Date   : Wed, 19 Aug 2009 21:25:06 +0100
From   : philb@... (Phil Blundell)
Subject: Retro Reunited: Econet Cabling, Demos, etc.

On Wed, 2009-08-19 at 20:39 +0100, Phill Harvey-Smith wrote:
> Take into account also that for most mains 
> fuses there is typically a time/current curent curve that determines 
> when they fail, so draw say 14A from a 13A circuit and it may take a 
> while to fail, the more current over the rated value the quicker it fails.

I think a BS1362 13A fuse will allow you to draw 14A pretty much
indefinitely.  As a general rule you need to overload them by a
significant multiple of their rating in order to be sure of having them
blow quickly, and for minor overloads there is no guarantee that they
will blow at all.

This is the reason that the earth loop impedance of the circuit matters:
if the earth resistance is too high, even a dead short between phase and
earth inside the appliance might not result in a high enough current to
rupture the fuse within an acceptable time.  For a circuit that's
protected solely by a 13A fuse, the earth loop impedance needs to be no
greater than about 2.5 ohms in order to be sure of clearing a fault
within 0.4 seconds.

If your earth loop impedance is too high to meet the requirement for
fault clearing time with a fuse alone then, as you say, adding an RCD is
one way to mitigate that.  This does often happen with TT installations
where the earth connection is via a local earth rod rather than
supplier-provided earth.

p.