MCB for AC and DC

Due to the absence of a current zero in DC, it is quite usual to use permanent magnet blowouts in DC MCBs.

In AC MCBs, these are not required.

i don't know what you want to ask, but i am sure the above answers some part of what you want. Whether you know it or not.

kvsridhar, would a breaker fitted with AC blow out coils work in the same way with DC?

This is just out of curiosity.

Of course yes ! Better sometimes...

Permanent magnet blowout can be polarised, demanding that the +ve and -ve terminations be in specific way. Some clever designs are there which overcome this handicap.

Permanent Magnet field strength being fixed, the blowout effect is dependant on the field strength of the current flowing.

In a blowout coil,

(a) the blowout effect is impervious to the connections, it is always upwards, and

(b) the blowout force would be proportional to the square of the current, since it would flow both thru the coil and the arc. At low currents, this can become low, causing the arc to stick in one place. Such a current is called 'critical current' at which the CB may not work.

In larger CBs, where space is no problem, coils can be used, but in small MCBs, the tiny space prohibits the coils, and so permanent magnets are usually used.

But then, you probably know all this already, if so, sorry .

Thanks kvsridhar, it's been years since I've worked on a DC breaker (it was a beast of a thing), I thought I was right in thinking there were coils in it. It burnt my legs when it opened under fault.

Again thanks.

Oh, i am sorry to hear that. Must have been pretty bad.

Hold on, your legs got burnt? The arc blew downwards? Strange. Can happen if the winding direction gets reversed. No poka-yoke in those days perhaps.

More likely to happen with Shunt blowout coils, not with Series ones.

Shunt:

Series:

Why on earth are three phases needed on DC?