Moulded case circuit breaker

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 ... 

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

Moulded case circuit breakers and moulded case switches up to 1000 V a.c. Rating more than 5000 A Moulded ease circuit breakers and their application, up to 1000 V a.c. Rating 5000 A and more Low-voliage cartridge fuses... 

Micro-computer based systems Moulded Case Circuit Breakers... 

Switchgear tends to be operated infrequently, whereas motor control centres operate frequently as required by the process that uses the motor. Apart from the incomers and busbar section circuit breakers, the motor control centres are designed with contactors and fuses (or some types of moulded case circuit breakers in low voltage equipment) that will interrupt fault currents within a fraction of a cycle of AC current. Circuit breakers need several cycles of fault current to flow before interruption is complete. Consequently the components within a circuit breaker must withstand the higher forces and heat produced when several complete cycles of fault current flow. 

The outgoing switching device in a high current, high fault level, switchboard will usually be a power circuit breaker if it feeds more that about 400 amps to the load. Below 400 amps the circuit could have a fuse-contactor combination, see sub-section 7.3.2 for comments on contactors and Chapter 8 on fuses. Therefore if the outgoing device must be a circuit breaker then the comments and discussion in sub-section 7.2.3 above apply. Low voltage switchboards often use moulded case circuit breakers... 

Moulded case circuit breakers are available in two basic modes of operation, current limiting and non-current limiting. It is difficult to design a moulded case circuit breaker to have a cut-off characteristic that is less than 0.01 second when a fully asymmetrical short-circuit current flows. However, there are such circuit breakers available, and care is needed when selecting these devices for a circuit that has a high prospective fault current. Some manufacturers are able to provide a cut-off in the order of 0.006 second. 

Moulded case circuit breakers are also available for incoming and busbar section purposes, with ratings up to 6000 A and service voltages between 220 V and 660 V. (At 415 V a 4000 A circuit breaker would satisfy the duty of a 2500 kVA feeder transformer with about 15% spare capacity.) These are also available as 4-pole units. Circuit breakers having ratings of 800 A and above are often provided with several adjustments that widely modify the shape of the complete protection curve, as described in Chapter 12. This enables the curve to coordinate with almost any other protective device or equipment that is immediately upstream or downstream of the circuit breaker. Some circuit breakers with the higher rated currents are also provided with integral earth fault protection facilities. 

Various external attachments can be fitted to moulded case circuit breakers, e.g. pad-locking tabs, shunt trip coils, hazardous area enclosure with an on and off operating handle, withdrawable rack mountings, undervoltage tripping units, auxiliary switches of the normally open and normally closed types, interlocking devices, ambient temperature compensation for the protection curves. Some... 

When fuses or moulded case circuit breakers are applied to a circuit it is necessary to ensure that their /-squared-t characteristics coordinate properly with the thermal capabilities of the downstream equipment, especially the cables. In order to determine the /-squared-t characteristics of a protective device it is assumed that the current in the device suddenly changes from a normal load value to the fault value in a very short period of time, i.e. similar to a step change in a control system. Hence for each value of current along the x-axis of the device s time-current characteristic the value of the current squared multiplied by the corresponding time can be plotted. For cables and busbars the /-squared-t function equals a constant (k) for each cross-sectional area of conductor, as explained... 

Figure 7.6 Cut-off and prospective current curves for a 40 A fuse and a 40 A moulded case circuit breaker.

Euses or moulded case circuit breakers have known current-time functions and for practical purposes these functions can be simply converted into their /-squared- characteristics by using the above method on as many sample points as can be conveniently transcribed. Figures 7.7 and 7.8 show the 1-t and the corresponding derived /-squared- characteristics for 32 A and 125 A fuses, 32 A and 125 A moulded case circuit breakers, and appropriate cables for the circuit. 

Where moulded case circuit breakers are chosen for a plant in favour of fuses the coordination of cascaded units becomes a little more difficult than with fuses. This difficulty arises from the fact that these circuit breakers have a definite or near definite minimum time limit to their time-current characteristic. This causes the lower part of the circuit breaker protection curve to be almost horizontal at a low value of time, typically in the range of 0.003 and 0.01 second. 

Figure 7.7 Clearing time versus fault current for fuses and moulded case circuit breaker curves.

Type of protection e.g., fuses, moulded case circuit breakers. 

It is therefore common practice to use 5 seconds in (9.7) as the disconnection time for motor cables. This choice also corresponds with standardised data given by the manufacturers of fuses and moulded case circuit breakers for their let-through current as calculated from their graphical data. 

Many modem installations favour the use of moulded case circuit breakers instead of fuses and separate relays. Monlded case circuit breakers are available with basic functions for small motors and more sophisticated fnnctions for large motors. 

The necessity for an instantaneous tripping function is the same as for a high voltage motor. This function can be provided by a magnetic repulsion device within the moulded case circuit breaker, by a (50) relay or by upstream fuses. If fuses are used then the contactor must be capable of carrying the duty until the fuse completes its function. To minimise the stressing of the contactor it should be coordinated with the fuses as recommended in IEC60947 Part 2, as a Type 2 requirement. 

The range of settings for moulded case circuit breakers is typically between 5 and 30 times the nominal current. The lower values e.g. 5, 7.5 and 10 are often fixed for a particular circuit breaker, whilst the higher values are adjustable. 

The purpose of negative phase sequence protection is the same as for high voltage motors. It is not normally needed for motor ratings less than approximately 22 kW. In the simpler designs of moulded case circuit breakers and relays the negative phase sequence detection is more in the form of single-phase protection, wherein a phase is completely lost. 

Some relays, for example as used with low voltage high current air circuit breakers and moulded case circuit breakers, have many adjustments to their parameters. Manufacturers often... 

The maximnm rating of any one of the loads mnst be limited by the rapid tripping or fnsing time of the device that protects the circnit. Fnses fnnction better than moulded case circuit breakers in these sitnations becanse they are not limited by a definite minimum time constraint. To ensnre that the protective device fnnctions rapidly the rated cnrrent of this device must be limited to about 30% of the rated current of the inverter npstream. Hence the ratio of load kVA to inverter kVA of each load snb-circnit will be a maximnm of abont 25%. 

Some British Standard numbers are written as BS EN followed by a number. This means that the British Standard is in harmony or agreement with the European Standards. These standards apply to all countries in the EEC, for example BS EN 60898 1991 is the British and European Standard for miniature and moulded case circuit breakers which were issued in 1991. 

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. 

Protection against overcurrent and short circuit current is required and is mainly provided by circuit breakers fitted with overload trips. At the initial planning stage the supply company should be asked for the prospective fault level at the supply intake so that adequately rated equipment may be selected. At this position, if a switchfuse is used to control the installation and the BS 88 FIBC fuses specified in BS EN 60439-4 are employed, there should be no problem as the breaking capacity is not less than 80 k A at 400 V. If a moulded case circuit breaker is used, however, it is necessary to select one of adequate fault breaking capacity as they are made for a range of ratings. 

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. 

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