Devices which provide overcur-
rent protection are:
● High breaking capacity (HBC) fuses 88-6.
● MCBs 60898. These are miniature circuit breakers (MCBs)
which may used alternative fuses for some installations.
The selected protective device should have current rating which not
less than the full load current the circuit but which does not exceed the
cable current rating. These were previously called re-
wirable fuses and are used mainly domestic installations having a
maximum fault capacity about 4kA. both cases the basic requirements for
protection are that the fault currents should interrupted quickly and the
circuit isolated safely before the fault current causes temperature rise or
mechanical effects which might damage the insulation, connections, joints
and terminations the circuit conductors their surroundings (IEE
Regulation 130. These are for indus-
trial applications having maximum fault capacity 80kA. short
circuit overcurrent resulting from fault negligible impedance con-
nected between conductors.Basic Electrical Installation Work
240
● the current carrying capacity the cable,
● the disconnection time requirements for the circuit.
● Cartridge fuses 1362.c.
Definitions
An overload current can defined
as current which exceeds the rated
value otherwise healthy circuit.3).
. The cable then fully protected against both overload
and short-circuit faults (Regulation 435.1). These are used 13A plug tops and have
a maximum fault capacity about 6kA.
● Semi-enclosed fuses 3036. Short-circuit currents may hundreds times
greater than the rated current.
The essential requirements for device designed protect against over-
current are:
● must operate automatically under fault conditions,
● have current rating matched the circuit design current,
● have disconnection time which within the design parameters,
● have adequate fault breaking capacity,
● suitably located and identified.
We will look these requirements below. An
overload current can defined current which exceeds the rated value
in otherwise healthy circuit. circuits industrial
and domestic installations having fault capacity about 30kA.
● Cartridge fuses 1361. Overload currents usually occur because the
circuit abused because has been badly designed modified.
An overload may result currents two three times the rated current
flowing the circuit. Short circuits usually occur result an
accident which could not have been predicted before the event. These are used for a.
A short circuit overcurrent result-
ing from fault negligible imped-
ance connected between conductors.
An overcurrent may overload current, short-circuit current
