9A General Requirements
(1) Overcurrent protection for circuit
(2) Examples of overcurrent protective devices (3) Requirements of overcurrent protective devices
9B Relation Between Circuit Conductors and Overcurrent Protective Devices (1) Overload protective devices
(2) Faults current protective devices
9C Breaking Capacity of Overcurrent Protective Devices (1) Overload protective devices
(2) Fault current protective devices 9D Position of Overcurrent Protective Devices
(1) General
(2) Overload protective devices (3) Fault current protective devices
Code 9
OVERCURRENT PROTECTIVE DEVICES
9A General Requirements (1) Overcurrent protection for circuit
Every circuit must be protected by one or more devices for automatic interruption of the supply in the event of overcurrent resulting from: (a) overload, or
(b) fault.
(2) Examples of overcurrent protective devices
The following devices are acceptable as protective devices against overcurrent:
(a) Miniature Circuit Breakers (MCB) (b) Moulded Case Circuit Breakers (MCCB) (c) High Breaking Capacity (HBC) Fuses (d ) Semi-enclosed Fuses
(e) Circuit Breakers incorporating overcurrent release, or in conjunction with fuse.
(3) Requirements of overcurrent protective devices
(a) Overload protective devices and fault current protective devices should satisfy the requirements of Codes 9B, 9C, 9D and 9E. (b) For devices providing protection against both overload current and
fault current, they should satisfy both the requirements of overload protective devices and fault current protective devices.
(c) The characteristics of devices for overload protection should be coordinated so that the energy let-through by the fault current protective device does not exceed that which can be withstood by the overload protective device without damage. For a circuit incorporating a motor starter, this CoP does not preclude the type of co-ordination described in IEC 60947-4-1, in respect of which the advice of the manufacturer of the starter should be sought.
9B Relation Between Circuit Conductors and Overcurrent Protective Devices
(1) Overload protective devices
(a) Overload protective devices should be capable of breaking any overload current flowing in the circuit conductors before such a current could cause a temperature rise detrimental to insulation, joints, terminations, or surroundings of the conductors.
(b) The nominal current or current setting of the devices should not be less than the design current of the circuit.
(c) The nominal current or current setting of the devices should not exceed the lowest of the current carrying capacities of any of the conductors in the circuit.
(d ) The current causing effective operation of the devices should not exceed 1.45 times the lowest of the current carrying capacities of any of the conductors of the circuit.
(Note: (i) If the device is a fuse to BS88 part 2 or part 6 or BS1361 or a circuit breaker to IEC 60898 or equivalent satisfying requirement (c), it is also considered to have satisfied requirement (d ).
(ii) If the device is a semi-enclosed fuse to BS3036, compliance with requirement (d) is afforded if its nominal current does not exceed 0.725 times the current carrying capacity of the lowest rated conductor in the circuit protected.)
(e) When the same protective device protects conductors in parallel, other than that of ring circuits, the value for ‘the lowest of the current carrying capacities’ mentioned in subparagraphs (c) and (d) may be taken as the sum of the current carrying capacities of those conductors in parallel provided that those conductors:
(i) are of the same construction, material and cross-sectional area, and are approximately the same length, and appropriate phase disposition;
(ii) have no branch circuits throughout their length; and (iii) are arranged so as to carry substantially equal currents. (2) Fault current protective devices
(a) Fault current protective devices should be capable of breaking any fault current in the conductors of each circuit before such current could cause danger due to thermal and mechanical effects produced in conductors and connections.
(b) The devices should be able to interrupt all currents caused by a fault occurring at any point of the circuit in a time not exceeding that which brings the cable conductors to their limiting final temperature. (Note: Table 9(1) gives the limiting final temperatures for some common
materials.)
9C Breaking Capacity of Overcurrent Protective Devices (1) Overload protective devices
Overload protective devices may have a breaking capacity below the value of the prospective fault current at the point where the device is installed provided that such devices are protected against fault current. (2) Fault current protective devices
(a) Fault current protective devices should have a breaking capacity not less than the prospective fault current at the point where the device is installed except if the following requirement in subparagraph (b) applies.
(b) The device may be permitted to have a lower breaking capacity provided that another protective device having the necessary breaking capacity is installed on the supply side. The characteristics of the devices should be coordinated so that the energy let-through of these two devices will not cause damage to the load side device and the conductors protected by these devices.
(c) The breaking capacities of protective devices against fault current should be assessed for all installations. Table 9(2) shows the minimum breaking capacities for general guidance only.
9D Position of Overcurrent Protective Devices (1) General
Overcurrent protective devices should be located in places readily accessible for maintenance.
(2) Overload protective devices
(a) Overload protective devices, subject to subparagraph (b), should be placed where a reduction occurs in the value of current carrying capacity of the conductors of the installation.
(b) The device may be placed at any point along the run of those conductors provided that the part of run between the point where the value of current carrying capacity is reduced and the position of the protective device has no branch circuits or outlets for the connection of current using equipment.
(3) Fault current protective devices
(a) Fault current protective devices, subject to subparagraph (b) and (c), should be placed where a reduction occurs in the value of current carrying capacity of the conductors of the installation.
(b) The device may be placed at any point along the run of those conductors provided that between the point where the value of
current carrying capacity is reduced and the position of the protective device provided that the conductors are:
(i) not exceed 3m in length, and
(ii) be erected in such a manner as to reduce the risk of fault, fire or danger to persons to a minimum.
(c) The device may be placed at a point other than specified in subparagraph (a) provided that the conductors between the device and the point of reduction in current carrying capacity are adequately protected against fault current according to Code 9B(2) by a fault current protective device installed on the supply side of the point of reduction.
9E Other Requirements of Overcurrent Protective Devices (a) Overcurrent protective devices should be placed in enclosure that are
free from easily ignitable materials.
(b) Every overcurrent protective device should be provided on or adjacent to it an indication of its intended nominal current as appropriate to the circuit it protects.
(c) Fuses which are likely to be removed or replaced whilst the circuits they protect are energised, should be of a type such that they can be thus removed or replaced without danger.
(d ) Suitable tools for safe withdrawal of fuses at a fuse board should be provided where necessary.
(e) Where circuit breakers may be operated by persons other than a registered electrical worker, they should be designed or installed so that it is not possible to modify the setting or the calibration of their overcurrent releases without a deliberate act involving either the use of a key or tool. A visible indication of the setting or calibration is recommended.
(f ) Operating handles of circuit breakers should be made accessible without opening any door or cover giving access to live parts. (g) All linked circuit breakers for overcurrent protection of equipment
run on polyphase supply must be purposely designed by the manufacturer to enable breaking of all related phase conductors simultaneously. Any modified miniature circuit breaker to achieve as a linked circuit breaker is not acceptable.
(h) When a consumer’s main switch or circuit breaker is connected directly to the electricity supplier’s distribution transformer, the overcurrent protection should discriminate with the electricity supplier’s high voltage protection settings.
Table 9(1)
Limiting Final Temperatures for Common Materials
Conductor material Insulation material
Assumed initial temperature °C Limiting final temperature °C Copper 70°C thermoplastic (general purpose pvc) 70 160/140*
90°C thermoplastic (pvc) 90 160/140* 60°C thermosetting (rubber) 60 200 85°C thermosetting (rubber) 85 220 90°C thermosetting 90 250 Impregnated paper 80 160 Copper Mineral
—plastic covered or exposed to touch 70 (sheath) 160 —bare and neither exposed to touch nor
in contact with combustible materials
105 (sheath) 250 Aluminium 70°C thermoplastic (general purpose pvc) 70 160/140*
90°C thermoplastic (pvc) 90 160/140*
60°C thermosetting (rubber) 60 200
85°C thermosetting (rubber) 85 220
90°C thermosetting 90 250
Impregnated paper 80 160
* Where two values of limiting final temperature are given the lower value relates to cables having conductors of greater than 300 mm2 cross-sectional area.
Table 9(2)
Minimum Breaking Capacities of Overcurrent Protective Devices
Types of supply to which the protective devices are connected
Current rating of back-up fuses (if provided) to
BS 88 or equivalent
Minimum three phase breaking capacities of the
protective devices (i) Supply directly taken from the
transformer within the premises in which the installation is situated.
no back-up fuse fitted 40 kA
not exceeding 160A 4.5 kA (with back-up fuses)
exceeding 160A but not
exceeding 400A 23 kA (with back-up fuses) (ii) Supply tapped from busbar rising
mains (for cable rising mains, the breaking capacities may be smaller in value depending on the design)
not exceeding 160A 4.5 kA (with back-up fuses)
exceeding 160A but not
exceeding 400A 23 kA (with back-up fuses) no back-up fuse fitted not less than the
prospective fault current shown in Table 9(3) (iii) Supply taken from electricity
supplier’s service box or overhead line
not exceeding 160A 4.5 kA (with back-up fuses)
exceeding 160A but not
exceeding 400A 18 kA (with back-up fuses) (Note: The single phase breaking capacity should be assessed by registered electrical workers of
Table 9(3)
Approximate Prospective Fault Current at Tap-off Positions of Busbar Rising Mains Installation in
kA (kilo-Amperes)
Length of Busbars (metres)
Rating of Rising Mains
200–300A 400–500A 600–800A 1000–2500A
kA kA kA kA 10 24 26 30 33 13 22 25 30 33 16 21 24 30 33 19 20 23 28 33 22 18 22 28 33 25 18 22 28 33 28 17 21 26 30 31 16 20 26 30 34 15 20 26 30 37 15 19 26 30 40 14 18 24 30 43 13 18 24 30 46 13 17 24 30 49 12 17 24 30 52 12 17 24 28 55 11 16 24 28 58 11 16 22 28 61 11 15 22 28 64 10 15 22 28 67 10 15 22 28 70 10 14 22 28 73 9 14 22 28 76 9 14 20 26 79 9 13 20 26 82 9 13 20 26 85 8 13 20 26 88 8 13 20 26 91 8 12 20 26 94 8 12 20 26 97 7 12 20 26
(Note: The information and values given in this Table are intended for general guidance only as the prospective fault current will vary for different kinds of busbars)
Table 9(4)
Classification of MCB to IEC 60898 according to the instantaneous tripping current
Type Instantaneously Tripping Current
B C D 3 In < I 5 In 5 In < I 10 In 10 In < I 20 In
Note: The information above is extracted from IEC 60898-1 ed.1.2