Circuit breakers MCCBs

Moulded case circuit breaker (MCCB) (Courtesy GE Power Controls)... 

The main switchboard is called a Main Distribution Assembly (MDA) and consists of an on load isolating switch which feeds a set of busbars to which are connected a number of moulded case circuit breakers (MCCBs) (see Fig. 11.2). For smaller sites and/or where it may be convenient, the ISA and MDA can he joined to form a combined incoming supply and main distribution assembly (ISDA). In this case the on load isolator is omitted. 

Electrical installations, whether temporary or permanent, together with electrical equipment and apparatus, should be protected by a range of measures including residual current devices (RCDs) which protect human beings from electrical shock, fuses and miniature moulded case circuit breakers (MCCBs) which provide protection to apparatus from overheating and short circuits. Arrangements should be in place to ensure that all electrical work to installations and/or electrical equipment is undertaken by only trained, qualified and competent electricians who are familiar with the systems involved. 

Varistor installation for auxiliary relays in reclosure. Independent molded case circuit breakers (MCCB) installation for dc circuits in reclosure. 

MCCB (moulded case circuit breaker) or MCB (miniature circuit breaker) and contactor] and the overload relay. These recommendations permit damage of components on fault to varying degrees as noted below ... 

If the 50 mm cable and a MCCB rating of 125 amps are chosen then the circuit earth loop impedance is still too high by a reduced ratio of about 1.19 1. Hence an earth leakage circuit breaker is still required. 

EVC Emergency Voice Communications MCCBs Mouided Case Circuit Breakers... 

A breaker, usually an MCCB or an MCB on an LT system, can be provided with backup HRC fuses to enhance their short-time rating. This may be done when the available MCCBs or MCBs possess a lower short-time rating than the fault level of the circuit they are required to protect, and make them suitable for the fault level of the circuit. But this is not a preferred practice and is seldom used. As a rule of thumb, the device that is protecting must be suitable to withstand electrically and endure mechanically the system fault current for a duration of one or three seconds, according to the system design. 

To summarize, HRC fuses or fuse-free MCCBs for LT and a breaker and IDMT relay arranged for 2 O/C and 1 G/F, with a short-circuit unit for HT capacitor banks, will provide a reliable protective scheme for short-circuit and ground fault protections. 

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