Friday, March 25, 2011

Miniature and Molded-Case Circuit Breaker

5. Description of different Circuit Breaker
5. 1. Air-break Circuit-breaker
5.1.2 Miniature and Molded-Case Circuit Breaker
       These are used extensively in low voltage domestic, commercial and industrial applications. They replace conventional fuses and combine the features of a good HRC fuse and a good switch. Figure 5 gives the internal details of a thermal magnetic miniature circuit breaker. For normal operation, it is used as a switch. During over loads or faults, it automatically trips off. The tripping mechanism is actuated by magnetic (part 7) and thermal sensing devices (part5) provide within the MVB.
       Tripping mechanism and the terminal contacts (part4) are assembled in a molded case, molded out of thermosetting powders. They ensure high mechanical strength, high dielectric strength and virtually no ageing. The current carrying parts (part3) are made out of electrolytic copper or silver alloy depending upon the rating of the breaker. All other metal parts are of non-ferrous, non rusting type. sufficient cross section for the current carrying parts is provided to ensure low temperature rise even under high ambient temperature environment. The arc chute (part8) has a special construction which increase the length of the arc by the magnetic field created by the arc itself and the arc chute is so placed in the breaker that the hot gases may not come in contact with any of the important parts of the breaker.
       The breakers has unit construction whereby multiple pole breakers can be made by assemble of single pole breakers.
Figure 5   The internal details of a 10 ampere European DIN rail mounted thermal-magnetic circuit breaker

Figure contents :
  1. Actuator lever - used to  manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On or Off /tripped). Most breakers are designed so they can still trip even if the lever is held or locked in the On position. This is sometimes referred to as "free trip" or "position trip" operation.
  2. Actuator mechanism - forces the contacts together or apart.
  3. contacts - Allow current to flow when touching and break the flow of current when moved apart.
  4. Terminals
  5. Bimetallic strip
  6. calibration screw - Allows the manufacture to precisely adjust the trip current of the device after assembly.
  7. Solenoid
  8. Arc divider / extinguished
Typical Rating of MCB :
       Current Rating : 5, 10, 15, 20, 30, 40, 50, 60 Amp.
                    also, 0.5, 0.75, 1, 2, 2.5, 3, 3.5, 6, 7.5, 8, 10, 12, 25, 35, 45, 55 Amp.
       Voltage Rating : 240 V/415 V AC; 50 V/11 V DC
       Rupturing Capacity :       AC: 3 KA at 415 V
                                             DC: 3 KA at 50 V ( non-inductive),
                                             1 KA at 110 V (non-inductive).

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