"Andrew Gabriel" wrote in message ...

Now the next issue is that MCB's and fuses can only break the current
flow at two points in a mains cycle, that's where the current passes
through zero. Current doesn't stop flowing just because two metal
contacts come apart, particularly at high current flow. An arc will
form which will extinguish when the current flow momentarily stops
as it passes through zero. (There are other ways to extinguish the
arc, but not used at normal mains voltage in domestic situations.)

Andrew, I think you'll find that all modern MCBs and most HBC fuses are
so-called 'current-limiting' designs which do interrupt arc current well
before it reaches its prospective peak. Compare the manufacturer's
published I^2*t let-through figures with the I^2 integral over a full
half-cycle at the full rated breaking capacity.

MK technical supplement: "The arc drawn beteen the contacts is moved by
magnetic forces into the multiple plate arc chamber where the arc is split,
rapidly cooled and extinguished, the total operating time of the MCB is
between 3 and 5 milliseconds."

Electrium-Wylex catalogue: "The high speed current limiting action ensures
that the MCB operates before the full prospective fault current is allowed
to develop. Under fault conditions, damage can be sustained to the
installation and associated equipment due to the amount of energy that
passes before the current is completely interrupted. The total energy
let-through depends on the value of current and the time for which it flows,
and is denoted by the symbol I2t. The high speed current limiting action of
MCBs ensures that the energy let-through and any subsequent damage is
minimised. This reduced energy let-through assists greatly with both back-up
and discrimination considerations."

But you still don't get discrimination on a high level s/c fault of course -
the trip mechanisms all fire together, then it's a race to see which
arc-quench chamber cuts off the current first.

--
Andy