This article from NICEIC & ELECSA looks specifically at the requirements and circumstances where the provision of overload protection may be omitted.
Omission of overload protection
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:
● For a conductor:
● on the load side of a point where a reduction in the value of current-carrying 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) (Regulation 433.3.1).
Overload protection of circuits can also be omitted for safety reasons, especially where unexpected disconnection of supply could cause danger or damage. Regulation 433.3.3 gives a number of examples of where this would be appropriate, including:
● the exciter circuit of a rotating machine
● the supply circuit of a lifting magnet
● the secondary circuit of a current transformer
● a circuit supplying a fire extinguishing device
● a circuit supplying safety services, such as a fire alarm or a gas alarm
● a circuit supplying medical equipment used for life support in medical locations where an IT system is incorporated.
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 (Regulation 433.3.1 (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 (Regulation 523.8). An example of this, as shown in Fig 1, would be where part of a cable run for the circuit passes through an area of significantly higher ambient temperature or through thermal insulation.
Rather than installing conductors of a single size which would be oversized for the majority of the circuit length, the cross-sectional area may be increased for only that part of the circuit run where there is a significant temperature increase. In such cases, protection against overload protection is not required at the point of reduction of cross-sectional area where the co-ordination requirements of regulation group 433.1 are met between the smallest conductor and the protective device at the origin of circuit.
Motor circuits (Regulation 433.3.1 (ii)) It is common to place the overload protection of a motor at the ‘load’ end of the circuit and usually 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 not operate when the motor starts as a result of the starting current.
Circuits supplying showers, instantaneous water heaters and limited loads (Regulation 433.3.1 (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 for the circuit is only required for fault protection. Similarly, the flexible cables of pendant lamps and other light fittings connected to a ceiling rose, as shown in Fig 2, will not be exposed to overload current due to the nature of the connected 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.
Consumer’s meter tails (Regulation 433.3.1 (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.
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 (Regulation 430.3). BS 7671 also allows for the omission of overload protection where its provision may present a risk of damage or danger (Regulation 433.3.3), protection is provided by another means, or where it is not possible for an overload to occur (Regulation 433.3.1).
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