(ET) Fuses

["Robert" <euclid delhitel net>]

Hi,
You might be able to use DC breakers designed for PV modules. My Outback 
system has several from 10A to 175A. Do a search for them.

Rob

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Today's Topics:

  1. Re:  Fuses made of silver? (David Roden)


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Message: 1
Date: Sun, 20 Jul 2014 03:56:40 -0400
From: "David Roden" <etpost drmm net>
To: elec-trak cosmos phy tufts edu
Subject: Re: (ET) Fuses made of silver?
Message-ID: <53CB3DF8 17668 50FF7B13 etpost drmm net>
Content-Type: text/plain; charset=US-ASCII

On 19 Nov 2013 at 13:30, David Roden wrote:

> The Avco R36 blew a mower motor fuse this past weekend.  Mouser wants
> $9.77 each plus shipping!

And yet another this past weekend.  Mouser is now charging $10.05 each.

I'd like to replace these 40a slow-blow fuses with circuit breakers.  I see
quite a few DC breakers on Ebay for 50% to 100% more than the MDL 40 fuse.
That looks like a pretty good deal.

The question is, what size breaker should I use?

Given the size of the wire connecting to the mower motors, I'm guessing 
that
GE oversized the fuses - and used slow-blow type - so they'd open on a
serious long-term overload, but not on a normal starting surge.  I need to
choose a breaker that will act the same way.  That might not necessarily be
a 40 amp breaker.

Problem is, I don't know the mower motors' starting surge. I have no
measuring device fast enough to catch that current peak.

Also, the fuse's delay curve is quite a bit different from the breakers'.
Oddly, Bussman only lists up to a 30a fuse in the MDL range; but
extrapolating, this is how long the 40a fuse should hold :

= 45 sec at 60a (150%)
= 25 sec at 80a (200%)
= 2 sec at 160a (400%)

An Airpax APL range DC breaker with standard (#52) delay, commonly 
available
at fairly reasonable prices, has these values :

= 3 to 30 sec at 150%
= 0.7 to 10 sec at 200%
= 0.18 to 2.5 sec at 400%

For example, a 60a Airpax would have these values :

= 3 to 30 sec at 90a
= 0.7 to 10 sec at 120a
= 0.18 to 2.5 sec at 240a


And here's an Airpax with delay #53.  These seem quite a bit harder to 
find.

= 30 to 300 sec at 150%
= 10 to 100 sec at 200%
= 1.5 to 15 sec at 400%

Here is a similar table for a Bluesea marine breaker :

= 1 to 150 (!) sec at 150%
= 0.3 to 20 sec at 200%
= 0.2 to 3 sec at 400%

If GE was expecting as much as a 160a surge (which the 40a MDL fuse would
hold for 2 sec), even a 60a delay 52 Airpax breaker isn't certain to hold
that long.  And at 80a, things look even worse.

The question is, what was GE's criterion for this fuse?

What do the EEs here think is a good choice for a breaker size?  Should I 
go
with 50a or 60a?  Something else?

Or am I overthinking this?  After all, the wires supplying those motors
don't look to be any larger than #16.  Maybe the wiring resistance will
limit peak current.  Still, GE must have used those slow fuses for a 
reason.

Ideas?  Suggestions?  All appreciated.  Thanks.


David Roden - Akron, Ohio, USA

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