[SI-LIST] Re: Can a thin PCB trace be used as a reliable fuse

> -----Original Message-----
> From: Kai Keskinen [mailto:kalevi@xxxxxxxxxx]
> Sent: Thursday, 26 August 2004 10:42
> To: Mark.Hofmann@xxxxxxxxxxxxxxx; a.ingraham@xxxxxxxx; 'Si-List'
> Subject: [SI-LIST] Re: Can a thin PCB trace be used as a reliable fuse
>=20
> Yes, you can use a thin trace as  fuse. How do you fix the thing when the
> "fuse" goes? Do you build a new pcb and populate it again? The whole poin=
t
> of a fuse is that you change the fuse if there is a fault. You don't thro=
w
> away the PCB.
>=20
> Cheers,
>=20


There are a range of applications where SMT or through-hole fuses are used =
(i.e. not the easily replaceable socketed fuses).  As pointed out by Bren=
t, they may be used because of their low cost or special characteristics =
and they are usually the last line of defence, protecting against some ca=
tastrophic event which very rarely occurs but can't be ignored, such as s=
afety or fire hazard concerns.

These days, the cost of the service technician just touching the board woul=
d be prohibitive because of the overheads involved, so usually the board =
or the whole box just gets replaced.  Who would want to repair something =
that costs, say, $25 when new?  Out with the old, in with the new!  Besid=
es, an event serious enough to blow the fuse, may have already caused oth=
er, not immediately obvious problems.  You don't want that board coming b=
ack in 3 weeks for yet another repair!

Sometimes non-repairable fuses can be found on telco grade interface boards=
.  Among other things, these fuses may protect from a short between the t=
ransmission cable and power lines.  Now, how often does that occur??!  St=
ill, as different cabling might meet in e.g. service tunnels, one does wa=
nt some protection and regulations may require it, accordingly. =20

The bulky, easily replaceable fuses sometimes may not be practical or simpl=
y can't be used.  So when that proverbial lightning strikes, the user is =
probably not too surprised to hear that his/her interface card is toast b=
ut you still want to be sure that that's all that goes in their system, s=
o you throw in a small, non-repairable fuse or two... 

Cheers,

Sandor

------
Sandor Daranyi
Snr Design Engineer


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