There is an fault in the mains supply to the electric heaters in the
community hall for which I am responsible. The heaters are fed from an
off-peak meter via a time switch. This switch is not working. This
morning an electrician diagnosed a possible fault with time switch which
he replaced. However, the replacement is not working either (it's not a
new switch but one he retained from a previous job.)

Several years ago something similar occurred but I could only remember
vaguely that it concerned a fuse which is in the power line to the time
switch. I went and had as look at the fuse myself. I was surprised to
find that the cartridge fuse had been by-passed by a piece of fuse wire.
No sign of either having blown - I didn't disconnect to check the
cartridge independently. Then I noticed that moulded onto the fuse
holder was a message saying "wire fuse should not be used". What could
be the problem??
The system is 30 years old or more. The fuse may not have anything to
do with the fault but there is something suspicious there.
Any suggestions
TIA
Frank

In article , WhinYett
writes
There is an fault in the mains supply to the electric heaters in the
community hall for which I am responsible. The heaters are fed from an
off-peak meter via a time switch. This switch is not working. This
morning an electrician diagnosed a possible fault with time switch which
he replaced. However, the replacement is not working either (it's not a
new switch but one he retained from a previous job.)

Several years ago something similar occurred but I could only remember
vaguely that it concerned a fuse which is in the power line to the time
switch. I went and had as look at the fuse myself. I was surprised to
find that the cartridge fuse had been by-passed by a piece of fuse wire.
No sign of either having blown - I didn't disconnect to check the
cartridge independently. Then I noticed that moulded onto the fuse
holder was a message saying "wire fuse should not be used". What could
be the problem??

Simply put, the fuseholder is not designed to safely interrupt its rated
fault current with a wire fuse. In a cartridge fuse the wire element
will be surrounded by an inert filler and be rated to contain any arc
flash. Under high fault conditions a wire fuse may either coat the fuse
holder with evaporated metal leading to ongoing conduction or expose it
to excess heat, resulting in damage. A wire rated fuse holder will have
some mechanism to contain the heat/flash/metal.

No suggestions as to the nature of the fault, you'll need to trace it
with a multimeter if you feel safe to do so. Volts and then continuity
with the power safely disconnected.

--
fred
it's a ba-na-na . . . .

So these are presumably storage heaters then. I've seen older types of these
where the elements just die for no apparent reason, or the thermostats, erm
dont and cook the elements with a similar result in the long run.

Brian

--
From the Sofa of Brian Gaff Reply address is active
"fred" wrote in message ...
In article , WhinYett
writes
There is an fault in the mains supply to the electric heaters in the
community hall for which I am responsible. The heaters are fed from an
off-peak meter via a time switch. This switch is not working. This
morning an electrician diagnosed a possible fault with time switch which
he replaced. However, the replacement is not working either (it's not a
new switch but one he retained from a previous job.)

Several years ago something similar occurred but I could only remember
vaguely that it concerned a fuse which is in the power line to the time
switch. I went and had as look at the fuse myself. I was surprised to
find that the cartridge fuse had been by-passed by a piece of fuse wire.
No sign of either having blown - I didn't disconnect to check the
cartridge independently. Then I noticed that moulded onto the fuse
holder was a message saying "wire fuse should not be used". What could
be the problem??

Simply put, the fuseholder is not designed to safely interrupt its rated
fault current with a wire fuse. In a cartridge fuse the wire element will
be surrounded by an inert filler and be rated to contain any arc flash.
Under high fault conditions a wire fuse may either coat the fuse holder
with evaporated metal leading to ongoing conduction or expose it to excess
heat, resulting in damage. A wire rated fuse holder will have some
mechanism to contain the heat/flash/metal.

No suggestions as to the nature of the fault, you'll need to trace it with
a multimeter if you feel safe to do so. Volts and then continuity with the
power safely disconnected.

--
fred
it's a ba-na-na . . . .

In article ,
WhinYett writes:
There is an fault in the mains supply to the electric heaters in the
community hall for which I am responsible. The heaters are fed from an
off-peak meter via a time switch. This switch is not working. This
morning an electrician diagnosed a possible fault with time switch which
he replaced. However, the replacement is not working either (it's not a
new switch but one he retained from a previous job.)

Several years ago something similar occurred but I could only remember
vaguely that it concerned a fuse which is in the power line to the time
switch. I went and had as look at the fuse myself. I was surprised to
find that the cartridge fuse had been by-passed by a piece of fuse wire.
No sign of either having blown - I didn't disconnect to check the
cartridge independently. Then I noticed that moulded onto the fuse
holder was a message saying "wire fuse should not be used". What could
be the problem??
The system is 30 years old or more. The fuse may not have anything to
do with the fault but there is something suspicious there.

A BS1361 HRC fuse is designed to safely contain the explosion as the
fuse blows on a fault current. The carrier and fuse way for the fuse
are not designed to contain the exploding fuse wire and resulting
arc. Burning metal may spill out and ignite nearby materials, and/or
the arc may contact some other metalwork, and cause a short circuit
of the supply.

A BS3038 rewirable fuse holder is designed to contain the exploding
fuse wire and stop the arc spreading to any other metalwork.

Another point is that the energy let through a rewirable fuse before
it blows can be significantly more than an HRC fuse because it has
nothing to quickly quench the arc. In some circumstances, this can
cause live to earth shorts to take too long to blow the fuse, and
cause overheating/damage of the cable - 2.5mm² ring circuits with
old T&E cable which has only a 1mm² CPC is an example of this. (This
is not an issue if there is also an RCD protecting the circuit.)

I would guess the breaking capacity of a BS1361 HRC fuse is higher
than that of exposed bare fuse wire, which might mean that in some
cases of a short circuit where a heafty supply is available, the
bare wire fuse will fail to interrupt the supply, and the next
fuse back up will blow, and/or the circuit wire will blow, which
might start a fire somewhere.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

(Andrew Gabriel) writes:

In article ,
WhinYett writes:
There is an fault in the mains supply to the electric heaters in the
community hall for which I am responsible. The heaters are fed from an
off-peak meter via a time switch. This switch is not working. This
morning an electrician diagnosed a possible fault with time switch which
he replaced. However, the replacement is not working either (it's not a
new switch but one he retained from a previous job.)

Several years ago something similar occurred but I could only remember
vaguely that it concerned a fuse which is in the power line to the time
switch. I went and had as look at the fuse myself. I was surprised to
find that the cartridge fuse had been by-passed by a piece of fuse wire.
No sign of either having blown - I didn't disconnect to check the
cartridge independently. Then I noticed that moulded onto the fuse
holder was a message saying "wire fuse should not be used". What could
be the problem??
The system is 30 years old or more. The fuse may not have anything to
do with the fault but there is something suspicious there.

A BS1361 HRC fuse is designed to safely contain the explosion as the
fuse blows on a fault current. The carrier and fuse way for the fuse
are not designed to contain the exploding fuse wire and resulting
arc. Burning metal may spill out and ignite nearby materials, and/or
the arc may contact some other metalwork, and cause a short circuit
of the supply.

A BS3038 rewirable fuse holder is designed to contain the exploding
fuse wire and stop the arc spreading to any other metalwork.

Another point is that the energy let through a rewirable fuse before
it blows can be significantly more than an HRC fuse because it has
nothing to quickly quench the arc. In some circumstances, this can
cause live to earth shorts to take too long to blow the fuse, and
cause overheating/damage of the cable - 2.5mm² ring circuits with
old T&E cable which has only a 1mm² CPC is an example of this. (This
is not an issue if there is also an RCD protecting the circuit.)

I would guess the breaking capacity of a BS1361 HRC fuse is higher
than that of exposed bare fuse wire, which might mean that in some
cases of a short circuit where a heafty supply is available, the
bare wire fuse will fail to interrupt the supply, and the next
fuse back up will blow, and/or the circuit wire will blow, which
might start a fire somewhere.

My recollection of the fusing curves found long ago in the main public
library (probably in a book which had taken the curves from the
then-current IEE regulations, or perhaps it was in the regs themselves)
is that rewirable fuses generally blow a little sooner than the newer
cartridge fuses.
It surprised me, until I began to think about the irritation of having
to pay higher prices for cartridge fuses.

--
Windmill, Use t m i l l
J.R.R. Tolkien:- @ S c o t s h o m e . c o m
All that is gold does not glister / Not all who wander are lost