Electric power distribution and environment: interference of power installation magnetic fields on computer systems

Electric power distribution and environment:interference of power installation magnetic

®elds on computer systems

Riccardo Tommasini*, Filippo Spertino

Department of Industrial Electrical Engineering, Politecnico di Torino,

Corso Duca degli Abruzzi, 24, 10129 Turin, Italy

Abstract

The present study analyzes the electromagnetic interference produced on visual displayunits (VDUs) in domestic and industrial environments. The main sources of disturbance may

be identi®ed in three-phase lines, unbalanced currents, currents in earthing systems, proximityof power installations, proximity of railway tracks, and presence of harmonics on the neutralconductor. Magnetic-®eld interference for PCs is practically limited to the de¯ection of thecathode-ray tube (CRT) electron beam. The vertical magnetic ®eld determines a horizontal

electron beam motion. The distortion depends linearly on magnetic ®eld intensity and on thedi�erence between the frequency of the CRT vertical raster scan and the frequency of theinterfering magnetic ®eld. Herein are analyzed two actual cases of interference produced by

the DC-supply line of a metropolitan tram network and by the public electric power dis-tribution network. # 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Electro magnetic interference (EMI); VDUs; Magnetic ®elds

1. Introduction

The problem of the direct and indirect environmental e�ects of industrial-fre-quency magnetic ®elds is taking on a growing importance [1,2]. In particular, theextra-low-frequency (ELF) magnetic ®eld produced by sources normally presentthroughout domestic and industrial environments may interfere with computerequipment, and in particular video display terminals, causing the phenomenon ofpicture instability or distortion. In some cases, this phenomenon is heightened by

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the harmonic pollution of the supply current, frequently caused by the computerequipment itself.This study illustrates and discusses the e�ects of interference produced by the

magnetic ®eld on visual display units (VDUs) and their levels of immunity. Thestudy then goes on to indicate and analyze the main sources of disturbance indomestic and industrial environments (three-phase lines, unbalanced currents, cur-rents in earthing systems, proximity of power installations, proximity of railwaytracks, and presence of harmonics on the neutral conductor).Finally, an analysis is carried out on two actual cases of interference, one of which

is produced by the DC supply line of a municipal tram network, and the other by apublic electric-power distribution network.

2. Interference on VDUs

Magnetic ®eld interference on a PC is practically limited to the de¯ection of thecathode-ray tube (CRT) electron beam.Several experimental tests conducted on di�erent types of monitors [3±5],

immersed in a 60 Hz uniform magnetic ®eld showed di�erent types of distortionaccording to the direction of the magnetic ®eld along the three orthogonal-axes. Avertical magnetic ®eld produces horizontal oscillations (jitters) of the picture. Ahorizontal magnetic ®eld parallel to the surface of the screen produces the com-pression and widening of the characters along the row. A horizontal magnetic®eld, which is perpendicular to the surface of the screen, causes a distortion byrotation.The amplitude of the disturbance varies linearly with the intensity of the applied

magnetic ®eld. To provide some indication, a vertical ®eld with a magnetic ¯uxdensity of 25 mT produces an oscillation having an amplitude of one character on arow of 80 characters of screen text.If the raster's vertical-scan has a frequency identical to the frequency of the inter-

fering magnetic ®eld, the distortion of one character width over the entire verticaldimension of the screen may not be noticeable. On the other hand, if the frequencydi�erence is 10 Hz (or 1/10th of a second scroll rate), then the character would appearfuzzy and unreadable. Therefore, it becomes important to know the vertical-scan rateof the monitor and the power-system's frequency. A monitor that is not susceptible inEurope (50 Hz) may not be acceptable in the United States (60 Hz). The vertical scrol-ling rate of the interference is determined by the reciprocal of the di�erence of the fre-quency of the CRT vertical raster scan and the frequency of the interfering magnetic-®eld.

3. Relevant standards

The problem of the interference of industrial-frequency magnetic ®elds on VDUsis covered by a number of national and international standards.

182 R. Tommasini, F. Spertino /Applied Energy 64 (1999) 181±193

Of particular interest is the ECMA (European Computer Manufacturer Associa-tion) Standard 199 (1993) `Ìmmunity of VDUs to power-frequency magnetic®elds'', which is based on the indications provided in the IEC Standard 1000-4-8(1993) `Èlectromagnetic Compatibility (EMC), Part 4: testing and measurementtechniques, Section 8: power-frequency magnetic ®elds: immunity tests'' and the ISOStandard 9241-3 (1992) `Èrgonomic requirements for o�ce work with visual displayterminals (VDTs), Part 3: visual display requirements''.The test ®eld waveform is the power-frequency sinusoid of 50 or 60 Hz depending

on the mains frequency of the market region. Di�erent frequencies are used forVDUs operating in the proximity of railway power-lines.The display terminal is immersed in the magnetic ®eld generated by an apparatus

(induction coil and generator) having the characteristics de®ned by the IEC Stan-dard, IEC 1000-4-8, in two orthogonal directions.The maximum peak-to-peak variation in the geometrical location of the character

(jitter) depends on the character size of the test pattern and may be computed usingthe following formula:

max Jitter �mm� � �character size �mm� � 0:33�=33:3

VDUs are divided into three classes, depending upon the conditions of use. Theseare: Class I Ð Residential/O�ce; Class II Ð Commercial/Light industrial; ClassIII Ð Heavy industrial.For each of the three classes, a di�erent level of test magnetic-¯ux density is

envisaged: 0.5 mT for Class I, 3 mT for Class II, and 10 mT for Class III. To facilitatethe jitter reading, magnetic ¯ux density values 50 times higher may be used. In thiscase, the maximum jitter allowed must be multiplied by 50.

4. Source of interference

The main sources of magnetic disturbance in both domestic and industrial envir-onments may be identi®ed in three-phase lines (with balanced or unbalanced cur-rents), currents in earthing systems, vicinity of power installations (transformers andother components), proximity of railway tracks, and presence of harmonics on theneutral wire. In what follows, the major sources of interference are speci®callyanalyzed.

4.1. Magnetic ®eld generated by electric lines

As is known, the magnetic ®eld produced by a conductor, in which a current In is¯owing, suspended at a height d above the ground, may be calculated by the prin-ciple of electrical images (Fig. 1). Considering a homogeneous ground resistivity �g,and assuming the conductor to be rectilinear and of inde®nite length, the followingorthogonal components of the magnetic-¯ux density vector B in the generic pointP(x,y,z) are obtained:

R. Tommasini, F. Spertino /Applied Energy 64 (1999) 181±193 183

Bxn � ÿ2� 10ÿ3Inyÿ dn� �r2cn

ÿ y� dn � �� r2m

� �Byn � 2� 10ÿ3In

xÿ hn� �r2cn

ÿ xÿ hn� �r2m

� � �1�

where:

rcn ��xÿ hn� �2� yÿ dn� �2

qrm �

��xÿ hn� �2� y� dn � �� 2

q� � 503

��2p ��

�gf

reÿj�=4

and f is the frequency of the current.On account of the symmetry of the case examined, the component along the z axis

is zero, i.e. Bzn=0.

In practical cases, the conductors are more than one, and the total magnetic ¯uxdensity may be evaluated by applying the principle of superposition. In the case of Nconductors, we obtain:

Bx �XNn�1

Bxn

By �XNn�1

Byn

�2�

When rcn << �, i.e. when the magnetic-¯ux density is evaluated at a short distancefrom the conductor (for industrial frequencies and for a ground resistivity �g of

Fig. 1. Rectilinear conductor parallel to the ground and its electrical image.


between 100 and 10,000 m, � is a few hundred metres), the contribution of the imageconductor may be neglected, and the formulaes amount to those of the magneto-staticcase.Magnetic ®elds near conductors are de®ned by a vector with constant direction,

oscillating with the power frequency. The resulting ®eld, close to a three-phase sys-tem, is also characterized by a vector oscillating with the power frequency, butadditionally alternating in direction. The tip of the rotating vector follows an ellipse.The calculation of the magnetic-¯ux density, with relations (1) and (2), shows that

the Bx decays with the square of the distance x from the line (1/x2); the component By

decays according to 1/x, the terms Bx and By being linked to the distance between theconductors having a more rapid decay. In the case where the phase currents arebalanced, the sum of the line currents is zero (i.e. I1+I2+I3=0); the dominant termBy (and, if the conductors are aligned, also the term Bx) goes to zero. The terms linkedto the distance between the conductors thus become predominant, and the magnetic-¯ux density shows a more rapid decay rate according to the distance from the line.

4.2. Current in a neutral wire

In general, in the neutral conductor, there may be present currents that are due tothe supply of non-balanced loads, or else that result from the presence of harmonics[5,6]. In some circuit situations, which may be encountered both in loads of adomestic type and in industrial systems, the neutral current may in part be throughthe earthing system. The resultant current on the active (phases and neutral) con-ductors may thus be other than zero and may be the source of a magnetic ®eldcausing interference.Unbalanced earth currents may occur both in domestic installations supplied by

TT systems (see Case 2 of the present study) and in industrial installations suppliedby TN systems. Fig. 2. shows a user installation supplied by a TN-S-type electricalsystem in which a connection (AB) is present between the neutral conductor and the

Fig. 2. Example of a TN±S distribution system in an industrial plant.


protection conductor (hybrid TN system). Part of the neutral current ¯ows thereforein the protection conductor and part ¯ows in the extraneous conductive parts con-nected to the ground (in the ®gure, a water pipe is shown). A similar situation isproduced in the circuit of Fig. 3, where an electrical system of the TN-C type isrepresented.

4.3. Presence of harmonics

The harmonic distortion of the line current contributes to the increase of the per-turbing e�ects of the magnetic ®eld. Examples of non-linear loads, that can produceharmonic distortion in the industrial environment, are, for instance, industrial fur-naces, ¯uorescent lamps and, in general, all the devices that use an electric arc;drives and control systems that make use of converters; UPSs having an AC/DCinput stage, transformers and electric rotating-machinery. In the civil and tertiarysphere, as well as in lighting systems and dimmers, the main distorting-loads are tobe identi®ed in personal computers, television sets, domestic appliances, and ingeneral in equipment that makes use of electronic-type power supplies. The dis-turbance e�ect is heightened in the case where the distortion produces harmonics of3k order, which, since they are mutually in phase, sum up in the neutral conductor,intensifying the e�ects of the perturbation. Illustrated in what follows is the systemaccording to which distortion occurs in some of the most common cases.

4.3.1. Recti®ers and switch-mode power supplies

Single-phase and three-phase electronic apparata, the input stage of which consistsof a recti®er bridge followed by a capacitive ®lter (for example, the input stage ofmany UPSs), absorb markedly distorted current from the mains network, as shownin Fig. 4.DC circuits of present-day electrical and electronic equipment are normally sup-

plied by devices of a Switch-Mode Power Supply type. Such power supplies consist

Fig. 3. Example of a TN±C distribution system in an industrial plant.


of a recti®er input stage, connected to a DC/AC converter of the chopper type througha capacitance. For this reason, the current absorption of these devices from the line issimilar to the case of the double-semiwave single-phase recti®er, dealt with previously.

4.3.2. Discharge lamps

The operation of these is based upon the maintenance of an electric arc betweenthe poles of the lamp. Owing to the non-linear characteristics of the arc, the voltage atthe terminals of the lamp is very distorted with respect to the sinusoidal supply-vol-tage. This also re¯ects on the waveform of the current, which is found to be a�ectedby a more or less accentuated harmonic distortion, depending on the characteristicsof the lamp and the current-stabilization circuit of the lamp.The harmonic distortion increases considerably for discharge lamps of the com-

pact type i.e. the ones that use power supplies of an electronic type. In this case, thelamp is supplied at high frequency (20±30 kHz) in order to reduce losses and theweight of the power supply and to improve the light e�ciency of the lamp. Suchelectronic power supplies present in fact normally an input stage of an AC/DC type,and thus fall within the case analyzed previously [1].

4.3.3. The proximity of electrical traction systems

In particular metropolitan ones, may produce interference on VDUs. The mag-netic ®eld is created by the current ¯owing in the contact line and its return throughthe tracks [3], or else by the supply lines of the contact line itself (see Case 1 of thepresent study).

5. Cases studied

In what follows, two practical cases of the perturbation of video terminals by themagnetic ®eld generated both by a DC electrical traction network and a public dis-tribution circuit, are illustrated.

5.1. Case 1 Ð metropolitan tram network

In an o�ce building situated in the vicinity of the electric power supply line of themetropolitan tramnetwork, faults were reported on the video terminals. The disturbance

Fig. 4. Current absorbed from the mains by (a) a single-phase bridge; (b) a three-phase bridge.


consisted of picture oscillations on the screen and colour changes lasting a few sec-onds, which were repeated at non-constant intervals ranging from a few dozen sec-onds to a few minutes.In the course of an observation period of ten minutes, a maximum amplitude of

horizontal translation (i.e. a jitter) of 4.5 mm was recorded on a 20-inch SVGAdisplay terminal.The colour variations, with a variable intensity over time, a�ected prevalently

those areas of the screen near the edges. Also the size and disposition of the areaswith altered colour were not constant, but varied slowly over time.A measurement was made of the perturbing magnetic ®eld using a meter equipped

with a Hall-e�ect probe (instrumental uncertainty, 0.5%).The probe, which is set perpendicular to the ground, was ®rst reset to zero by

putting it in a shielded chamber. It was then set sequentially at various measurementpoints, and the resultant magnetic-®eld variation over an observation period of ®veminutes was recorded.Only the vertical component of the magnetic ¯ux density was measured, and hence

the measurement made indicates an underestimate of the environmental magnetic-®eld. The measurement was made, inside the building, at four di�erent pointsmarked by the letters A, B, C and D in Fig. 5. The points A, B, and C are situatedon the ground ¯oor, the point D is in the basement.The results of the measurements are tabulated in Table 1.The magnetic ®eld recorded was produced by a supply line of the tram electric

power network, laid underground at a distance of a few metres from the building, inwhich the unbalanced currents ¯owed between the negative and positive cables.The tram's electric power network runs at a voltage of 600 Vdc, and is supplied by

the medium-voltage network via a current transformation and conversion system.

Fig. 5. Location of the measurement points.


Fig. 6 is a schematic representation of an electric power supply installation and itsconnections to the tram network. The tramcars are connected to the electric powersupply on the one hand by the overhead contact line connected to the positive poleof the converter, and on the other through the tram-lines, which are connected tothe negative pole of the converter.The contact line network is divided into 52 electrically insulated areas, supplied by

14 electric power installations. The tram-line tracks of all the areas are connectedtogether. Some connections moreover link up the two tracks of each line (Fig. 7).The power supply of the contact lines of each area is provided by means of

underground cables (positive cables) which start from the stations and, broadlyspeaking, follow the path of the tracks. The connection to the contact line is madeevery 200±300 m. Also starting from the electric power supply installation are thenegative cables connected to the tracks, which have the same section as the positivecables and a length of approximately 400 m.The negative cables are permanently connected, on the one hand to the network of

the tracks, and on the other to the converter. It is, therefore, possible that part of thecurrent that ¯ows from the positive cables does not close through the negative cablesof the same cable duct, but is taken up by other negative cables that reach theinstallation from di�erent directions.

Table 1

Measurements of magnetic ¯ux density

Point Maximum variation of

magnetic ¯ux density

Maximum estimated

slope �B=�t

A 44 mT 14mT/sB 35 mT 7mT/sC 20mT 5mT/sD 85.5mT 22mT/s

Fig. 6. Schematic representation of the tram-line electric-power supply system.


This phenomenon is represented in schematic form in Fig. 8 in the case of just twonegative cables and one positive cable.The load current (Ic) ¯ows along the positive cable up to the tramcar. Along the

return path, the current divides into two fractions, In1 and In2, which re-closethrough two distinct negative cables, until they join up at the power supply station(of course, in fact all the current that leaves the positive pole of the converter mustin some way re-enter at the negative one).The result is that the conductors inside the cable duct, indicated by the letter A in

Fig. 8, are not traversed by two equal and opposite currents, but by the currents Icand In1 which are in general of di�erent intensities.This imbalance is not constant over time, but varies according to the current

absorption produced by the passage of the tramcars travelling in the area.In fact, the phenomenon here exempli®ed is far more complex in so far as a few

dozen cables are considered. In addition, part of the current does not ¯ow throughthe tracks, but through the ground.However, the terms of the problem do not change, and similar considerations to

those expressed previously likewise apply here.

Fig. 8. Closing of the circuit via negative cables.

Fig. 7. Interconnection of the areas of tram network.


The reduction of the disturbance emitted by the tram network lines is an opera-tion that is feasible but rather laborious and costly.In general, the solutions could consist of one, or a combination, of the following:

. Overhaul of the electric-power distribution system at output from the powersupply stations, for example sectionalizing some negative cables, so as to getthe return current of one area, as far as possible, to ¯ow through the samecable duct as that of the output current from the converter.

. Removal of the cable duct (i.e. source of disturbance), or of some of the con-ductors contained therein, from the area in which the computer equipment isinstalled. The magnetic ®eld is inversely proportional to the distance of thesource causing the disturbance.

. Shielding of the perturbing source (cable duct) or of the ``victims'' (displayterminals). In practice, however, this solution is costly and relatively ine�ec-tive, especially in the case of shielding of the cable duct.

5.2. Case 2 Ð public electric-power distribution network

The pictures on the video terminals in an o�ce presented oscillations of varyingamplitude in the course of the day. This disturbance was independent of theabsorption of electric current of the o�ce itself.The disturbance noted had to be related to the presence of a three-phase power

line, within the building perimeter wall, belonging to the public-utility distributingelectric power.An evaluation of the perturbing magnetic ®eld carried out with a magnetic-coil

magnetic-®eld meter (band width 10±1000 Hz) at the points shown in Fig. 9 indi-cated the values given in Table 2.

Fig. 9. Position of measurement points.


The magnetic ®eld decays roughly inversely proportional to the distance and is rela-ted to an imbalance of currents produced by closure of the neutral current to earth.The electric-power line considered belonged to an electric-power distribution sys-

tem of the TT type, which supplied various loads (Fig. 10). The star point of theMV/LV transformer was earthed inside the transformer substation via the resistanceRN. For greater safety, the neutral conductor was earthed by the distribution utility,also at intervals along its path, via the resistance R0N. When the load RL of the userB absorbs the current IL, one part of the neutral current Ig returns to the generatorthrough the earthing system. The vector sum of the line currents of the user A is thusother than zero, so producing a perturbing magnetic ®eld. Ig depends on the resis-tances RN and R0N and on the length of the power supply line; in addition, Ig mayvary in the course of the day, depending upon the variation of the load RL.In the conductor protecting the electric power network, a current of approxi-

mately 10 A, variable during the course of the day, was recorded.

References

[1] Ghirardi M, Perego P. Lampade a scarica: loro e�etti sulla rete di alimentazione e corretto impiego.

Giornata di studio AEI-ANIE Apparecchi utilizzatori a componenti elettronici. Milan, March 1989.

Table 2

Magnetic-¯ux density values measured inside the building

Position of points Bx[mT] By[mT] Bz[mT] B[mT]

A1 9.25 0.18 3.89 10.03

A2 7.46 0.19 4.56 8.74

A3 5.0 0.57 5.25 7.47

A4 2.94 0.23 5.53 6.27

A5 1.09 0.39 4.98 5.11

B1 9.6 0.1 3.8 10.3

B2 6.4 0.39 4.4 7.77

B3 4.0 0.8 4.7 6.2

B4 2.1 0.99 4.55 5.1

B5 1.2 0.78 4.15 4.38

Fig. 10. Example of a TT distribution system in a residential plant.


[2] Magnetic ®elds from electrical-power lines: theory and comparison to measurements. Report of the

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p. 2127±35.

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Electric power distribution and environment: interference of power installation magnetic fields on computer systems