In article ,
Aidan Karley writes:
In article , Andrew Gabriel
wrote:
Also, in the case of an extension cable, beware of max length
due to earth fault loop impedance. In the case of an extension
cable fused at 13A, that is 15m for 1.5 mm² and 25m for 2.5 mm².

Oh dear. Does that mean that the 30m extension lead that I made
for the electric lawn mower has significant electrical risks? Naturally

Possibly -- it depends on a lot of things.

Also, quite apart from the earthing problems with long cables,
you might be dropping a significant proportion of the mains
voltage across the cable when the mower is running. Most electric
mowers use cheap universal motors which won't mind this too much,
but better quality mowers use induction motors, which don't like
running at reduced voltage on-load (could overheat).

I used a properly sealed connector to "puddle-proof" the joint between
the cables (I can't remember the IP rating of the connector, but it was
suitable for regular immersion to a couple of metres, but not for
diving with. I'd have to cultivate an ROV technician to obtain higher
ratings. Which could be done.). The cost of a 30 or 50m reel of mains
flex at B+Q was considerably higher (as in, times several) than the
cost of 2 X 15m extension leads.

Yep -- you would do better at an electrical wholesaler for that.
(Conversely, they'll charge much more for the extension leads;-).

The risk is what? That in the event of a fault at the remote
socket, current travelling back down the earth line will raise the
potential of the remote end of the earth (Ohms Law), which leaves the
person at the remote end holding something live while standing in a
puddle?

Yes. Also the resistance of the long conductors will reduce the
fault current which flows so it takes significantly longer to
blow the fuse (or in really bad cases, might not blow the fuse
at all).

90% of the time I remember to plug the extension lead into an
RCD through-plug before I take the lawn mower out of the cellar. Maybe
I'd better tie-wrap or gaffer-tape the extension lead and RCD together
to make it 100% (including when the wife/ daughter does the lawn while
I'm away).

Yes.
Better still, mount an RCD-protected socket outside, so you are
always going to use an RCD protected supply outdoors. (I would
suggest using a separate RCD and socket, with the RCD mounted
inside in the dry, and a plain waterproof socket outdoors. That
way, if moisture does get into the outdoor part which it often
does eventually, you don't also write off an expensive outdoor
RCD socket.)

--
Andrew Gabriel