In message ,
Aaron wrote:

If you were to take say 7amps on a 5amp MCB it may not trip straight
away. The MCB uses an electro magnet that generates magnetic flux
depending on the amount of electrons (current) flowing round the coil.
As I understand it, they are designed not to trip when just over the
threshold current is being drawn, because you may well get trips if
say you had a fan light or tube that draws more current than usual
when it starts up.

From tables in the back of BS7671:

6A type B MCBs need 30A to trip in 0.4s. They need 13 or 14A to trip
under 100 seconds. 9A looks like about 2,000 seconds. Less than about
8.5A is unlikely to cause it to trip at all. 1mm2 cable is rated up to
16A in normal ("clipped direct") installations, or 12A partly in
insulation.

A 5A BS3036 (rewireable) fuse link needs 24A to blow in 0.4s. 10A will
blow it in about 100 seconds.

Hwyl!

M.

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Martin Angove: http://www.tridwr.demon.co.uk/
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