Mccb ratings and general specification

Molded case circuit breakers (MCCBs), insulated case circuit breakers (ICCBs),and power circuit breakers(PCBs) are used by electrical engineers in power distribution system designs primarily to protect low-voltage electrical equipment and circuits. Safe application requires an understanding of their performance and ratings. Proper application of these devices can ensure continuity of power during system disturbances. So we must know basic MCCB ratings and spesification
A. General Standard for MCCB

1. MCCBs will be manufactured to meet the general requirements of IEC/EN60947-2
2. MCCBs will have isolation characteristics to fully satisfy the requirements of IEC/EN60947-2and main switch characteristics to fully satisfy the requirements of IEC/EN 60 204-1
3. MCCBs will satisfy the requirement for “Protective Measures” as defined in chapters 41 – 48of IEC/EN 60 364. That is:

• Protection against thermal influences
• Protection of cables and wiring against over-current
• Protection against under-voltages
• Provision for isolation and switching
• Provision for the application of additional protective measures
• Selectivity of protective measures determined by external influences

B. Ratings mccb
a). Voltage Ratings

• • Ue = Rated operational voltage
• • Ui = Rated insulation voltage
• • Uimp = Rated impulse withstand voltage

b).Current ratings

• • In = Rated current
• • Ith = Conventional free –air thermal current
• • Ithe = Conventional enclosed thermal current

c). Short Circuit ratings

• • Icm = Rated short circuit making capacity
• • Icu = Rated ultimate short circuit breaking capacity
• • Ics = Rated service short circuit breaking capacity
• • Icw = Rated short time withstand current

d). Trip unit ratings

• • Ir = Adjustable overload setting current
• • 1.05xIr = Conventional non –tripping current
• • 1.30xIr = Conventional tripping current
• • Isd = Short time delay tripping setting current
• • Ii = Instantaneous tripping setting current