3.6.2 -  Overload

Overload currents occur in circuits which have no faults but are carrying a higher current than the design value due to overloaded machines, an error in the assessment of diversity, and so on. When a conductor system carries more current than its design value, there is a danger of the conductors, and hence the insulation, reaching temperatures which will reduce the useful life of the system.

The devices used to detect such overloads, and to break the circuit for protection against them, fall into three main categories:
1. - Semi-enclosed (rewirable) fuses to BS 3036 and cartridge fuses for use in plugs to BS 1362.
2. - High breaking capacity (HBC) fuses to BS 88 and BS 1361. These fuses are still often known as high rupturing capacity (HRC) types.
3. - Circuit breakers, miniature and moulded case types to BS EN 60898.

Examination of the characteristics of these devices {Figs 3.13 to 3.19} indicates that they are not the 'instant protectors' they are widely assumed to be. For example, an overloaded 30 A semi-enclosed fuse takes about 100 s to 'blow' when carrying twice its rated current. If it carries 450 A in the event of a fault (fifteen times rated current), it takes about 0.1 s to operate, or five complete cycles of a 50 Hz supply.

HBC fuses are faster in operation, but BS 88 Part 2 specifies that a fuse rated at 63 A or less must NOT operate within one hour when carrying a current 20% greater than its rating. For higher rated fuses, operation must not be within four hours at the same percentage overload. The latter are only required to operate within four hours when carrying 60% more current than their rated value.

Circuit breakers are slower in operation than is generally believed. For example, BS EN 60898 only requires a 30 A miniature circuit breaker to operate within one hour when carrying a current of 40 A. At very high currents operation is described by the BS as 'instantaneous' which is actually within 0.01 seconds.

All protective devices, then, will carry overload currents for significant times without opening. The designer must take this fact into account in his calculations. The circuit must be designed to prevent, as far as possible, the presence of comparatively small overloads of long duration.

The overload provisions of the Regulations are met if the setting of the device:
1. - exceeds the circuit design current
2. - does NOT exceed the rating of the smallest cable protected

In addition, the current for operation must not be greater than 1.45 times the rating of the smallest cable protected.

The overload protection can be placed anywhere along the run of a cable provided there are no branches, or must be at the point of cable size reduction where

Fig 3.12 Time/current characteristics
a) 30 A semi-enclosed fuse     b) 30 A miniature circuit breaker type 3

this occurs. There must be NO protection in the secondary circuit of a current transformer, or other situation where operation of the protective device would result in greater danger than that caused by the overload. Fuses and circuit breakers controlling a small installation are commonly grouped in a consumer's unit at the mains position. Backless types are still available, and they must be filled with a non-combustible back on installation.

There are some circuits which have widely varying loads, and it would be unfortunate if the protection operated due to a severe but short-lived overload. In such cases, the heating effect of the currents must be taken into account so that the overload setting is based on the thermal loading.