Co-ordination between conductors and overload

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Co-ordination between conductors and overload protective devices

At first sight, many engineers and electricians may find the requirements of Regulation 433.1.1 for co-ordination between the conductors of a circuit and an overload protective device a little daunting. This article aims to explain the reasons behind these requirements and to demonstrate that, in the majority of cases, compliance with them will be achieved by following accepted cable sizing procedures.

Regulation 433.1.1 indicates that the operating characteristics of a device protecting a conductor against overload must satisfy the following conditions: (i) The rated current or current setting of the protective device (I n) is not less than the design current (Ib) of the circuit (that is, I n ≥ Ib), and (ii) the rated current or current setting of the protective device (I n ) does not exceed the lowest of the current-carrying capacities (I z) of any of the conductors of the circuit (that is, I n ≤ I z), and (iii) the current (I2) causing eff ective operation of the protective device does not exceed 1.45 times the lowest of the current-carrying capacity (I z) of any of the conductors of the circuit (that is, I2 ≤ 1.45 I z ).

Reasons behind conditions (i), (ii) and (iii) of Regulation 433.1.1

The object of condition (i) is to avoid unwanted operation of the overload protective device when it is carrying the steady-state load current (Ib) of the circuit. This is achieved by requiring the rated current of the protective device (I n ) to be at least equal to Ib. (Note, however, that a higher value of I n may be necessary for loads with high starting or inrush currents – see Regulation 533.2.1).

Condition (ii) is intended to give protection under normal load conditions such that the steady-state temperature of the conductors will not exceed the maximum admissible value (such as 70 °C for thermoplastic insulated conductors). This is achieved by requiring the conductors to have suffi cient current-carrying capacity (I z) to carry the rated current of the circuit protective device (I n) (and hence to carry the design current of the circuit (Ib)).

Condition (iii) is intended to give overload protection when the circuit carries overload current, by causing operation of the protective device within a time suitable to avoid damage to the insulation and surroundings of the conductors due to high conductor temperatures caused by the overload current.

Small overloads of long duration

Regarding conditions (i) and (ii), it is important that the circuit is designed so that small overloads of long duration are unlikely to occur (Regulation 433.1 refers). In other words, care should be taken that the amount of simultaneously operating load connected to the circuit (Ib), and the current rating of the conductors (I z) and protective device (I n), are such that there is unlikely to be a prolonged overload current lower than the current causing eff ective operation of the protective device (I2).

Meeting the requirement of condition (iii)

Provided the requirements of conditions (i) and (ii) are met, the requirement of condition (iii) is deemed to be met where the protective device is a type (gG) fuse to BS 88-2, a fuse to BS 88-3, a circuit-breaker to BS EN 60898 or BS EN 60947-2, or an RCBO of any type to BS EN 61009-1. Regulation 433.1.100 refers.

Where the protective device is a semi-enclosed (rewireable) fuse to BS 3036, the requirement of condition (iii) is met where I n does not exceed 0.725 times the current-carrying capacity of the circuit conductors (I z), provided the requirements of conditions (i) and (ii) are also met. Regulation 433.1.101 refers. This is to compensate for the high value of I2 of a BS 3036 fuse, which can be up to 2 × I z .

For cables either buried directly in the ground or buried in ducts where current-carrying capacity is based on an ambient temperature of 20 °C, compliance with condition (iii) is aff orded where I n does not exceed 0.9 times I z, provided the requirements of conditions (i) and (ii) are also met. Regulation 433.1.102 refers.

Cable sizing procedures in Appendix 4 of BS 7671

Section 5 of Appendix 4 in BS 7671 lays out the procedure for determining the size of live conductors that will generally give compliance with the requirements of Regulations 433.1.1 to 433.1.102, discussed in this article.

Where overcurrent protective devices other than the fuses and circuit-breakers referred to in Regulations 433.1.100 and 433.1.101 are used, such as a thermal overload protective device in a motor circuit, it will be necessary for the electrical installation designer to refer to the device

Table 1 – Requirements of Regulation 433.1.1

characteristics in order to ensure compliance with all aspects of Regulation 433.1.1.

Worked example

A circuit supplying a 30 A single-phase load is wired using fl at twin and earth 70 °C thermoplastic insulated and sheathed cable installed ‘clipped direct’ (tabulated current-carrying capacities given in column 6 of Table 4D5 of Appendix 4 of BS 7671). The circuit cable is not grouped with any other loaded cables (C g = 1.0), the ambient temperature is 30 °C (C a = 1.0), the cable is not installed in thermal insulation (Ci = 1.0), and the overload protective device is a 30 A BS 3036 fuse (Cf = 0.725). Rating factors C c and Cd, relating to buried cables, do not apply, and hence their value is 1.0.

Use the procedures described in Appendix 4 of BS 7671 to determine the minimum size of cable required for the circuit from considerations of normal load and overload. Also, for the purpose of this example only, show that the circuit meets the requirements of Regulation 433.1.1 when this size of cable is used.

Using Equation 1 of section 5.1.1 of Appendix 4:

As indicated in section 5.1.1 of Appendix 4, the size of cable to be used is to be such that its tabulated current-carrying capacity (It) is not less than that given by Equation 1 (41.4 A, in this case). Fig 1 – Diagram of proposed circuit

From column 6 of Table 4D5 of Appendix 4, the minimum size of cable for which It is not less than 41.4 A is 6 mm2 (It = 47 A).

Thus, using the procedures described in Appendix 4, the minimum required size of cable from considerations of normal load and overload is 6 mm2 .

Now, checking that this size of cable meets the requirements of Regulation 433.1.1 (see Table 1, above), the proposed circuit can be represented by Fig 1, above.