Full text of DOI:10.1093/oxfordjournals.rpd.a006697

Radiation Protection Dosimetry
Vol. 97, No 4, pp. 401–404 (2001)
Nuclear Technology Publishing
EVALUATION OF LONG-TERM EXPOSURE TO THE
MAGNETIC FIELD PRODUCED FROM POWER LINES
G. Licitra, N. Colonna and C. Chiari
ARPAT Provincial Department of Livorno
Via Marradi 114, Livorno, Italy
Abstract — For some years, the Italian Regional Agencies for Environmental Protection (ARPA) have been engaged in the
evaluation of long-term exposure to 50 Hz magnetic fields for receptors close to power lines. In this study, experiences and
procedures are suggested in order to estimate long-term exposure levels in the case of magnetic induction produced contempor-
aneously by different power lines. In the simplest cases, the influence of a single line is evaluated by verifying the correlation
factor between the variation of the current in each line and the measured magnetic induction. When the prevalence of a single
line is not clear and systematic, much additional data (voltage, current, phases and geometrical configuration) have to be con-
sidered. The case of a school in Livorno near two lines, placed on the same supports, is described and a range of the most
probable exposure values is given, corresponding to the phase change of the two current fluxes.
INTRODUCTION
actual evaluation of the short-term exposure (days or
week), but it cannot be extended to a longer period (for
example a year). Calculation models are quite quick and
reliable, but they require much electric, technical and
geometrical input data and have to be validated by pro-
per measurements.
In the simplest situations — as for points close to just
one power line (simple tern) — it is possible to correlate
the measured magnetic induction values with the current
circulating on the line during the measurements. As a
consequence it is possible, by simple proportion, to
extrapolate the yearly exposure level from the yearly
average current on the line. An intrinsic and not elimin-
able limit of these inquiries is that it is only possible to
foresee the values of expected current on a power line
by relying on the statistical analysis of the recorded cur-
rents in previous years.
In the past twenty years epidemiological studies(1)
have pointed to a possible correlation between child-
hood leukaemia and prolonged exposure, at low levels,
to ELF fields. Even if these studies were not definitive
and did not recognise cause and effect relationships, in
Italy, as well as in other European countries, there is
growing pressure to limit exposure, chiefly where chil-
dren remain for long periods (e.g. schools, nurseries,
funfairs, etc.). Recent Italian regulations have fixed
maximum sanitary limits, attention values and quality
objectives. Moreover, such decrees provide for the issue
of guidelines for measuring magnetic induction (for
example CEI(2)) and highlight the necessity of estimat-
ing magnetic induction levels before power lines are
built. In order to verify the compatibility of its insertion
in the territory it is fundamental to calculate the yearly
exposure levels near specific buildings or in wide areas.
A recent study(3), showed the lack of suitable calculation EXPOSURE LEVELS FROM SEVERAL LINES
models for evaluating long-term exposure from various
power lines at the same place.
The simplest case
In this study, experiences and procedures are sug-
gested in order to estimate long-term exposure levels in
complex situations. In particular, the case is reported of
a school situated under a span of two power lines sup-
ported on the same poles, even though they are absol-
utely independent.
When the magnetic induction levels from several
power lines can be attributed to only one line, the influ-
ence of the single line is determined by verifying the
correlation factor between the trends of the current on
each line and the measured magnetic induction. An
almost direct dependence should be highlighted between
the trend on a single line and the induction values. Such
dependence can be verified by a linear best-fit procedure
and, by computing the correlation factor, it is possible
to estimate the long-term exposure value by the yearly
average value of the current on that line.
This procedure is useful when data and information
are inadequate for an in-depth analysis and when it is
necessary to estimate long-term exposure. There is an
uncertainty owing to a lack of sufficient information, the
peculiarity of the investigated situation and the com-
plexity of the involved parameter.
EXPOSURE LEVELS FROM A SINGLE LINE
Regional Agencies for Environmental Protection
(ARPA) have been studying long-term exposure to mag-
netic induction at 50 Hz in the case of receptors close
to a power line by using an instrumental survey and a
provisional calculation model. The farmer supplies the
Contact author E-mail: fisica.liȰarpat.toscana.it
401

G. LICITRA, N. COLONNA and C. CHIARI
The example in Figure 1 shows the trends of currents tion of the energy fluxes, to the phase shift between the
and the measured magnetic induction of two lines that two currents and to intensity.
are close to the measure point. It is clear that just one
According to the statements of the Manager of the
line influences the exposure values. Figure 2 shows the National Transmission Network (G.R.T.N. S.p.A.), over
best fit between the measured magnetic induction and a long peroid, the energy flux of line no 593 goes, for
the current values on the two lines. This provides the 90% of the time, from Livorno Lodolo to Livorno East
relation for evaluating long-term exposure.
and, for 10%, in the opposite direction. The energy flux
of line no 532 goes, for 60% of the time, from Livorno
Marzocco to Rosignano and, for 40%, in the opposite
direction (Figure 3).
Complex case
When the prevalence of a single line is not so clear
and systematic, the situation becomes complex. The
induction measurements and the use of a provisional
calculation model are described next by considering the
case of a school in Livorno.
INSTRUMENTS AND METHODS
Measurements were carried out by the triaxial meas-
urer Enertech model EMDEX II.
The ENEL power lines no 593 (132 kV) ‘Livorno
Lodolo – Livorno east’ and no 532 (132 kV) ‘Livorno
Marzocco – Rosignano’ go through the inhabited area of
Livorno and they are placed on the same supports. They
part near the industrial area and go towards their sub-
stations. These lines are completely independent, though
they are placed on the same poles for 2 km with the
double tern configuration, because they have no electric
characteristic in common, and they do not end at the
same substation. The two lines, then, are not in phase
and the phase shift between the currents can change
unpredictably in an instant. On the two lines, the direc-
tion of the energy fluxes changes according to the utilis-
ation conditions of the net. The phase angle of a single
line depends on the reaction between the active power
and the reactive power. As a result, the measured levels
of magnetic induction also vary according to the direc-
0.40
y = 0.0021x + 0.0278
0.35
R2 = 0.3334
0.30
0.25
0.20
y = 0.0004x + 0.0458
0.15
0.10
0.05
0
R2 = 0.9459
0
100 200 300 400 500 600 700 800
Circulating current (A)
Figure 2. Example of best-fit between the measured values of
magnetic induction and the values of the dominant current in
the presence of two power lines. (̅) line 297, (࡯) line 217.
800
700
0.50
0.45
0.40
600
500
400
300
200
100
0
Magnetic induction value relative
to the yearly average current
of the most influential line
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Time
Figure 1. Example of temporal trend of the magnetic induction depending on just one current in the presence of two power lines.
(࡯) line 217, (ć) line 297, ( — — ) measured magnetic induction.
402

EXPOSURE TO ELF MAGNETIC FIELDS FROM POWER LINES
In order to verify, at first, the magnetic induction THE SIMULATION
levels in the school building, a series of point measure-
The span of the power lines was simulated by using
ments were carried out in the different classrooms.
Then, the Enertech was placed in a room to monitor the
induction levels for a long period. The sampling was
carried out by acquiring a datum each minute. The
monitoring was in two stages, the first lasted about 4
days and the second, according to the suggestion of the
G.R.T.N., about 2 days. It was thus possible to charac-
terise the two different configurations of the circulating
currents in the lines: energy fluxes with the same and
the opposite direction. During the first stage, the rms
value of the magnitude of the magnetic induction vector
was recorded. In the second, the single components x,
y and z were also recorded.
the calculation software of the C.N.R. (National Council
of Research, Research Institute for the Electromagnetic
Fields of Florence). All the geometrical and technical
data of poles and lines were loaded in the calculation
software (voltage, current, disposition and number of
conductors, phases, and height of the conductors above
ground level). In this way, the resultant of the magnetic
induction vector in the point in which the measurement
was carried out was calculated for each pair of currents.
All the conductors were considered straight, horizon-
tal, infinitely long and running in parallel. Moreover, it
was assumed that the ground is perfectly transparent to
the magnetic field. Based on these hypotheses it is poss-
ible to turn the calculation of the magnetic field from a
power line into a two-dimensional problem.
CURRENT–INDUCTION CORRELATION
The magnetic field H produced in Q by n conductors,
numbered by 0 to n − 1, is:
In order to correlate the levels of measured magnetic
induction with the currents in the lines, the G.R.T.N.
was requested to provide the current values, each 15
minutes, and the direction of the energy flux on the two
lines during the monitoring periods. Moreover, in order
to calculate the yearly averaged exposure, it was
requested that all the hourly data of the currents circulat-
ing on the two lines during the 12 previous months be
provided. On the basis of these data, the yearly average
currents of the two lines were obtained and they were,
respectively, 185 A for line no 593 and 135 A for line
no 532.
By comparing the current values of the two lines with
the magnetic induction values, it was seen that, when
the differences between the two currents were over 100
A, the magnitude of magnetic induction followed per-
fectly the temporal trend of the dominant current (Figure
4). There was not a good correlation when the differ-
ences were below 100 A. Consequently, it was neces-
sary to improve the investigation by means of theoreti-
cal calculations.
n−1
1
4␲
i
rk3
H = −
rk ϫ dl
͸
Ͷ
k=0
C
k
The above hypotheses allow the integration to be perfor-
med easily and the following expression to be obtained:
n−1
1
4␲
ikz ϫ (Q − Pk)
H = −
͸
2
͉Q − Pk͉
k=0
where ik is the current on the kth conductor, Pk is the
position of the kth conductor and the Z axis is on the
direction of the conductors.
The model was validated minimising the uncertainties
on the distance by using the measured values of mag-
netic induction and the corresponding current values in
a period in which one of the two lines was not in use.
In order to estimate the phase-shift effect and its inci-
dence on the magnetic induction levels, several simula-
tions were carried out obtaining the spatial distribution
of the magnetic induction levels from the yearly average
400
350
300
250
200
150
100
50
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
Line
593
Fluxes of energy
with opposite direction
Line 593
Livorno
Est
Livorno
Lodolo
School
Livorno
Marzocco
Line 532
Line
532
M.I.
Fluxes that disagree
Fluxes that agree
Rosignano
0
11:00 13:00 15:00 17:00 19:00 21:00 23:00
Time
Figure 3. Schematic representation of the power lines and Figure 4. Temporal trend of the magnetic induction measures
directions of their energy fluxes. and of the currents travelling on the two lines.
403

G. LICITRA, N. COLONNA and C. CHIARI
currents both in the cases of in-phase and out-of-phase the magnetic induction was different by a factor of 3.
energy fluxes. For the simulations, the phase of one line Moreover, the point where the magnetic induction has
was maintained unchanged whereas the phase of the the maximum moves if the phase-shift is varied
other was increased progressively by adding an angle (Figure 5).
from 0° to 330° with a step of 30°.
By measuring the time variation of X, Y and Z
The results indicated that an increase in the phase components of the magnetic induction vector it was
shift involves an increase in the generated magnetic possible to verify this behaviour of the phase-shift. In
induction. The minimum was obtained when the two fact, phase-shift variations imply time changes in direc-
lines were exactly in phase. The magnetic induction tion and magnitude of the magnetic induction vector.
values varied between the case of concordant fluxes
As far as it can be seen, the parameters required for
(minimum induction) and the case of discordant fluxes determining the averaged exposure level are numerous
(maximum induction). In these two extreme situations (current fluxes, phase shift). Then it is useful to recog-
nise the most probable variability range for the averaged
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
induction level, considering that it was not possible to
obtain from G.R.T.N. accurate values for temporal
change of phases of two lines. The extremes of this
interval can be calculated in the cases of discordant and
concordant median fluxes. In this way it was evaluated
that the range of highly probable values for the yearly
averaged exposure is 0.24–0.72 ␮T.
0.35
CONCLUSIONS
0.30
0.25
0.20
0.15
0.10
0.05
0
The estimate of long-term exposure to 50 Hz mag-
netic induction is an open problem in the case of com-
plex situations. Here, an inquiry method has been
described that is affected by the limited availability of
technical data relative to the circulating currents and the
20
power line.
Point of measurement
-20
-15
-10
-5
0
5
10
15
Any regulations that set new limits for long-term
exposures will have to require the presence of systems
for adequately monitoring the current in power lines.
Such systems will need to be managed by the con-
trolling authorities, otherwise it will not be possible to
work independently and with the necessary accuracy for
assessing the effects of exposure to the resulting mag-
netic fields.
Distance from the axis of the line
Figure 5. Trend of the magnetic induction when the phase
between the two lines varies from 0° (in phase) to 330°. The
energy fluxes have opposite directions.
In phase,
f150,
f30,
f180,
f60,
f210,
f90,
f120,
f240,
f270,
f300,
and
f330.
REFERENCES
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2. CEI 211–6. Guida per la misura e per la valutazione dei campi elettrici e magnetici nell’intervallo di frequenza 0–10 kHz
con riferimento all’esposizione umana (2001).
3. Gambato, E. and Rosa, M. Studio dei modelli per il calcolo dei campi elettromagnetici generati da elettrodotti. Task 4.2.1
Centro Tematico Nazionale (November 2001).
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