House of Horrors

"tony sayer" wrote in message

In article , Max Demian
writes

Moot as the fuse is likely to be made of a different metal than a
mains conductor.

And what sort of metal would that be then, the fuse that is?..

Low rating fuse wire is pure tin; I suppose a tin alloy could be used in
other cases. High amperage fuses might be copper.

--
Max Demian

"Brian Gregory [UK]" wrote in message

"Max Demian" wrote in message
...

Moot as the fuse is likely to be made of a different metal than a
mains conductor.

Duh!

It will be a metal with a higher resistance than copper, not lower,
and it's *still* thinner even then.

There's melting point as well as resistance to consider. For a 500 mA fuse
copper would be much too thin mechanically so tin could be used.

--
Max Demian

"Max Demian" wrote in message
...
"Brian Gregory [UK]" wrote in message

"Max Demian" wrote in message
...

Moot as the fuse is likely to be made of a different metal than a
mains conductor.

Duh!

It will be a metal with a higher resistance than copper, not lower,
and it's *still* thinner even then.

There's melting point as well as resistance to consider. For a 500 mA fuse
copper would be much too thin mechanically so tin could be used.

I have noticed that low amperage fuses have measurable resistance. This will
not be significant in terms of their application, though, I suppose.

What's the thing about the rating of a fuse versus the actual current needed
to blow it immediately? There's some sort of standard about this I believe.
Years ago we used to use a little RS 12VDC psu which had a max output of
1.2A. If the line amps that they fed were affected by lightning they would
present a short circuit to the psu, less the resistance of the co-axial
trunk, which would typically be about 1 ohm. So the psu would sit there
producing 1.2A. There was nothing internally to tell it to shut down. I took
to fitting 1A fuses, the load being typically 700mA. But I soon found that
the fuses did not not blow immediately when the fault occurred. Sometimes
I'd find them unblown, hot, and passing 1.2A. On one occasion I had used one
of those cheap in-line plastic fuse holders and it had started to melt. By
the time the problem had come to my attention we had just about stopped
using DC line power anyway, so I never resolved it. I think nowadays I would
fit a bigger psu anyway -- maybe a 3A one.

Bill

In article , Bill Wright
wrote:

"Max Demian" wrote in message
...
"Brian Gregory [UK]" wrote in message

"Max Demian" wrote in message
...

Moot as the fuse is likely to be made of a different metal than a
mains conductor.

Duh!

It will be a metal with a higher resistance than copper, not lower,
and it's *still* thinner even then.

There's melting point as well as resistance to consider. For a 500 mA
fuse copper would be much too thin mechanically so tin could be used.

I have noticed that low amperage fuses have measurable resistance. This
will not be significant in terms of their application, though, I suppose.

The question is, should they have a measurable resistance? We once had a
serious problem due to faulty batch of fuses being high resistance and
dropping a significant voltage. a quick reference to memory says 0.2ohm,
which at 5A drops 1v.

--
From KT24 - in "Leafy Surrey"

Using a RISC OS computer running v5.11

In article , Max Demian
writes
"tony sayer" wrote in message

In article , Max Demian
writes

Moot as the fuse is likely to be made of a different metal than a
mains conductor.

And what sort of metal would that be then, the fuse that is?..

Low rating fuse wire is pure tin; I suppose a tin alloy could be used in
other cases. High amperage fuses might be copper.


So in days of Yore when my dad blew a fuse in our old house.. that roll
of tinned copper wire wasn't a fuse then as it was used as such and none
of the wiring blew out lighting or power!..

And in all the years I've been working with leccy in one form or another
I've yet to see a mains cable or flex that a fuse even a larger one than
that needed didn't protect the cable!....




--
Tony Sayer

On Fri, 01 Jun 2007 10:35:12 +0100, charles
wrote:

In article , Bill Wright
wrote:

"Max Demian" wrote in message
...
"Brian Gregory [UK]" wrote in message

"Max Demian" wrote in message
...

Moot as the fuse is likely to be made of a different metal than a
mains conductor.

Duh!

It will be a metal with a higher resistance than copper, not lower,
and it's *still* thinner even then.

There's melting point as well as resistance to consider. For a 500 mA
fuse copper would be much too thin mechanically so tin could be used.

I have noticed that low amperage fuses have measurable resistance. This
will not be significant in terms of their application, though, I suppose.

The question is, should they have a measurable resistance? We once had a
serious problem due to faulty batch of fuses being high resistance and
dropping a significant voltage. a quick reference to memory says 0.2ohm,
which at 5A drops 1v.

5 watts dissipated in a fuse? It must have got b????? hot!

In article , tony sayer
writes
In article , Max Demian
writes
------------Cut----------
Moot as the fuse is likely to be made of a different metal than a mains
conductor.

And what sort of metal would that be then, the fuse that is?..

Try an alloy of tin, silver, copper, and antinomy. Others maybe used
depending on the required rupture performance. As a point of curiosity,
the inside of the fuse might be coated with an organic compound that
releases a spark quenching gas with the heat of rupture; fuses are a
complex subject, and are not just a 'bit of wire'.
--
Ian G8ILZ
There are always two people in every pictu the photographer and the viewer.
~Ansel Adams

In article , Bill Wright
writes
I have noticed that low amperage fuses have measurable resistance. This will
not be significant in terms of their application, though, I suppose.

What's the thing about the rating of a fuse versus the actual current needed
to blow it immediately? There's some sort of standard about this I believe.
Years ago we used to use a little RS 12VDC psu which had a max output of
1.2A. If the line amps that they fed were affected by lightning they would
present a short circuit to the psu, less the resistance of the co-axial
trunk, which would typically be about 1 ohm. So the psu would sit there
producing 1.2A. There was nothing internally to tell it to shut down. I took
to fitting 1A fuses, the load being typically 700mA. But I soon found that
the fuses did not not blow immediately when the fault occurred. Sometimes
I'd find them unblown, hot, and passing 1.2A. On one occasion I had used one
of those cheap in-line plastic fuse holders and it had started to melt. By
the time the problem had come to my attention we had just about stopped
using DC line power anyway, so I never resolved it. I think nowadays I would
fit a bigger psu anyway -- maybe a 3A one.

As I recall there is, or was, a difference between British and USA
specifications. A USA fuse should blow at the rated current (limit
value), I am not sure about how quickly, and time delay fuses would
obviously have a delay to allow a brief over current. British fuses
should blow at 1.7 times the rated current and pass the rated current
indefinitely (operating value, think of a 3kW fire with a 13A fuse).
Obviously there are time and ageing factors again. This difference could
make repairing USA manufactured equipment in the UK problematic.

--
Ian G8ILZ
There are always two people in every pictu the photographer and the viewer.
~Ansel Adams

In article , charles
writes
In article , Bill Wright
wrote:

"Max Demian" wrote in message
...
"Brian Gregory [UK]" wrote in message

"Max Demian" wrote in message
...

Moot as the fuse is likely to be made of a different metal than a
mains conductor.

Duh!

It will be a metal with a higher resistance than copper, not lower,
and it's *still* thinner even then.

There's melting point as well as resistance to consider. For a 500 mA
fuse copper would be much too thin mechanically so tin could be used.

I have noticed that low amperage fuses have measurable resistance. This
will not be significant in terms of their application, though, I suppose.

The question is, should they have a measurable resistance? We once had a
serious problem due to faulty batch of fuses being high resistance and
dropping a significant voltage. a quick reference to memory says 0.2ohm,
which at 5A drops 1v.

Obviously they must have a resistance, or they would not get hot. What
is less obvious is that 'identically' rated fuses from different
manufactures can have substantially different resistance, I remember
testing fuses from several manufactures in order to find one with a very
low resistance so as to minimise the effect on source impedance of a
signal generator and not cause a significant drop in the output voltage.


--
Ian G8ILZ
There are always two people in every pictu the photographer and the viewer.
~Ansel Adams

"brightside S9" wrote in message
...
On Thu, 31 May 2007 16:59:57 +0100, "Graham." wrote:

Remember those round kettle plugs that auto-ejected if the
kettle boiled dry?

Just fitted a new element, with the auto eject safety feature, on my
Swan kettle yesterday.

Did you have the correct tool, or did you have to file a notch
in a screwdriver?

--
Graham
%Profound_observation%