Basics of MCB

Miniature circuit-breakers are used for:

i) overload protection ii)short circuit protection

In electrical circuits as well as ?protection against electric shock by automatic Disconnection

The disconnection is performed by two different releases.

i) Electromagnet for Short-Circuit protection

The instantaneous tripping by electro-magnetic release provides protection against short circuits.

?ii) Bi-metal for Overload protection

?Two metals with a different temperature coefficient of expansion are fused together

?With heating one of them expands more & another expands less.

?This action results in the bending of the bimetal.

The combination of the electromagnetic release and the thermal bi-metal release tripping curves results in an overall tripping curve for overload protection.

This curve – referred to as the individual tripping characteristic – represents the time/current behavior of a miniature circuit-breaker

The desire for the best protection, which requires miniature circuit-breakers to be highly sensitive, has to be reconciled with the different operating characteristics of the loads to be protected.

?Load current peaks must be permitted to pass unhindered, yet at the same time, a disconnection must be ensured in the event of relatively low, but continuous, overloads.

Overload protection in accordance with IEC 60364-4-43

?For protection against overload, the protective device must be selected

based on the current-carrying capacity Iz of the cable:

Ib ≤ In ≤ Iz (1)

I2 ≤ 1.45 x Iz (2)

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Ib = Design current of a circuit

In = Rated current of the protective device

Iz = Current carrying capacity of the cable in accordance with IEC/HD 60364-5-52

I2 = Current ensuring effective operation in the conventional time of the protective device

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