Handbook of Electrical Installation Practice - BeKnowledge

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The circuit earth loop impedance
The prospective fault current at the point of installation.
The main advantage fuses have over circuit-breakers is:
• Cost
• BS 88 fuses can be used where there is a high prospective fault current.
The main disadvantages are:
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Control and Protection of Low Voltage Installations 595
They are costly to replace after a fault
Because fuses can readily be replaced and easily interchanged a person may, by
fitting the wrong type of fuse, compromise the safety of the installation.
The main advantage of circuit-breakers is they are easy to reset after a fault and
cannot be tampered with.
Circuit-breaker types
Type B circuit-breakers are mainly suitable for domestic and commercial installations
where there are few or no switching surges, and where multicore cables with
reduced size protective conductors are used.
Type C circuit-breakers are for use in commercial and industrial applications
where there are inductive lighting motor loads with switching surges. For these much
lower circuit earth fault loop impedances are needed to achieve the required disconnection
time and all insulated wiring systems may not be able to achieve the
disconnection time without an rccb.
Type D circuit-breakers are for use in applications with abnormally high inrush
currents, such as main frame computers, welding equipment, transformers and Xray
equipment. Again, all insulated wiring systems may not be able to achieve the
disconnection times.
Allowing for the constraints of prospective fault current at the point of installation
and circuit impedances, fuses and circuit-breakers offer the same quality of
protection and are, therefore, equally safe. As a rule, circuit-breakers are preferable
to fuses where operation, maintenance and replacement is likely to be by
unskilled persons. It is important that the correct circuit-breaker type is selected for
the load and installation conditions but the choice is very often a matter of personal
preference.
Rccbs
These devices should be used where the circuit earth loop impedances are too
high for the required disconnection times, in places where there is an increased risk
of electric shock (damp/wet situations) and where there are TT or IT earthing
systems. Rccbs are often required in addition to fuses or circuit-breakers to provide
shock protection and where high surge currents require slow-acting overcurrent
protection.