This article looks specifically at circumstances where the provision of overload protection may be omitted.
Paul Collins | Technical Director
In general, a device for overload protection is required at the point where a reduction occurs in the current carrying capacity of the conductors of the installation. However, except where a location presents a risk of fire or explosion, overload protection need not be provided:
BS 7671 permits the variation of conductor size where heat dissipation varies along the length of a circuit but requires the current-carrying capacity of the cable to be appropriate for the part of the route where it is installed (523.8). An example of this is where part of a circuit passes through an area of significantly higher ambient temperature or through thermal insulation (see Fig 1).
Rather than installing conductors of a single size which would be oversized for the majority of the circuit length, cross-sectional area may be increased for only that part of the circuit run where this is necessary.
In such cases, protection against overload protection is not required at the point of reduction of crosssectional area where the coordination requirements of regulation group 433.1 are met between the smallest conductor and the protective device at the origin of circuit.
Motor circuits (433.3.l(ii))
It is common to place the overload protection of a motor at the 'load' end of the circuit as an integral part of the starting equipment. The circuit supplying the starter is therefore incapable of being overloaded. Fault protection is provided by the overcurrent protective device (OCPD) at the origin of the circuit. The benefit of such an arrangement is that certain OCPDs, if correctly selected/set to provide overload protection will operate when the motor starts as a result of the starting current.
Circuits supplying showers, instantaneous water heaters and limited loads (433.3.l(ii))
Circuits supplying 'electric' showers and other instantaneous water heaters cannot by their nature cause an overload. As a result, overload protection is not necessary and the OCPD need only provide fault protection.
Similarly, the flexible cables of pendant lamps and other light fittings connected to a ceiling rose will not be exposed to overload current due to the nature of the load and so overload protection does not need to be provided despite the reduction in conductor size and current-carrying capacity when compared to that of the fixed wiring of the lighting circuit (see Fig 2).
Consumer's meter tails (433.3.l(iii))
Overload protection does not need to be provided for the meter tails between the supplier's metering equipment and the first piece of consumer's equipment where the conditions specified by the distributor are met; see indent (iii) of Regulation 433.3.1 (see Fig 3).
A protective device must be provided to break any overcurrent in the circuit conductors before the overcurrent causes damage to insulation, connections, joints, terminations or the surroundings of the conductors (430.3).
BS 7671 also allows for the omission of overload protection where its provision may present a risk of damage or danger (433.3.3), protection is provided by another means, or where it is not possible for an overload to occur (433.3.1).
- For a conductor:
- on the load side of a point where a reduction in the value of currentcarrying capacity occurs if the conductor is effectively protected against overload by a protective device installed on the supply side of that point, or
- which, because of the characteristics of the load or the supply, is not likely to carry overload current;
- Where the Distributor agrees that their overload device(s) provide(s) overload protection between the origin and the main distribution point of the installation (so long as overload protection is provided at that point) (433.3.1).
Examples of omission of overload protection for all or part of a circuit in accordance with regulation 433.3.1
Varying conductor sizes in a circuit (433.3.l(i))BS 7671 permits the variation of conductor size where heat dissipation varies along the length of a circuit but requires the current-carrying capacity of the cable to be appropriate for the part of the route where it is installed (523.8). An example of this is where part of a circuit passes through an area of significantly higher ambient temperature or through thermal insulation (see Fig 1).
Rather than installing conductors of a single size which would be oversized for the majority of the circuit length, cross-sectional area may be increased for only that part of the circuit run where this is necessary.
In such cases, protection against overload protection is not required at the point of reduction of crosssectional area where the coordination requirements of regulation group 433.1 are met between the smallest conductor and the protective device at the origin of circuit.
Motor circuits (433.3.l(ii))
It is common to place the overload protection of a motor at the 'load' end of the circuit as an integral part of the starting equipment. The circuit supplying the starter is therefore incapable of being overloaded. Fault protection is provided by the overcurrent protective device (OCPD) at the origin of the circuit. The benefit of such an arrangement is that certain OCPDs, if correctly selected/set to provide overload protection will operate when the motor starts as a result of the starting current.
Circuits supplying showers, instantaneous water heaters and limited loads (433.3.l(ii))
Circuits supplying 'electric' showers and other instantaneous water heaters cannot by their nature cause an overload. As a result, overload protection is not necessary and the OCPD need only provide fault protection.
Similarly, the flexible cables of pendant lamps and other light fittings connected to a ceiling rose will not be exposed to overload current due to the nature of the load and so overload protection does not need to be provided despite the reduction in conductor size and current-carrying capacity when compared to that of the fixed wiring of the lighting circuit (see Fig 2).
Consumer's meter tails (433.3.l(iii))
Overload protection does not need to be provided for the meter tails between the supplier's metering equipment and the first piece of consumer's equipment where the conditions specified by the distributor are met; see indent (iii) of Regulation 433.3.1 (see Fig 3).
Summary
A protective device must be provided to break any overcurrent in the circuit conductors before the overcurrent causes damage to insulation, connections, joints, terminations or the surroundings of the conductors (430.3).BS 7671 also allows for the omission of overload protection where its provision may present a risk of damage or danger (433.3.3), protection is provided by another means, or where it is not possible for an overload to occur (433.3.1).
