If you mean fitting a larger fuse carrier in place of the smaller
fuse carrier (e.g. fitting a 15A fuse carrier into a 5A fuse bridge)
it's actually impossible without the assistance of a "Birmingham
Perhaps I'm a little out of the ordinary in keeping a set of
correctly wired fuse carriers spare to facilitate fuse replacement.
Perhaps I'm also a little unusual in only re-wiring a blown fuse with
the correct fusewire.
The practice of miswiring fuses with the wrong fuse wire seems to be
one only followed by those aiming to earn a "Darwin Award". The fuse
carriers are clearly marked with their fuse rating amperage and the
fusewire card clearly identifies each wrap of fusewire and it's not
exactly 'Rocket Science' to figure how to rewire a Wylex fuse carrier.
The fact that Niglon cards of replacement fusewire were readily
available from almost any hardware store or chandlers does suggest
that this was a job that was deemed to be within the competence of the
typical literate householder of the day.
to run this wire into a double plug socket. (Then I can put plug on end
of wiring for light also and just plug this is leaving me with one space
Feed the supply to a 5A fused spur unit and feed that to the 13A socket,
and label the socket 'Max load 5A'. That's what they usually do for
aerial amps. You do need to pick up an earth from the lighting circuit
though, so if there isn't one you're stuffed.
Years ago I went to a block of flats to find out why the TV system
didn't work very well. There was no amp in loft above the communal
stairs, but I could see coaxes passing across that loft into the loft of
a flat, so I got into there and found the amp, which was strategically
placed midway between a lighting terminal box and a steel housing which
was something to do with the warden call system. The mains cable for the
amp had been stripped back by about two feet. The brown wire went to the
lighting terminal box and the blue one went to one of the screws that
held the lid on the metal box. This was not the cause of the fault,
which was simply that the amp was overloaded by aerial signal. However I
made myself unpopular by disconnecting the amp (which left the residents
with no reception rather than poor reception) pending the arrival the
next day of an electrician.
Assuming we're dealing with 'modern' lighting circuits which are
required to include a protective earth, if you're going to use a FIVE
AMP fuse inline, you might as well forego the luxury of such a fuse
box connection altogether for your 13A socket spur and just use an
ordinary joint box to tap into the lighting circuit.
The 6A fuse or CB at the CU will still provide the same level of
fault protection on the lighting circuit and exceed safety
requirements for the attached 13A Socket circuit. With a 5A fuse
you're just as likely to see the 6A fuse blow (or CB trip) at the CU
as you are the 5A cartridge fuse in the spur fuse box.
The extra fuse protection only makes sense with 3A or lower rated
fuses fitted. You can always fit a 13A fuse on a temporary basis to
allow you to use a powertool or whatever that draws just less than the
6A limit on the lighting circuit with all bar the one lamp switched
off whenever you want to avoid the hassle of digging out and setting
up a mains extension cord from a regular mains socket.
 If you're going to swap out the regular 1, 2 or 3 amp fuse, you
might as well fit a 13A fuse and work to the 6A limit of the lighting
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