3.6.4 -  Circuit breakers

Circuit breakers operate using one or both of two principles. They are:

1. - Thermal operation relies on the extra heat produced by the high current warming a bimetal strip, which bends to trip the operating contacts,

2. - Magnetic operation is due to the magnetic field set up by a coil carrying the current, which attracts an iron part to trip the breaker when the current becomes large enough.

Fig 3.15 Time/current characteristics of cartridge fuses to BS 88 Part 2

Thermal operation is slow, so it is not suitable for the speedy disconnection required to clear fault currents. However, it is ideal for operation in the event of small but prolonged overload currents. Magnetic operation can be very fast and so it is used for breaking fault currents; in many cases, both thermal and magnetic operation are combined to make the circuit breaker more suitable for both overload and fault protection. It must be remembered that the mechanical operation of opening the contacts takes a definite minimum time, typically 20 ms, so there can never be the possibility of truly instantaneous operation. A typical time/current characteristic for a circuit breaker is shown {Fig. 3.12(b)}

All circuit breakers must have an indication of their current rating. Miniature circuit breakers have fixed ratings but moulded case types can be adjusted. Such adjustment must require the use of a key or a tool so that the rating is unlikely to be altered except by a skilled or instructed person.

There are many types and ratings of moulded case circuit breakers, and if they are used, reference should be made to supplier's literature for their characteristics. Miniature circuit breakers are manufactured in fixed ratings from 5 A to 100 A for some types, and in six types, type B giving the closest protection. Operating characteristics for some of the more commonly used ratings of types 1, 3, B and D are shown in {Figs 3.16 to 3.19}. The characteristics of Type C circuit breakers are very similar to those of Type 3.

BS3871, which specified the miniature circuit breakers Types 1 to 4 was withdrawn in 1994 and has been replaced with BS EN 60898:1991 (EN stands for "European norm"), although it is possible that circuit breakers to the old standard will still be on sale for five years from its withdrawal. In due course, it is intended that only types B, C and D will be available, although it will be many years before the older types cease to be used. Short circuit ratings for the newer types will be a minimum of 3 kA and may be as high as 25 kA - the older types had short circuit ratings which were rarely higher than 9 kA.

The time/current characteristics of all circuit breakers {Figs 3.16 to 3.19} have a vertical section where there is a wide range of operating times for a certain current. Hence, with a fixed supply voltage, the maximum earth fault loop impedance is also fixed over this range of time. The operating current during the time concerned is a fixed multiple of the rated current. For example, a Type 2 MCB has a multiple of 7 (from {Table 3.3}) so a 30 A device of this type will operate over the time range of 0.04 s to 8 s at a current of 7 x 30 A = 210 A.

Table 3.3 Operating time ranges and current multiples for MCBs
over fixed current section of characteristic

MCB Type	Range of operating times (s)	Current multiple of rating
1	0.04 to 20	x4
2	0.04 to 8	x7
3	0.04 to 5	x10
B	0.04 to 13	x5
C	0.04 to 5	x10
D	0.05 to 3	x20

Table 3.4 A comparison of types of protective device

Semi-enclosed fuses	HBC fuses	Miniature circuit breakers
Very low initial cost	Medium initial cost	High initial cost
Low replacement cost	Medium replacement cost	Zero replacement cost
Low breaking capacity	Very high breaking capacity	Medium breaking capacity

Table 3.5 Comparison of miniature circuit breaker types

Type	Will not trip in	Will trip in	Typical application
	100ms at rating	100ms at rating
1	2.7 x	4 x	Low inrush currents (domestic installations)
2	4 x	7 x	General purpose use
3	7 x	10 x	High inrush currents (motor circuits)
B	3 x	5 x	General purpose use (close protection)
C	5 x	10 x	Commercial and industrial applications with fluorescent fittings
D	10 x	50 x	Applications where high in-rush currents are likely (transformers, welding machines)

{Table 3.4} shows a comparison of the three main types of protective device in terms of cost, whilst {Table 3.5} compares the available types of MCB.