Page 125 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Thermal magnetic MCB
The switching mechanism is housed in a moulded housing with phenolic moulded high mechanically strong
switching dolly. This type of MCB is also provided with bimetallic overload release (Fig 1).
The electric current gets through two contact tips one each on moving and fixed contact of silver graphite.
An arcing chamber incorporating de-ionising arc chutes for control and quick suppression of the arc is provided in
the gap between two contacts. It has a ribbed opening closed by metal grid which allows ventilation and escape
of gases.
For protection against over-load and short circuit, MCB’s have thermal magnetic release unit. The overload
is taken care of by bimetallic strip, short circuit currents and over loads of more than 100% are taken care by
solenoid.
Working
The bimetallic strip when flexing due to temperature rise caused by increasing normal rated current beyond 130%
rotates a trip lever carrying an armature to which it is to brought into field of a solenoid. The solenoid is designed
to attract the armature to full position at about 700% overload or instantaneous short circuit current.
For initial portion of current wise (130% to 400%) tripping of circuit breaker is due to thermal action, between 400
to 700% tripping is due to combined thermal and magnetic action and beyond 700% due to fully magnetic action.
Categories of MCBs
Certain manufacturers like Indo Kopp manufacture the MCBs in three different categories namely ‘L’ series, ‘G’
series, and ‘DC’ series.
‘L’ series MCBs
‘L’ series MCBs are designed to protect circuits with resistive loads. They are ideal for protection of equipment
like Geysers, ovens and general lighting systems.
‘G’ series MCBs

‘G’ series MCBs are designed to protect circuits with inductive loads. G series MCBs are suitable for protection
of motors, air conditioners, hand tools, halogen lamps etc.,
‘DC’ series MCBs

‘DC’ series MCBs are suitable for voltage upto 220V DC and have a breaking capacity up to 6kA.
The tripping characteristics are similar to ‘L’ an ‘G’ series. They find extensive application in DC controls,
locomotives, diesel generator sets etc.,

Advantages of MCB
1 Tripping characteristic setting can be done during manufacture and it cannot be altered.
2 They will trip for a sustained overload but not for transient overload.
3 Faulty circuit is easily identified.

4 Supply can be quickly restored.
5 Tamper proof.
6 Multiple units are available.
Disadvantages
1 Expensive.
2 More mechanically moving parts.

3 They require regular testing to ensure satisfactory operation.
4 Their characteristics are affected by the ambient temperature.
Moulded Case Circuit Breakers (MCCB): Moulded case circuit breakers are similar to thermo magnetic type
MCBs except that these are available in higher ratings of 100 to 800amp at 500V 3-phase.

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CITS : Power - Electrician & Wireman - Lesson 20-25