse will be
cheaper and smaller.

--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

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Registered
Joined
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70 Posts
Discussion Starter #4
Thanks Lee and Dennis for your feedback. Special thanks to Lee for
demystifying the spec sheet.

As my pack will be 128V 200AH Thundersky and my controller 144V 500A EVHelp,
the 150V 300A Ferraz Shawmut looks like it will protect the pack and
controller quite well.

Here's the time to blow rating for the 150V 300A fuse according to the spec
sheet:

Indefinitely at 300 amps
300 seconds at 475 amps
100 seconds at 500 amps
30 seconds at 530 amps
10 seconds at 560 amps
1 second at 650 amps
0.3 seconds at 700 amps
0.1 seconds at 760 amps
0.03 seconds at 830 amps
0.01 seconds at 950 amps

So if the pack shorts for any reason, the fuse should blow well before the
current hits 4000 amps. My controller limits the current draw to a maximum
of 500 amps and that can be tweaked downward if I feel so inclined.

Many thanks again,
Hart Penn


-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Lee Hart
Sent: Sunday, October 10, 2010 6:38 PM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] High-current Inter-Pack Fuse Selection

Hart Penn wrote:
> I've read a little about fuse sizing but still it seems like an art
> form.

It's science; but any sufficiently well-developed science looks like
art. :)

> the inter-pack fuse... From what I've read, I should use a
> fast-acting type fuse such as a Ferraz Shawmut A30QA500-4 which is
> 300V 500A. These are fairly expensive at ~$50 per fuse.

Fuses are chosen to protect whatever wiring and parts come after them.

The fuse you put in a battery pack is there in case there's a short
inside the pack, in the wiring coming out of the pack shorts, or the
load connected to it. In general, the batteries and their wiring are
pretty tough, and can stand very high currents for at least several
seconds. Therefore, you don't need an expensive fast-blow fuse.

For example, you said your batteries were rated for 4000a short circuit.
If the wires coming from the pack shorted, that means something like
4000a would flow for a few seconds. Then, a battery would melt or catch
fire or explode! So, you need a fuse that will open *before* the
batteries fail.

Or, it could be that your wiring is sized so it would melt before a
battery fails. Copper is expensive, so let's say you're using #0 wire
which melts at something like 2500 amps.

> The fuse should carry the rated current for indefinite periods
> without opening and should open at the interrupt rating... Here's the
> spec url for the 150V/400A fuse:
> http://www.discountfuse.com/v/vspfiles/downloadables/Ferraz/PDF/a15qs.pdf

Sadly,
>
it isn't that easy. The current at which a fuse blows is a
function of both current *and* time. Look at the "Melt Time" chart in
the data sheet you referenced above: A 150 amp fuse opens at:

300 seconds at 250 amps
30 seconds at 275 amps
3 seconds at 300 amps
0.3 sec at 350 amps
0.01 sec at 475 amps

There is a *huge* tolerance in these times; they can vary 3:1 in either
direction. So at 300 amps, this "150 amp" fuse might take from 1 to 10
seconds to blow.

Also, a "150 amp" fuse *can* blow at 150 amps. It may take hours or days
to do it, though. To make sure it never blows due to normal currents,
size the fuse at about double the normal current.

> Is the short-circuit current of a battery the max discharge current

No; it is the current that *will flow* if you short it. The battery is
also virtually guaranteed to fail at this current!

The reason for giving you this current is so you can pick a fuse that
can safely interrupt a 4000 amp short (without itself exploding). The
fuses you referenced here have a 100,000 amp interrupt rating ("100kA
I.R.) on the first page of the data sheet, so they can *easily* stop a
4000 amp current without fail.

> If the short-circuit current, say 4000A, is less than the
> fuse's interrupt rating, say 100KA for the A30QS500, will the fuse
> open up?

Yes. The above chart tells you how long the fuse takes to open at
various currents above its rated current.

> I'm just trying to figure out if I can go with a lower rated
> voltage like 150V which is closer to my 120V and a lower rated
> current like 400A.

You *definitely* want a fuse with a current rating far lower than the
short circuit current of the pack and its wiring! Otherwise, it won't
blow when it needs to.

For these Thunderskys, I don't think I'd use a fuse over 300a. Even less
might be OK, depending on your controller.

The fuse has to be DC rated, or at least rated for an AC voltage about 4
times higher than your pack voltage (i.e. a 150vdc fuse is fine, and a
600vac fuse can be used in a pinch). A lower voltage fuse will be
cheaper and smaller.

--
Lee A. Hart | Ring the bells that still can ring
814 8th Ave N | Forget the perfect offering
Sartell MN 56377 | There is a crack in everything
leeahart earthlink.net | That's how the light gets in -- Leonard Cohen

_______________________________________________
| REPLYING: address your message to [email protected] only.
| Multiple-address or CCed messages may be rejected.
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| OPTIONS: http://lists.sjsu.edu/mailman/listinfo/ev

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| Multiple-address or CCed messages may be rejected.
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