On Wed, 08 Feb 2006 10:18:36 +0000 someone who may be Ben Blaukopf
wrote this:-

MCBs disconnect in the event of a fault current
much faster than fuses

Far too sweeping a generalisation to be correct, though it is a
common misconception often expressed with absolute certainty. There
are fuses and fuses, there are MCBs and MCBs and there are faults
and faults.

Never forget that an MCB takes an irreducible minimum time to
operate. The magnetic coil has to attract a bit of iron, this has to
release a mechanism, springs have to go boing, things have to pivot
and contacts have to move some distance apart before the circuit is
opened. No matter how high the current this minimum time is
irreducible for a particular design. In contrast with all this
mechanical stuff, all a fuse wire has to do is melt. Rapid melting
is something HBC fuse links do very rapidly at high currents, faster
than the fastest MCBs. In fact even rewirable fuse wires melt
rapidly at high currents, though (like many ranges of MCB) they are
unable to break the fault current.

With a small overload of long duration the position is reversed. The
irreducible minimum operating time of the MCB is now irrelevant, as
the controlling factor is how quickly the thermal part of the
mechanism activates the contacts. HBC fuses will take longer to
operate (though one can argue this is an advantage in a properly
designed circuit). Rewirable fuses are rather uncertain in such
circumstances.

MCBs do have advantages of ease of operation and resetting, though
if one operates more than once in a blue moon then there is probably
something wrong with the circuit or something connected to it. They
are thus particularly suitable for those who don't know much about
electricity, provided that the correct range is selected to break
the prospective fault current (or adequate backup provided).

On the other hand some people sleep a bit better at night knowing
that they live near a substation, but if a serious fault was to
develop in their wiring a HBC fuse will react incredibly quickly to
disconnect the particular circuit concerned in far less than a
quarter of a cycle and limit the fault current. With typical MCBs
the same fault would probably have caused the main "electricity
board" fuse to operate, cutting off all electricity, hopefully
before the MCBs had caught fire because they were unable to break
the fault. Some MCBs are a lot better than typical MCBs, but there
are still limits on how fast a series of mechanical components can
move.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54