On 18/03/2017 14:47, Andrew Gabriel wrote:
In article ,
John Rumm writes:
On 17/03/2017 19:39, Alan wrote:
On Fri, 17 Mar 2017 16:55:19 +0000, John Rumm wrote:

there is a more general danger with cascaded extension leads that the
earth loop impedance can rise too much, and then in the event of a fault
(say crushing a flex under a chair leg), the fault may not be cleared
fast enough to prevent something getting too hot and catching fire.
Having said that, with short leads that's unlikely.

Surely that is what the 13A plug fuse is for?

It much depends on the lead lengths (and a little on the quality of the
sockets etc). In the case of short multi-way leads its very unlikely to
be an issue unless the circuit feeding the socket is well out of spec.

2.5 Ohms max ZS for a 13A fuse, almost double the requirement for a 32A
circuit.

For a 13A fuse you probably want 95A of fault current to be sure of
being inside the typically required 0.4 secs disconnect time. So that
means about 2.4 Ohms at 230V. You could lose 1.44 ohms off that before
you even get to the plug on 32A circuit at maximum length.

So 40m is about your limit. Use multiple cascaded leads and it could be
quite significantly less due to the extra plug and socket connections.

You need to add the resistance of the appliance flex too.

Indeed, although that's a bit harder to factor without knowing what is
plugged in.

The rules don't really allow for extension leads.
Hence the max length of an extension lead is 12m for 1.25mm²,
and 15m for 1.5mm² cable (PAT testing guidelines).

Yup, I expect my 50m 1.5mm^2 JoJo lead is probably out of spec more
often than its in it! ;-)

Rules don't allow for adding of contact and fuse resistances
either, hence a reason not to daisy-chain them, and they're
almost never the quality of a proper 13A wall outlet.

I managed to pick up a few from an old employer for peanuts
where the extension lead sockets are actually MK 13A round
panel mounting sockets, but these were damn expensive to make,
and the company manufacturered them itself. There was enough
space inside to add a mains relay per socket, so I have each
socket individually switchable via my home automation.

Handy...


--
Cheers,

John.

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