In article t,
Chip writes:
On 31 Jul 2005 23:00:48 GMT,it is alleged that
(Andrew Gabriel) spake thusly in uk.d-i-y:
For a short circuit, to get 26A in a 0.75m cable, the short
circuit would have to be 149m from the plug. Again, that's
much longer than an appliance flex is allowed to be.

I meant to stay out of this thread, however, something similar has
happened to me while using a hedgetrimmer with a ~50m x 0.75mm sq
flex, this being one occasion when such a length is unavoidable in
large gardens. Unless we install several outdoor sockets, and this
thread really *doesn't* need to get onto outdoor electrics, ring or
radial g. Cut through the flex part way such that it was jammed in
the blades, evidently the short wasn't _short enough_ to blow the 5A
plug fuse. Ended up throwing the hedgetrimmer away from me and running
to pull the plug, which was hot. Even with the blade of the
hedgetrimmer + sparking flex in contact with wet grass, the RCD hadn't

50m x 0.75mm² is 3.1 ohms which is 77A, so it can't have
been a short circuit as that would have blown the fuse within
10 seconds (possibly less -- I don't have the curves for BS1362
fuses). The sparking would also imply it wasn't a dead short,
as that would produce no continuous sparking.

As I said before (probably in the previous thread), extension
cords are probably one of the most serious sources of danger,
and that's from several perspectives. It's not just a question
of making them "idiot proof" -- most people with no electrical
engineering or similar background really have no idea what the
safety issues with them are, and many of the issues can hardly
be said to be intuitive in any case.

In your case, you should really be using a much thicker extension
cord for the bulk of that cable run. A rough guideline for 13A
extension cord length is:
1.25mm² cable, 12m max
1.5mm² cable, 15m max
2.5mm² cable, 25m max.

By reducing the fuse and max current, you could proportinally
increase the length, so a 2.5mm² 50m cable with a 5A fuse
would be within this. (Note however that you won't get 2.5mm²
flex into the cord grips of most 13A plugs or line sockets.)

Something else that I've done is replaced my hedge trimmer
cable with 3-core earthed. The earth is not connected at the
hedge timmer end (like all garden tools, it's double insulated),
but it does make it much more likely the RCD will trip in the
type of fault you had.

However, again as I said before, electrical installations
would be better designed to remove the requirement for extension
cords at all. Shortly before Part P came in, as part of replacing
my Consumer Unit, I included provision of outside sockets such
that no part of the garden is more than 15m from a socket. They
are on a dedicated TT-system circuit, and RCD protected at 10mA.

--
Andrew Gabriel