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)
3. - Circuit
breakers, miniature and moulded case types to BS EN 60898.
Examination of the characteristics of these
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
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
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