Handbook of Electrical Installation Practice - BeKnowledge

Excess current can be in the form of either overload current or fault current. An
overcurrent occurs when a conductor’s load current is greater than its installed
current rating. A fault current occurs when there is a short-circuit between live conductors
or when there is an insulation breakdown or failure between a phase
conductor and an earthed conductor.
Generally, all circuits and equipment have to be protected against the effects of
overload and fault currents, earth leakage and conditions that may cause dangerous
electric shocks. Under certain conditions, it is possible to dispense with overload
protection if the equipment connected to the circuit is unable to create an
overload condition.
In many installations overcurrent devices can provide protection against all of
the conditions. With other installations, essentially those with TT or IT earthing
systems, residual current devices (rcds) have to be used for earth leakage and shock
protection.
The effectiveness of the protective device is measured by the time it takes to
operate under overload or fault conditions. The operating time is dependent on the
impedances of the system’s circuit conductors and protective conductors.
Circuit protective devices may be either fuses or circuit-breakers. A fuse is
designed to be destroyed by the element melting at a current in excess of its rated
current, without given time parameters. A circuit-breaker is a mechanical device
with switching contacts designed to open at a current in excess of its rated current,
or under certain conditions, when there is imbalance between live conductors, again
within given time parameters. For these devices to be effective they must operate
rapidly to cut off the current before danger can arise.
Types of protective devices
The following types of protective devices can be used:
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Control and Protection of Low Voltage Installations 591
Overcurrent and earth leakage: fuses and circuit-breakers
Earth leakage only: residual current circuit-breakers (rccbs)
Combination units: combined overcurrent and residual current circuit-breakers.
(rccbs)
All protective devices have to comply with the relevant British Standards. British
Standards set out the requirements to which a supplier must design, build and test
the equipment. This includes:
• Conditions for in-service operation
• Classification
• Characteristics:
rated voltage, current and frequency, rated power dissipation, rated power
acceptance of fuse holders
• Limits of time–current characteristics:
breaking range and breaking capacity, cut-off current and I2t characteristics
• Markings