Re: Re: Re: Fuses for 12v system

On Tue, 6 Dec 2005 23:13:16 +0000 (UTC), "Allan Jones" <sot177-urw1@xxxxxxxxxxx>
wrote:

>
>Thank goodness the voice of sanity appears sometimes. Thank you Peter.
>
>Maybe you can answer one question that has bothered me for some time. About
>25 years I tested a number of fuses from a variety of manufacturers, they
>seemed to be in two categories. When a fuse said, for example, 5 Amps
>sometimes this meant that it would carry 5 Amps forever without blowing, and
>sometimes it meant that it would blow more-or-less instantly at 5 Amps. The
>former would blow instantly at about 10 Amps, the latter would carry only
>about 2.5 Amps indefinitely.
>
>Does this situation still exist or have the manufacturers standardised
>things since then?

Fuses are rated at their carrying capacity PLUS their speed of breaking and
their rupture capacity.

They generally come into headings that describe their basic ratings:

SF1A - Superfast 1A - very fast blowing fuse 1A (sometimes FF1A)
F1A - Fast 1A quick blowing fuse 1A
T1A - Time Lag 1A - delayed blowing fuse

The carrying capacity is that current which the fuse will carry indefinitely at
a specified ambient. The fuse will heat up during operation of the circuit it is
protecting so the thermal rise has to be taken into consideration, which is why
fuses sometimes have to be derated for high ambients or when in an enclosed
carrier. Thermal graphs are published for all makes.

HRC fuses are a slightly different animal and are generally used for industrial
circuit protection. They still comply with the basics though, and come in the
basic HRC or the 'motor rated' types which are a time lag version for high
starting current applications. High-voltage versions are also made, we use some
500A 1000V fuses for 750V DC supplies, they run into the hundreds of pounds
each....

Rupture ratings are the fault current that the fuse will pass before blowing,
and can run into thousands of VA for an industrial HRC fuse. The smaller glass
bodied types also have this rating but it tends to be much lower as the actual
fuse isn't as mechanically strong as the industrial types.

Vehicle 'tab' fuses are another type, they have all the ratings published and
tend to be towards the fast blowing end of the scale.

Semiconductor fuses are a special breed which are used to protect items like
IGBT's and Thyristors in power control circuits. They are a very fast-blowing
power fuse, a bit intolerant of surges and expensive! but better blow a £10 fuse
than a £150 thyristor... These are particularly sensitive to temperature and do
not like running hot as their ratings are seriously affected by increased
temperature.

Fuses are like people, they have individual characteristics which need to be
understood before they are fitted to a circuit. Once there and fitted/sized
correctly they do their job very well.

This page shows some of the smaller glass-bodied Schurter (Swiss) fuses with
..pdf files for data:
http://www.schurter.ch/wwwsc/con_pg01_2.asp?page_nr=2&page_res=20

GE took over the GEC fuse products:
http://www.gepowercontrols.com/10086/10089/10119/10120/index.html

One of the better explanations of fuse ratings:
http://www.circuitprotection.ca/fuseology.html

Peter

--
Peter & Rita Forbes
Email: diesel@xxxxxxxxxxxxx
Web: http://www.oldengine.org/members/diesel
.