Electromagnetic Effects for Cables a.c. Circuits

Contents

1. Introduction

2. Generation of heat

3. Single-core cables armoured with steel wire or tape not to be used

4. Conductors installed in a ferromagnetic enclosure

5. Conductors entering a ferromagnetic enclosure

1. Introduction

Electromagnetic effects from incorrectly installed cables of alternating current(a.c.) circuits can cause heat to be generated in the metal of ferromagnetic enclosures, such as steel conduit or steel equipment housings. To prevent such heat, which may damage cables and other materials, Regulation 521-02-01 of BS 7671 calls for the following requirements to be met:

(i) Single-core cables armoured with steel wire or tape must not be used for a.c. circuits.

(ii) Conductors of a.c. circuits installed in ferromagnetic enclosures (such as steel conduit, trunking or ducting) must be arranged so that the conductors of all phases and the neutral conductor (if any) and the appropriate protective conductor of each circuit are contained in the same enclosure.

(iii) Where such conductors enter a ferromagnetic enclosure, they must be arranged so that they are not individually surrounded with ferromagnetic material, or other provision must be made to prevent eddy (induced) currents.

Requirements (i), (ii) and (iii) above are explained in items 3, 4 and 5, respectively, of this topic.

Item 2 of this topic explains the mechanism by which heat can be generated in ferromagnetic materials surrounding conductors carrying alternating current.

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