3.6.5 -  Protecting conductors

The prime function of overload protection is to safeguard conductors and cables from becoming too hot. Thus the fuse or circuit breaker rating must be no greater than that of the smallest cable protected. Reference to the time/current characteristics of protective devices {Figs 3.13 to 3.19} shows that a significantly greater current than the rated value is needed to ensure operation.

Thus, the current at which the protective device operates must never be greater than 1.45 times the rating of the smallest cable protected. For example, consider a cable system rated at 30 A and protected by a miniature circuit breaker type 3, rated at 32 A. Reference to (Fig 3.17 shows that a prolonged overload of about 38 A will open the breaker after about 104 seconds (about two and a half hours!). The ratio of operating current over rated current is thus 38/30 or 1.27, significantly lower than the maximum of 1.45. All circuit breakers and HBC fuses listed in {3.6.2 sections 2 and 3} will comply with the Regulations as long as their rating does not exceed that of the smallest cable protected.

Semi-enclosed (rewirable) fuses do not operate so closely to their ratings as do circuit breakers and HBC fuses. For example, the time/current characteristics of

Fig 3.16 Time/current characteristics for some miniature circuit breakers Type 1

Fig 3.17 Time/current characteristics for some miniature circuit breakers Type 3.
Type C -MCBs have very similar characteristics to Type 3

{Fig 3.13} show that about 53 A is needed to ensure the operation of a 30 A fuse after 10,000 s, giving a ratio of 53/30 or 1.77. For rewirable fuses, the Regulations require that the fuse current rating must not exceed 0.725 times the rating of the smallest cable protected. Considering the 30 A cable protected by the 32 A miniature circuit breaker above, if a rewirable fuse replaced the circuit breaker, its rating must not be greater than 0.725 x 30 or 21.8 A.

Since overload protection is related to the current-carrying capacity of the cables protected, it follows that any reduction in this capacity requires overload protection at the point of reduction. Reduced current-carrying capacity may be due to any one or more of:

Fig 3.18 Time/current characteristics for some miniature circuit breakers Type B

Fig 3.19 Time/current characteristics for some miniature circuit breakers Type D

1. - a reduction in the cross-sectional area of the cable
2. - a different type of cable
3. - the cable differently installed so that its ability to lose heat is reduced
4. - a change in the ambient temperature to which the cable is subjected
5. - the cable is grouped with others.

{Figure 3.20} shows part of a system to indicate how protection could be applied to conductors with reduced current carrying capacity.

Fig 3.20 Position and rating of devices for overload protection

In fact, the calculated fuse sizes for {Fig 3.20a)} of 72.5 A, 21.75 A and 7.25 A are not available, so the next lowest sizes of 60 A, 20 A and 5 A respectively must be used. It would be unwise to replace circuit breakers with semi-enclosed fuses because difficulties are likely to arise. For example, the 5 A fuse used as the nearest practical size below 7.25 A is shown in {Fig 3.13} to operate in 100 5 when carrying a current of 10 A. Thus, if the final circuit is actually carrying 10 A, replacing a 10 A circuit breaker with a 5 A fuse will result in the opening of the circuit. The temptation may be to use the next semi-enclosed fuse size of 15 A, but that fuse takes nearly seven minutes to operate at a current of 30 A. Clearly, the cable could well be damaged by excessive temperature if overloaded.

The device protecting against overload may be positioned on the load side of (downstream from) the point of reduction, provided that the unprotected cable length does not exceed 3 m, that fault current is unlikely, and that the cable is not in a position that is hazardous from the point of view of ignition of its surroundings. This Regulation is useful when designing switchboards, where a short length of cable protected by conduit or trunking feeds a low-current switch fuse from a high current fuse as in {Fig 6.2}.

All phase conductors must be protected, but attention must be paid to the need to break at the same time all three line conductors to a three-phase motor in the event of a fault on one phase, to prevent the motor from being damaged by 'single-phasing'. Normally the neutral of a three phase system should not be broken, because this could lead to high voltages if the load is unbalanced. Where the neutral is of reduced size, overload protection of the neutral conductor may be necessary, but then a circuit breaker must be used so that the phases are also broken.