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ELECTROMAGNETIC EFFECTS E E105-1Cables for a.c. circuits

Electrical Safety Council ELECTRICAL SAFETY COUNCIL TECHNICAL MANUAL October 2007 1 of 6

ELECTROMAGNETIC EFFECTS - Cables for 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.) circuitscan cause heat to be generated in the metal of ferromagnetic enclosures, such as steelconduit or steel equipment housings. To prevent such heat, which may damage cablesand other materials, Regulation 521-02-01 of BS 7671 calls for the following requirementsto 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 steelconduit, trunking or ducting) must be arranged so that the conductors of all phasesand the neutral conductor (if any) and the appropriate protective conductor of eachcircuit are contained in the same enclosure.

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

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

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

2. Generation of heat

When a conductor, such as a single-core cable, carries alternating current, the conductor isencircled by a magnetic field that alternates (changes in magnetic polarity) at the samefrequency as the current. Where the conductor is also surrounded with metal, thealternating magnetic flux cuts the metal, inducing voltages in it which in turn causecirculating currents called eddy currents to flow (see Fig 1). The eddy currentsproduce power in the metal, due to its electrical resistance. In a single-phase orthree-phase circuit the magnetic fields cancel each other out unless one or more of theconductors (phase or neutral) is outside the enclosure, in which case eddy currents will beproduced.



Where metal surrounding a conductor (or some of the conductors) of an a.c. circuit isferromagnetic, such as iron or steel, the power dissipated in the metal as a result of theeddy currents can cause the metal to heat up. This is because, with a ferromagneticmetal, the flux density of the alternating magnetic field is greater than with anon-ferromagnetic metal, such as aluminium or copper, due to the high relativepermeabilities of ferromagnetic metals. Hence the eddy currents, and consequently thepower dissipated in the metal, are greater if the metal is ferromagnetic, making the metalliable to heat up. If the metal is non-ferromagnetic, no noticeable heating of it due to eddycurrents will occur.

Metal surrounding a single conductor carrying alternating current

Fig 1

Note: Eddy currents (as shown in Fig 1) are still produced if the metal surrounds some, but not all, of the

conductors of an a.c. circuit, as the magnetic fields surrounding the conductors do not then cancel each other out.

Eddy current heating of metal does not occur if it surrounds all the conductors of ana.c. circuit collectively, even if the metal is ferromagnetic. This is because, in suchcircumstances, the overall sum of the magnetic fields encircling the conductors is equal tozero. In this context, it should be noted that BS 7671 regards the circuit protectiveconductor as one of the circuit conductors (unless formed by the enclosing metal, such asconduit), as it has to carry current in certain circumstances.

Conductor

Typical eddy current loop

Alternating magnetic field

Metal surroundingconductor (eg tube)

Alternating current

Warning!

This arrangement is not permittedby BS 7671 where the metal surrounding the conductor is ferromagnetic (such as steel)



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

Regulation 521-02-01 does not permit the use of single-core cables armoured with steelwire or tape for a.c. circuits. Such use would automatically result in conductors beingindividually surrounded in ferromagnetic material, with the consequent risk of thearmouring being heated up as a result of eddy currents.

4. Conductors installed in a ferromagnetic enclosure

Where a ferromagnetic enclosure, such as steel conduit, trunking or ducting, is used tocontain conductors of a.c. circuits, Regulation 521-02-01 requires the conductors of allphases and the neutral conductor (if any) and the appropriate protective conductor of eachcircuit to be contained in the same enclosure. This is so that the overall sum of themagnetic fields encircling the contained conductors when carrying a.c. current will be zero,so there will be no resulting eddy current heating of the enclosure.

The rule may appear to be broken in the case of a steel conduit or other ferromagneticenclosure taking the cables to a lighting switch position. However, so long as all theassociated unswitched and switched phase conductors are contained in the sameenclosure, as shown in Fig 2, the magnetic fields add up to zero, and hence no eddycurrents (or heat) are produced in the metal of the enclosure.

Lighting switch wiring contained in a ferromagnetic enclosure

Fig 2

Luminaires

Lighting switch(2 - gang)

Phase

Ferromagnetic enclosure (eg steel conduit)

All associated unswitched and switched phase conductorscontained in the same enclosure

Neutral



5. Conductors entering a ferromagnetic enclosure

Where conductors of an a.c. circuit enter a ferromagnetic enclosure, Regulation 521-02-01requires that one of the following two arrangements is provided:

The conductors are arranged so that they are not individually surrounded withferromagnetic material. An example of this is given in Fig 3, which shows cablessupplying an item of switchgear having a ferromagnetic enclosure, entering theenclosure from steel trunking though a single entry hole.

Some other provision is made to prevent eddy currents. For example, theconductors may enter the enclosure through separate holes of a plate made of eithernon-ferromagnetic metal (such as aluminium, brass or some types of stainless steel*)or non-metallic material (see Fig 4), or through separate holes of a ferromagneticplate (such as steel) having a slot which joins the holes to each other, so that theyeffectively form a single hole (see Fig 5). It should be remembered that where aplate containing such a slot is used to protect against contact with live parts(protection against direct contact), the width of the slot must be small enough tomeet the requirements of Regulation Group 412-03.

Cables all passing through the same entry hole

Fig 3

* Most stainless steels are not ferromagnetic but some are. The application of a magnet will determinewhether or not a particular sample is ferromagnetic.

The material should be selected to meet the requirements of Regulation 526-03-02 for enclosure ofconnections.

To satisfy Regulation Group 412-03, the width of a slot must be less than 1 mm (IP4X) if in a readilyaccessible top surface, or not more than 12 mm (IP2X or IPXXB) if in any other surface.

Steel trunking

Item of switchgearhaving a ferromagneticenclosure

L1 L2 L3 N



Cables passing through separate holes of a non-ferromagnetic metal plate or a non-metallic plate

Fig 4

Cables passing through separate holes of a ferromagnetic metal plate having a slot

Fig 5

Conductors (sheathed with non-ferromagnetic material)

Plate of non-ferromagnetic metalor non-metallic material

Ferromagnetic enclosure

L1L2

L3N

Conductors (sheathed with non-ferromagnetic material)

Slotted ferromagnetic plate

Ferromagnetic enclosure

L1L2

L3N

Slot joining holes



Topics referred to in this text:

None

Topics not referred to in this text, which are related and may be of interest:

None listed

BS 7671 (Requirements for electrical installations)Some of the most important requirements are found in:

Cables for a.c. circuits electromagnetic effects 521-02

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