INTRODUCTION OF DIFFERENT CURVES IN MCBs

Peoples are confused at some point while buying MCBs for protraction in there house/ office/industries etc.

What is meant by B, C, D, K and Z curves in MCBs?

MCB is a device designed to protect a circuit from short circuits and over currents. Trip curves of MCB's (B, C, D, K and Z curves) tell us about the trip current rating of Miniature Circuit breakers. Trip current rating is the minimum at which the MCB will trip instantaneously. It is required that the trip current must persist for 0.1s.

An MCB with trip curve class B means that the MCB trips at as soon as the current rises above 3 to 5 times its rated current In.   Similarly, MCB with trip curve class C means that the MCB trips at as soon as the current rises above 5 to 10 times its rated current In and so on..

In some applications, frequent current peaks occur for a very short period (100ms to 2s). For such applications class K type fuses shall be used. Class K type fuses are used in circuits with semiconductor devices. 

TRIP CURVE CLASS B:         Above 3 to 5 times rated current. Suitable for cable protection.

TRIP CURVE CLASS C:        Above 5 to 10 times the rated current. Suitable Domestic and residential                                                              applications and electromagnetic starting loads with medium starting                                                                   currents

TRIP CURVE CLASS D:   Above 10(excluding 10) to 20 times the rated current. Suitable for inductive                                                       and motor loads with high starting currents.

TRIP CURVE CLASS K:    Above 8 to 12 times the rated current. Suitable for inductive and motor                                                               loads with high inrush currents.

TRIP CURVE CLASS Z:    Above 2 to 3 times the rated current. These type of MCBs are highly                                                                     sensitive to short circuit and are used for protection of highly sensitive                                                                 devices such as   semiconductor devices.

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What should be a Marking On Circuit Breakers...?

Marking On Circuit Breakers

Each circuit-breaker shall be marked in a durable manner.

a)  The following data shall be marked on the circuit-breaker itself or on a nameplate or nameplates attached to the circuit-breaker, and located in a place such that they are visible and legible when the circuit-breaker is installed;

1. Rated current (In).
2. Suitability for isolation, if applicable, with the symbolic Indication of the open and closed positions, with O and I respectively, if symbols are used.

b) The following data shall also be marked externally on the circuit-breaker, except that they need not be visible when the circuit-breaker is installed;

1. Manufacturer’s name or trade mark;
2. Type designation or serial number;
3. IEC 60947-2 if the manufacturer claims compliance with this standard;
4. Utilization category;
5. Rated operational voltage
6. Rated impulse withstand voltage;
7. Value (or range) of the rated frequency
8. Rated service short-circuit breaking capacity at the corresponding rated voltage;
9. Rated ultimate short-circuit breaking capacity at the corresponding rated voltage
10. Rated short-time withstand current, and associated short-time delay, for utilization category B;
11. Line and load terminals, unless their connection is immaterial;
12. Neutral pole terminals, if applicable, by the letter N;
13. Protective earth terminal, where applicable, by the symbol
14. Reference temperature for non-compensated thermal release, if different from 30 ‘C.

c) The following data shall either be marked on the circuit-breaker as specified in item b), or shall be made available in the manufacturer’s published information:

1. Rated short-circuit making capacity,
2. Rated insulation voltage, if higher than the maximum rated operational voltage,
3. Pollution degree if other than 3;
4. Conventional enclosed thermal current if different from the rated current,
5. IP Code, where applicable
6. Minimum enclosure size and ventilation data (if any) to which marked ratings apply;
7. Details of minimum distance between circuit-breaker and earthed metal parts for circuit-breakers intended for use without enclosures;
8. Suitability for environment A or environment B, as applicable,
9. R.M.S. sensing, if applicable

D)  The following data concerning the opening and closing devices of the circuit-breaker shall be placed either on their own nameplates or on the nameplate of the circuit-breaker; alternatively, if space available is insufficient, they shall be made available in the manufacturer’s published information:

1. Rated control circuit voltage of the closing device and rated frequency for alternating current
2. Rated control circuit voltage of the shunt release and/or of the under-voltage release, and rated frequency for Alternating current;
3. Rated current of indirect over-current releases;
4. Number and type of auxiliary contacts and kind of current, rated frequency and rated voltages of the auxiliary switches, if different from those of the main circuit.
5. Terminal marking.

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