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ELECTROMAGNETIC EFFECTS
Cables for a.c. circuits
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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.) 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.
2.
Generation of heat
When a conductor, such as a single-core cable, carries alternating current, the conductor is
encircled by a magnetic field that alternates (changes in magnetic polarity) at the same
frequency as the current. Where the conductor is also surrounded with metal, the
alternating magnetic flux ‘cuts’ the metal, inducing voltages in it which in turn cause
circulating currents – called eddy currents – to flow (see Fig 1). The eddy currents
produce power in the metal, due to its electrical resistance. In a single-phase or
three-phase circuit the magnetic fields cancel each other out unless one or more of the
conductors (phase or neutral) is outside the enclosure, in which case eddy currents will be
produced.
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ELECTRICAL SAFETY COUNCIL TECHNICAL MANUAL
October 2007
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Cables for a.c. circuits
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Where metal surrounding a conductor (or some of the conductors) of an a.c. circuit is
ferromagnetic, such as iron or steel, the power dissipated in the metal as a result of the
eddy currents can cause the metal to heat up. This is because, with a ferromagnetic
metal, the flux density of the alternating magnetic field is greater than with a
non-ferromagnetic metal, such as aluminium or copper, due to the high relative
permeabilities of ferromagnetic metals. Hence the eddy currents, and consequently the
power dissipated in the metal, are greater if the metal is ferromagnetic, making the metal
liable to heat up. If the metal is non-ferromagnetic, no noticeable heating of it due to eddy
currents will occur.
Metal surrounding a single conductor carrying alternating current
Warning!
This arrangement is not permitted
by BS 7671 where the metal surrounding
the conductor is ferromagnetic (such as steel)
Metal surrounding
conductor (eg tube)
Alternating
magnetic field
Conductor
Typical eddy current loop
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 an
a.c. circuit collectively, even if the metal is ferromagnetic. This is because, in such
circumstances, the overall sum of the magnetic fields encircling the conductors is equal to
zero. In this context, it should be noted that BS 7671 regards the circuit protective
conductor as one of the circuit conductors (unless formed by the enclosing metal, such as
conduit), as it has to carry current in certain circumstances.
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ELECTRICAL SAFETY COUNCIL TECHNICAL MANUAL
October 2007
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ELECTROMAGNETIC EFFECTS
Cables for a.c. circuits
3.
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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 steel
wire or tape for a.c. circuits. Such use would automatically result in conductors being
individually surrounded in ferromagnetic material, with the consequent risk of the
armouring 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 to
contain conductors of a.c. circuits, Regulation 521-02-01 requires the conductors of all
phases and the neutral conductor (if any) and the appropriate protective conductor of each
circuit to be contained in the same enclosure. This is so that the overall sum of the
magnetic 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 ferromagnetic
enclosure taking the cables to a lighting switch position. However, so long as all the
associated unswitched and switched phase conductors are contained in the same
enclosure, as shown in Fig 2, the magnetic fields add up to zero, and hence no eddy
currents (or heat) are produced in the metal of the enclosure.
Lighting switch wiring contained in a ferromagnetic enclosure
Neutral
Ferromagnetic enclosure
(eg steel conduit)
Phase
Luminaires
All associated unswitched
and switched phase conductors
contained in the same enclosure
Lighting switch
(2 - gang)
Fig 2
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ELECTRICAL SAFETY COUNCIL TECHNICAL MANUAL
October 2007
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ELECTROMAGNETIC EFFECTS
Cables for a.c. circuits
5.
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Conductors entering a ferromagnetic enclosure
Where conductors of an a.c. circuit enter a ferromagnetic enclosure, Regulation 521-02-01
requires that one of the following two arrangements is provided:
•
The conductors are arranged so that they are not individually surrounded with
ferromagnetic material. An example of this is given in Fig 3, which shows cables
supplying an item of switchgear having a ferromagnetic enclosure, entering the
enclosure from steel trunking though a single entry hole.
•
Some other provision is made to prevent eddy currents. For example, the
conductors may enter the enclosure through separate holes of a plate made of either
non-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 ferromagnetic
plate (such as steel) having a slot which joins the holes to each other, so that they
effectively form a single hole (see Fig 5). It should be remembered that where a
plate 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 to
meet the requirements of Regulation Group 412-03 ‡.
Cables all passing through the same entry hole
Item of switchgear
having a ferromagnetic
enclosure
L1
L2
L3
N
Steel trunking
Fig 3
*
Most stainless steels are not ferromagnetic but some are.
The application of a magnet will determine
whether or not a particular sample is ferromagnetic.
†
The material should be selected to meet the requirements of Regulation 526-03-02 for enclosure of
connections.
‡
To satisfy Regulation Group 412-03, the width of a slot must be less than 1 mm (IP4X) if in a readily
accessible top surface, or not more than 12 mm (IP2X or IPXXB) if in any other surface.
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ELECTRICAL SAFETY COUNCIL TECHNICAL MANUAL
October 2007
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Cables for a.c. circuits
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Cables passing through separate holes of a non-ferromagnetic metal plate or a non-metallic plate
Conductors (sheathed
with non-ferromagnetic material)
Plate of non-ferromagnetic metal
or non-metallic material
Ferromagnetic enclosure
N
L3
L2
L1
Fig 4
Cables passing through separate holes of a ferromagnetic metal plate having a slot
Conductors (sheathed
with non-ferromagnetic material)
Slotted ferromagnetic plate
L1
N
L3
L2
Ferromagnetic enclosure
Slot joining holes
Fig 5
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October 2007
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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
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ELECTRICAL SAFETY COUNCIL TECHNICAL MANUAL
521-02
October 2007
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