NICEIC & ELECSA look at the general requirements of BS 7671 relating to protection against overload current, whether provided in isolation or in conjunction with fault current protection.
Basic requirements for overcurrent protection
A protective device must be provided in order to disconnect any overcurrent in live (that is, line and neutral) circuit conductors before such current could cause danger due to thermal or electromechanical effects which may be detrimental to insulation, connections, joints, terminations or the materials immediately surrounding the conductors (430.3).
Fig 1. Co-ordination between a conductor and an overhead protective device
The term overcurrent encompasses currents resulting from both fault and overload conditions. BS 7671 permits both the omission of overload (see Regulation group 433.3), or fault protection (Regulation group 434.3) in certain prescribed situations. It is common however, for a single protective device to provide both types of overcurrent protection in a circuit.
Where this is the case, if the overload protective device meets the co-ordination requirements of Regulation group 433.1 and has a rated short-circuit breaking capacity not less than the prospective fault current at that point in the circuit then, in general, it can be assumed that the overload device can also provide the necessary fault protection for conductors downstream from its point of installation.
Such co-ordination of overload and fault current protection is not necessarily provided in the case of fault current protection of conductors in parallel (434.4), or where non-current-limiting circuit-breakers are used. Where either of the above applies, compliance should be confirmed on a case-by-case basis (435.1).
In most cases, where a neutral conductor has a cross-sectional area at least equivalent to that of the associated line conductors and the current in the neutral is not expected to exceed that in the line conductors, it is not necessary to provide either overcurrent detection or a disconnecting device in the neutral conductor (431.2.1).
However, overcurrent detection – as opposed to disconnection – is required in a neutral conductor of a polyphase circuit in situations where the harmonic content of the associated line currents may cause the current in the neutral conductor to exceed its as-installed current-carrying capacity. The overcurrent detection must cause disconnection of the line conductors, but not necessarily the neutral conductor (431.2.3).
Co-ordination between a conductor and an overload protective device
To prevent the persistence of a small overload of long duration the following conditions must be met:
(i) The rated current or current setting of the protective device (In) must not be less than the design current (Ib) of the circuit, and
(ii) The rated current or current setting of the protective device (In) must not exceed the lowest of the current-carrying capacities (Iz) of any conductor in the circuit, and
(iii) The current causing effective operation of the protective device (I2) should not be more than 1.45 times the current-carrying capacities (Iz) of any conductor in the circuit (433.1.1).
Regulation 433.1.201 states, where conditions (i) and (ii) are met (and Ib ≤ In ≤ Iz), condition (iii) will also be met where the following protective devices are employed:
● general-purpose (gG) fuse to BS 88-2
● fuse to BS 88-3
● circuit-breaker to BS EN 60898 or BS EN 60947-2
● RCBO to BS EN 61009-1.
Additionally, where a circuit is protected by a rewireable fuse to BS 3036, condition (iii) will be met where the rated current (In) of the fuse does not exceed 0.725 times the lowest of the current-carrying capacities (Iz) of any conductor of the circuit.
In ≤ 0.725 x Iz (433.1.202)
For cables buried, either directly in the ground or within ducts, where tabulated current-carrying capacity is based on an ambient temperature of 20°C, compliance with condition (iii) is achieved where the rated current or current setting of protective device (In) does not exceed 0.9 times the lowest of the current-carrying capacities (Iz) of any conductor of the circuit (433.1.203).
A ring final circuit is deemed to comply with the requirements of Regulation 433.1.1 if the following conditions are met:
● Accessories such as socket-outlets and connection units to BS 1363 are used, and
● The circuit is protected by a 30/32 A device complying with the BS 88 series, BS 3036, BS EN 60898, BS EN 60947-2 or BS EN 61009-1, and
● Copper line and neutral conductors are used, having a cross-sectional area of either: – 2.5 mm2, or – in the case of mineral insulated cables, 1.5 mm2, and
● The lowest of the current-carrying capacities (Iz) of any conductor of the circuit corrected for ambient conditions is not less than 20 A, and
● The load current in any part of the circuit is unlikely to exceed the current-carrying capacity of the cable for long periods (433.1.204).
Fig 2. Overload protective device positioned at the point of reduction in current-carrying capacity of conductors
Devices providing protection against both overload and fault current
Any device providing both overload and fault current protection, except as permitted by Regulation 434.5.1, must be capable of breaking, and for a circuit-breaker making, any overcurrent up to and including the maximum prospective fault current at the point where the device is installed (432.1). Utilising a single device to provide both overload and fault protection is not always a practical proposition. In such cases, a designer may choose to use different devices for the two functions.
Devices providing protection against overload only
A device providing protection against overload current only may have a rated short-circuit breaking capacity lower than the prospective fault current at its point of installation, although the operating characteristics of the device must be co-ordinated with the conductor being protected in accordance with Section 433 of BS 7671 as described previously in this article (432.2).
Location of overload protective devices
In general, a device for overload protection is required at the point where a reduction occurs in the current-carrying capacity of the conductors because of a change in cross-sectional area, the type of cable/conductors used and method of installation, or changes in environmental conditions (433.2.1).
If there are no outlets or spurs after the reduction in cross-sectional area, Regulation 433.2.2 permits the installation of the protective device along the run of that conductor provided that:
● Protection against fault current is provided, or
● The length of run before the overload protection device does not exceed 3m, and the circuit is installed in a manner that reduces to a minimum the risk of:
– a fault, and
– fire or danger to persons.
Summary
A protective device must be provided to break any overcurrent as a result of an overload or fault in the circuit conductors before the overcurrent causes damage to insulation, connections, joints, terminations or the surroundings of the conductors (430.3). In prescribed circumstances, BS 7671 permits protection against fault currents and overloads to be provided by a single protective device or by separate means (Section 432).
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