Human Exposure to Electromagnetic Fields Produced ...

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Advances in Electrical and Computer Engineering

Volume 14, Number 1, 2014

Human Exposure to Electromagnetic Fields Produced by Distribution Electric Power Installations Marian COSTEA1, Nicolae GOLOVANOV1, Ioana Marina GRINŢESCU 2, Elena-Luminiţa STĂNCIULESCU3, Ştefan GHEORGHE4 1 University Politehnica Bucharest, 060042, Romania 2 University of Medicine and Pharmacy "Carol Davila" Bucharest, 020022, Romania, 3 Emergency Hospital Bucharest, 014461, Romania 4 University Valahia of Targoviste, 130024, Romania [email protected] Abstract—The long term effects of electromagnetic fields on the human body are far from being well understood. Special concerns arise if this exposure is relatively prolonged such as in the case of professionals working in electric power installations. In this paper the authors focus on electric power distribution because of the high number of such installations and also their proximity to residential areas. We present the main recommendations and standards regarding the admissible limits and also the procedures applied to assess human exposure in distribution installation workplaces. Theoretical considerations and measured values in typical distribution installations from Romania are presented and discussed. Measurements have shown that, in distribution installations, in terms of limits set by specific regulations, permissible levels are not exceeded for occupational exposure to power frequency fields; only in some points the public exposure limits can be overcome. Medical considerations on short or long term exposure of workers are also discussed, based upon up to date knowledge. Index Terms—electromagnetic field, electrical engineering, human exposure, medical condition, occupational medicine.

I. INTRODUCTION In recent years concerns about electromagnetic field effects were raised both for professionals and for the public. An important role in the debate was played by a constantly increasing number of scientific works which refer to this problem [1-6] – and also by the media. This concern is justified by the increasing number and density of radiation electromagnetic sources and also by the easy access to or use of them. Based on the researches about acute effects on human body, International Commission on Non-Ionizing Radiation Protection (ICNIRP) releases periodically recommendations about the admissible limits for human beings in the different range of frequencies. These recommendations serve as a basis for national or regional standards in the field of work safety or public protection against the effects of electromagnetic fields. In the European Union, the 40/2004/EC Directive [7] established these limits for the workplaces, in a range of frequencies between 0 and 300 GHz. Some requirements of this Directive are still postponed because of the impossibility to be applied at a specific category of equipment without affecting their proper operation. Regarding the public exposure the limits are stipulated in the 519/1999/CE Recommendation [8]. Above cited documents present admissible values for the quantities which have

direct biological effects (such as induced density current in the body or specific absorption rate) and for external/measurable quantities such as field strengths or flux densities. Thus, the biological effects of professional exposure are called "exposure limit values” and the measurable corresponding quantities are called "action values". These documents do not provide the measuring procedure, the requirements for the devices or apparatus which can be used to perform these measurements. These documents state only the limit values of the above specified quantities, without explaining how the compliance should be checked. To complete this task one needs to follow a defined procedure, to use a measuring system with agreed performances, and also to perform the measurements in specified atmospheric conditions or, if not possible, to apply certain corrections. One must also specify how should collected, interpreted and processed the data. These limit values are not detailed with respect to whether they are instantaneous, timed and/or space-averaged values or if they must be correlated with operation characteristics of the source. II. STANDARD REQUIREMENTS As far as the procedure to be followed in order to demonstrate compliance of a workplace with the exposure limit values and action values stated in the Directive 2004/40/EC, first of all one must follow the provisions of European Standard EN 50499/2010 [9], because this is its declared scope. In the list of normative references of EN 50499 it is cited among others the basic standard EN 50413/2008 [10] also dedicated to the measurement and calculation procedures for human exposure in the frequency range covered by the Directive (0 ... 300 GHz). This related normative (now under review) is the only one which deals with the problem of exposure at low frequency fields in the workplace. Other related standards (given in EN 50499/2010) refer to the exposure to high frequency fields, the calculation of induced currents at low frequencies or with electromagnetic compatibility requirements of equipment used by workers (electric hand tools, transceivers, etc.). It must be emphasized that EN 50499/2010 stipulates that the compliance with Directive 40/2004/CE can be proved either by calculations or by measurements.

Digital Object Identifier 10.4316/AECE.2014.01005

29 1582-7445 © 2014 AECE


Advances in Electrical and Computer Engineering On the other hand, some locations can be excluded from exposure assessment being considered implicitly compliant. Among these locations there are sites where the exposure is under the maximum levels admitted for the public and also workplaces with equipment who comply with the EMC manufacturing product requirements - for example, IT and office equipment, instruments and control devices, lighting equipment (with some exceptions) etc. For workers in electric power installations, the site is considered compliant with the Directive: (a) regarding the electric field, if it is generated by “any underground or insulated cable circuit, rated at any voltage, any overhead bare circuit rated at a voltage up to 200 kV, or overhead line up to 250 kV, oversailing the work place, or at any voltage where the workplace is indoors”; (b) regarding the magnetic field, if it is generated by “any electrical installation with a phase current rating of 500 A or less, any individual circuit within an installation, with a phase current rating of 500 A or less, any circuit where the conductors are close together and having a net current of 500 A or less, all components of the networks satisfying the criteria above are covered, (including the wiring, switchgear, transformers etc.), any overhead bare conductors of any voltage”. The power distribution installations, with rated voltages of up to 110 kV, meet the above requirements first of all in terms of electric field (only accidentally it could be exceeded admissible values for public exposure, as resulted also by the measurements performed in situ and presented in this paper). Few considerations regarding the features of magnetic field: the slowest decrease of the magnetic field strength (H) with distance (r) from a source is recorded in the case of a single conductor (the return conductor is located far away and has no influence on the resultant value of the field). In the case of multi-conductor systems (e.g. bi-phase or three-phase) or equipment, the magnetic field decreases proportionally to 1/r2 or 1/r3, considerably faster than in the case of single conductor; this empirical “law” is valid only if the distance to the observation point from the system of conductors is larger comparing with the distances between the conductors themselves. But for a single conductor, the effective (r.m.s.) value of H is given by H =I/2r, I being the effective value of the current through the conductor. As a result, if the current which flows through the conductor is 500 A, the distance at which the field strength decreases under the admitted value for public exposure (80 A/m) is 1 m and for professional exposure (400 A/m) is 0.2 m. Workers or the general public should not be found at such a small distance from the magnetic field source, except in atypical circumstances and only when the source is an insulated monophase cable, posed in air (not buried) regardless its rated voltage. The minimum safety clearances for bare conductors are always greater than the distances at which the magnetic field, for the mentioned value of the current, can reach the admissible exposure limits. Because the exposure limit values are usually expressed in units of magnetic flux density, the minimum distance, in [m], from a single conductor carrying a current I given in [A] (r.m.s.) and for a magnetic flux density expressed in [µT] can be obtained using a simple relation. 30

Volume 14, Number 1, 2014 However, one must do additional investigations around the dry-type power transformers, air cored reactors and cables (or insulated conductors) carrying intense currents. As the standard EN 50499 describe the procedure to be followed to assess the occupational exposure, the procedures regarding the evaluation of public exposure to power frequency (50 Hz or 60 Hz) fields are described in IEC 62110/2009 [11]. The CIGRE brochure 375/2009 [12] describes the procedures used to evaluate human exposure near overhead lines. III. MAIN PROPERTIES OF LOW FREQUENCIES FIELDS PRODUCED BY ELECTRIC POWER INSTALLATIONS Properties of low frequency electric and magnetic fields have been thoroughly studied and reported in many papers or standards [9], [12], [13], [14]. At power frequency, the regime of electromagnetic field is quasi-static and electric and magnetic fields can be considered decoupled, the variation of one of them having no influence on the level of the other one (the case of region so called non radiative or reactive near-field). As a result, each field can be studied and calculated independently. The electric field in this regime has a unique source: the electric charges on the conductors, as a result of applied voltage. Due to this fact, the calculation of electric field is based on the methods used in electrostatics. An electric field will appear in the neighborhood of an overhead line once it was energized even if it's not loaded. As such, the electric field has a variation into a restricted range of values because the network operation code imposes the maintaining of voltage in prescribed limits. Also, at power frequency the unique source of the magnetic field is the current flowing through conductors. At low frequency, the induced currents in the soil do not influence, practically, the resultant field above the ground. The magnetic field strength follows the curve load of the consumer. Consequently, the magnetic field near an electric power installation could have large variation during a day. A common feature of the electric and magnetic fields of power frequency: under three-phase overhead lines or under air insulated long busbars, they have elliptical polarization, meaning that the (rotating) field vector describes an ellipse during a cycle of voltage or, respectively, current. In particular positions versus field sources (conductors), the ellipse could degenerate into a circle or into a right line. The time variation of the resultant field is non sinusoidal, although its components (horizontal and vertical) show sinusoidal variations. Also, the resultant it’s not influenced by the phase angle between the field axial components but only by their effective values. As a result of real time variation shape, the electric and magnetic fields have a significant DC component. It is described above the case of a 2D problem. But in an air insulated substation, due to multiple sources of field (busbar systems, over or under crossings, connections of equipment terminals), the field polarization in a given point cannot be easily described, the field strength being the resultant of all three space components (in Cartesian coordinates). This is a 3D problem, more complicated to be solved analytical. Particular properties of low frequency electromagnetic field components produced by an electric power installation are mentioned below.


Advances in Electrical and Computer Engineering (a) Particular properties of electric field Under usual conditions of the suspension height of the conductors, the electric field nearby the ground is almost uniform and its vertical component predominates. Generally, regarding the overhead power lines where the suspension height of conductors lies from few meters in the case of MV lines up to tens meters for high voltage lines, in a strip of about 0...2 m above the ground, the electric field is almost uniform. The field strength depends on the applied voltage and the geometry of the line: the relative position and distances between conductors, the suspension heights, the real radius of conductors or the equivalent radius in case of bundled conductors and of course the position of the measuring point.

Volume 14, Number 1, 2014 the middle of segments, were retained. For a segment that exceeds 25 m in length, the error drops under 5% in the case of a suspension height of 4.5 m and about 8% for the suspension height of 6.1 m. The relative errors are negative, the parallel problem hypothesis giving conservatives values. There must be also mentioned that the maximum magnetic field strength - considering the busbar infinitely long - for the suspension height of 4.5 m and a current (theoretical) of 1 kA is, in the point of observation, of 39.5 A/m. Consequently, applying the Biot-Savart's theorem it is possible to calculate the acceptable values for the magnetic field strength in a substation, with the condition of knowing the currents flowing through the conductors at a considered time. 0 Hsusp = 4.5 m Hsusp = 6.1 m

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IV. CONSIDERATIONS ABOUT FIELD EVALUATION BY CALCULATIONS The electric and magnetic field produced by an overhead line or only the magnetic field generated by a cable can be calculated easily by accepting the hypotheses of "plan parallel symmetry" (in plans parallel to each other and perpendicular to the longitudinal axis of the line, the field values are identical in the same coordinate points belonging to the considered plans). For example, in the case of an overhead line, the results obtained are very close to the real ones at the midspan, where the field strength has also the maximum level because of the maximum sag of conductors. But in the case of a substation, this method couldn't be applied because of the finite length of the conductor and of the proximity of equipment or mounting accessories with different conductive or magnetic properties. An example of the influence of finite length of conductors on the magnetic field calculated values is presented in Fig. 1. It shows the errors between real values of magnetic field calculated by applying the Biot Savart's theorem for a finite length of conductors and the values obtained considering that the busbar length is infinite. The variable was the length of considered busbar. The comparison is performed for a case of a busbar in a substation of 110 kV, where the distance between phases is 2.4 m and considering two distances of suspension height of conductors to the ground (4.5 m and 6.1 m) commonly encountered in Romanian substations. The observation point was at 1 m height from the ground and the maximum values of the transversal profile, found in

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40 50 60 Busbar length [m]

70

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Figure 1. Relative error between magnetic field strength calculated using Biot-Savart theorem and calculated considering infinite length of conductors, versus segment conductor lengths (used in the first hypothesis).

Fig. 2 presents the magnetic field variation (and its axial components) in transversal profile (perpendicular to the busbar) in the middle of a busbar segment of 20 m long and with the above specified geometry (busbar suspension height of 4.5 m, and observation point 1 m above ground). Near the vertical axis of symmetry of the busbar the vertical component of the field is predominant and then, at a greater distance, the horizontal component predominates. 40 resultant horizontal component vertical component

35 Relative magnetic field strength [A/m/kA]

In the vicinity of the ground there is no rule regarding the predominant component of the magnetic field produced by three-phase sources: it could be the horizontal or vertical one, depending on the relative position of the source conductors and the position of the measuring point. On the other hand, the field strength decrease from the conductors to the ground, where the minimum value is recorded. Comparing with the electric field produced by the same of conductors’ arrangement, the magnetic field show greater non-uniformity in the same area located close to the ground. The magnetic field strength depends on the current (it is recommended to be expressed relative to a unit of current, usually 1 kA for high voltage systems), the geometry of the line and the position of the observation point.

Relative error [%]

(b) Particular properties of magnetic field

30 25 20 15 10 5 0 -15

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0 x [m]

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Figure 2. The transversal profile of the relative magnetic field strength (r.m.s. values) calculated in the middle of a 110 kV busbar segment of 20 m length at 1 m above ground level.

As could observe from Fig. 2, the magnetic field reach its maximum value in the symmetry axis of three phase conductors and consequently in order to record this value it’s necessary to place the measuring probe in this point. 31




H ave

 100  % 

value measured at h height above the ground, and Have is the average of the three values Hh , measured at 0.5 m, 1 m and 1.5 m. 25

N o n -u n i fo rmi ty [% ]

40 30 20 10 0 -10 -20 -30

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0

0.002 0.004 0.006 0.008

0.01 0.012 0.014 0.016 0.018 Time [s]

0.02

Figure 4. The instantaneous magnetic field components on the axes and resultant value during an AC cycle in a particular point under busbar system. The geometry of the busbar system is the same as used to compute the lateral profile presented in Fig. 2.

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10

5 -6

resultant horizontal component vertical component DC component

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As a result, most of the data on the distribution of electric field (and also the magnetic field) in substations were collected by measurements. -4

-2

0 x [m ]

2

4

6

Figure 3. The calculated non-uniformity of magnetic field in the middle of a 110 kV busbar segment of 20 m length (2.4 m between phase conductors which lies at 4.5 m height) at 1 m above ground level.

The maximum non-uniformity is about 24 %, but the difference between the averaged value (obtained from three values measured at different heights) and that recorded at 1 m height it’s only of 2.3 % (of course, calculated values). Fig. 4 presents the time variation (during a cycle of power frequency current – 50 Hz) of components on axes and also the resultant value, in a point located in the vertical symmetry axis of busbar system considered above, at the same observation height from the ground (1 m). As stated above, the variation of the resultant is not sinusoidal (but the time variation of axial components is always sinusoidal). The continuous (DC) component (marked in the figure), calculated according to the known relation, for periodic T

signals, H DC  1 / T  H t dt presents a significant level 0

(T = 20 ms, the period corresponding to power frequency signal of 50 Hz). The analytical calculation of electric field in a substation is a more complicated problem and the obtained results are less accurate by accepting some equivalent distributions of electric charges. To accurately calculate the electric field distribution computing programs based on numerical solution of field equations could be used. But the effort to discretize adequately all substation components, to find and introduce their material constants, to put all the boundary conditions and also the required computation time makes this solution uneconomical. On the other hand, it is not necessary to know the field distribution at any point of the installation: on human exposure it's enough the knowledge of the field distribution at the points where the personnel has access for switching, monitoring and maintenance operations. 32

50

Relative magnetic field strength H [A/m/1kA]

But in this point, the non-uniformity reaches also its maximum value as could be observed in the Fig. 3. The field non-uniformity reaches also its maximum value that , where Hh is the results from the equation H h  H ave

V. MEASURING PROCEDURES The above cited standards describe the procedures used to evaluate human exposure regarding professionals and the public. There are some differences between the stipulated requirements, generally referring to the minimum admissible distances between the field probe and permanent obstacles or equipment. Below we present and comment upon common stipulations of these standards, pertaining to the measurement of low frequency fields:  the measurements must be performed in time domain and in conditions of undistorted field (in the absence of objects able to influence the local value of the field);  the measurements must be expressed as the effective values of the resultant; as a consequence, triaxial probes are recommended because they provide the results directly by using the three spatial measured (in r.m.s.) components (for example for an electric field Eres  E x2  E y2  E z2 );  the probes must have a relatively small size (the upper limits of their areas are specified - for example, the area of a magnetic field probe should not exceed 100 cm2); generally, the intrinsic uncertainty of a probe decreases with increasing probe surface, but its accuracy is increased only in uniform fields - their software is built and their calibration is performed under these conditions.  minimum lateral distances (generally expressed as a function of the diagonal or diameter of the probe) must be kept from conductive surfaces and/or with magnetic properties;  the evaluation uncertainty of effective values of the measuring system only, in uniform fields, must be less than  10% as stipulates the standard IEC 61786 [15]; the basic standard EN 50413/2008 offers a calculation procedure for the overall uncertainty, expressed as "expanded uncertainty" (including all factors which could influence the measurements, such as atmospheric conditions, influence of operator etc.) but without stipulating the admissible limits for human exposure.


Advances in Electrical and Computer Engineering Details on this issue are given in the brochure CIGRE 375 [8] which specifies that the measurements of electric field can be affected by an overall uncertainty which must not exceed 15% + 10 V/m, while for the magnetic field (expressed in units of magnetic flux density) this value must not exceed 10% + 20 nT. Concluding, a prudent approach would be to ensure that the overall measurement uncertainty does not exceed 10% for any of the power frequency evaluated field.  the bandwidth of the measuring system must be appropriate for the frequencies content of the measured field (generally, up to 500 Hz; a narrow bandwidth could be accepted if the harmonics content could be neglected). As the EN 50413 states, “a preliminary scan may be performed to determine the distribution of the field”. The results of this scan should be documented. After the point of maximum exposure was detected it’s necessary to perform detailed measurements at this point and to conclude what is the “exposure level”. In accordance with the requirements of the standards which deal with the problem of human exposure, a preliminary scan must be performed at a specified height above the ground (1 m). If the field is uniform, the exposure level in a given point is even the measured value. In the case of a non-uniform field, one must perform measurements at 0.5 m, 1 m and 1.5 m and the average of the three values is the exposure level. In the proximity of equipment or above a buried cable, the procedure implies three or five measurements and the exposure level is then calculated using an averaging formula. In the case of magnetic field measuring no special requirements regarding the influence of the operator on the field probe, because the human body does not magnetic properties, but specific precautions must be taken regarding electric field measurements. In order to reduce the operator's influence on the measured value, the distance between him and the field probe must be at least 1.5 m, but the recommended one is 3 m. As a consequence, the electric field probe must be placed on a dielectric tripod or support and connected to the central unit of measuring system via an optical fiber cable. On the other hand, the electric field measurement is affected when relative the humidity exceeds 70% and involved errors are only positive, this influence being acknowledged in EN 50499. Therefore, if the measured values obtained under high relative humidity do not exceed the reference levels, the compliance with the prescribed values is confirmed. Otherwise the measurements must be repeated in acceptable humidity conditions which not exceed the mentioned threshold. VI. MEASURED DATA In order to confirm or refute the requirements of the EN 50499 standard for the occupational exposure and to identify the maximum exposure points, we carried out a large measurement campaign that included air insulated and indoor substations with rated voltage of 110 kV and also MV/LV transformer substations owned by a Romanian company of power distribution. We used measuring professional type equipment with an intrinsic uncertainty of 3%, both for electric and magnetic field probes under adequate atmospheric conditions

Volume 14, Number 1, 2014 (fair weather, relative humidity under 60%). In the substations, measurements were performed both on access paths, with a step of 1 m (in order to identify the maximum values), and in the proximity of equipment such as power transformers, actuator devices for switching equipment, relay cabins. In a 110 kV substation none of the measurements reached the limit for public exposure to a magnetic field. Even considering the maximum values of the currents (for 110 kV, in normal operating conditions, the currents cannot exceed 600 A) the maximum strength of magnetic field remained under the exposure limits admissible for the public. The maximum values of the magnetic field strength were recorded under MV busbar (built by bare conductors) which connect the 110 kV/MV transformer to the distributor, but even this value was substantially below the limit exposure values for the public. No unusual values were recorded regarding the field value in the proximity of power transformers even at a distance of 1 m against their case. Despite of what one might think about their presence as magnetic field sources, transformers are constructed so as to present magnetic flux leakage as little as possible, in order to increase their efficiency. On the other hand, theirs metallic cases acts also as shields against magnetic field. Thus, measurements performed close to the 110 kV/MV transformers (having apparent power between 16 MVA and 40 MVA) showed values of field strength between 1.5 A/m and 4.6 A/m. Generally, on the access path and in vicinity of oil cooled transformers the magnetic field strength was up to 10 A/m, with the exception of, as mentioned before, the area strictly located under MV busbar from the power transformers. The higher values (tens of A/m or around 100 A/m) of magnetic field strength were recorded in particular points of the MV/LV transformer substation. For example, close to the low voltage cables (at 0.2 m) or distribution panels (at 0.5 m from bare live conductive parts) were recorded values up to 115 A/m. In this last case (low voltage distribution panel), measurements were performed also on the first three odd harmonics. Comparing with the fundamental, the rank 3 harmonic has represented 13...17 % and the rank 5 harmonic has between 5 and 6%. The aforementioned measurements were performed using the filter function embedded in the measuring system (which has a narrow passband of 0.1 Hz, at – 3 dB, around a selected frequency). At this level of harmonics, the error committed by measuring only the fundamental is less than 2%. Therefore, the attention should be focused on electric field assessment. First of all it must mentioning that the action value for professional exposure at power frequency electric fields is 10 kV/m (the reference level for public exposure cannot exceed 5 kV/m). The above values are specified for AC installations of 50 Hz. For 60 Hz, the limit values are lower: 8.3 kV/m for professional exposure and respectively 4.2 kV/m, for public exposure. Because from the point of vue of the effects of these fields on the human body the induced currents are important (and their distribution in different organs) the limit value of external fields stipulated by different regulations decreases with the increase of frequency. 33




In four air insulated substations we explored the electric field strength in 11 main access paths (summing approx. 1,000 m); the maximum measured values can be summarized as follows: on a single path – 4.87 kV/m; on four paths values ranged between 3.22...3.47 kV/m and on six paths values ranged between 1.25...2.68 kV/m. Electric field distribution on an access path in a 110 kV substation is shown in Fig. 5. The superposition of this distribution on the top view of substation allows the identification of the points with maximum exposure on the explored path. 3 .5

Electric field strength [kV/m]

3

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140

x[m]

Figure 5. The measured electric field distribution on an access path in a 110 kV substation.

The measurements were performed at 1 m above ground level. The results were those expected. For comparison, the Fig. 6 shows the lateral profile of electric field generated by a 110 kV busbar system considered of infinite length, with 2.4 m between phases, the suspension height of 4.5 m and the observation point situated at 1 m above ground level. The cross section of conductors was considered to be of 300 mm2. The calculated non-uniformity of the field, in the above described hypotheses is 11.7%. Could be remarked the acceptable concordance between the maximum values of the electric field measured and calculated. An exception is the point where the access path is over-crossed by a cross coupling bay of the substation, where the measured value exceed 3 kV/m. 3

E-resultant [kV/m] (r.m.s.)

2.5

2

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1

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0 -15

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-5

0 x [m]

5

10

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Figure 6. The lateral profile of electric field strength (r.m.s. values) calculated under a 110 kV busbar (considered infinitely long).

For suspension heights of the busbar higher than 4.5 m the electric field will decrease accordingly. But, in real cases, it can be higher due to the influence of other nearest sources of electric field (connections to equipment, parallel 34

busbar or which over pass the considered system) due to higher equivalent radius of the conductors (as is the case of bundled conductors – solution adopted in three of the explored substations) or due to reduced distance above the ground or caused by different external influences (high temperature, frost deposits and so on). In the points where the maximum value was detected at 1 m, we performed measurements at three heights (as specified above) and we calculated the “exposure level” as average of the three values. In the 110 kV air insulated substations the measured non-uniformity degree was between 14% and 28%, but generally, the exposure level determined as average of three value was very close to the measured value at 1 m (the differences were of only few percentage points, in the limits of measuring uncertainty). Higher values of the non-uniformity degree of the electric field were recorded on access paths of an indoor substation. Regarding the values of the field strength in indoor transformer substations (measured at 1 m above the floor) ranged between few V/m and up to 140 V/m. Around pole-mounted distribution transformer, connected to overhead lines of 20 kV, the measured values were up to 50 V/m. Usually, under 20 kV overhead lines the electric field strength was up to about 120 V/m, but the metallic parts of the transformer substation acts as an electric shield and reduce, near the ground, the recorded value. Thus, even for the public the recorded values were under the admissible exposure limits. VII. MEDICAL CONSIDERATIONS As it is well known, the interaction between the electromagnetic fields and the living organisms has different consequences according to the characteristic parameters and to the frequency range of the electromagnetic fields. The intensity values of the electric and magnetic field become restrictive parameters that should be taken into consideration in the design and implementation of electrical equipment. Parameters that highlight the direct biological effects of the electromagnetic field on the human body bear distinct names in the mentioned documents, issued by the European Council. The values for occupational exposure which cause biological effects are called ‘exposure limit values’, and the measurable ones ‘action values’. In the case of public exposure, the values which cause biological effects are called ‘basic restrictions’, and the measurable ones ‘reference levels’. The monitoring of the exposure dose of the personnel to the electric field allows assessing the risk posed by a particular job [6]. The international standards settled limit values for the characteristic parameters regarding the exposure of the human body to the action of the electric field produced to the ground, by high-voltage electric installations. The human body has not been considered to be affected by values under 5 kV/m. On the ground, under the high voltage transmission lines, in the areas where the electric field reaches the maximum value, it should not be exceeded a level of 10 kV/m, imposed by design. The human presence in electric fields with a strength over 10 kV/m is allowed only if special measures of protection are taken.


Advances in Electrical and Computer Engineering The provisions of the World Health Organization regarding the right of way of overhead lines recommend not to exceed the value of 2 kV/m outside this area, meanwhile inside it the maximum allowable level prescribed is of 10 kV/m. Many rules require the limiting of current density, induced by the environmental electric field in the human body at a level of up to 10 mA/m². This value is considerably lower than the level of stimulation of nerve and muscle tissue that is of 1000 mA/m². The study of the biological effects of electric field exposure requires the knowledge of electric current induced in the body and its distribution in different parts of the body. The biological effects of human body exposure to the magnetic field are differentiated according to exposure time: cumulative, for the long-term exposure and acute, for the short-term exposure [16]. The significant characteristic parameter for exposure with cumulative effects is the dose of magnetic field acquired by people over large periods of time - years. The dose of magnetic field is defined as the product between the magnetic flux density B and the time t of human body exposure to the magnetic field. The research in the field of bioelectromagnetism is based on experiments that highlight its possible negative effects or, on the contrary, the potential therapeutic effect, for example the controversial therapies in oncology, developed by Antoine Priore and the therapy of old fractures with a difficult evolution with the aid of low-frequency electromagnetic radiation through the technique initiated by Andrew Bassett [17]. Nowadays the role of low-frequency electromagnetic fields in orthopedics is recognized, as well as the positive results in Parkinson's disease. The study concluded that there is no important association between occupational exposure and cutaneous malignant melanoma [18]. The limits of magnetic field exposure were established by the World Health Organization and the International Commission on Non-Ionizing Radiation Protection [19]. In occupational studies, the type of work performed by electricians determines different exposures to the electric and magnetic field and, consequently, different risk factors [20-23]. Also, several studies have been devoted to the effects of radiofrequency fields [24-27]. The automation of electrical processes from substations and transformer substations (SCADA - the remote management of the installations, equipment without maintenance or with reduced maintenance) or the use of advanced technologies to control power lines (thermo vision) determines the reduction of workers’ exposure time and thus the reduction of risk factors. At the moment, there are no results for the monitoring over long periods of time (years) of the human body exposure to electric and magnetic fields produced by electric power distribution facilities and this is a challenge for future researches. From the medical point of view is not yet sufficiently explored the issue of human exposure (and its consequences) to magnetic fields produced by domestic appliances, if we refer only to the low frequency electromagnetic radiation. As example, even if they was significantly upgraded [28] the heater equipment are the

Volume 14, Number 1, 2014 most important source of low-frequency magnetic field in a residential building. The effects of long-term exposure of the human body to electric and magnetic fields of low frequency require systematic interdisciplinary studies, until now the results not being conclusive. An important medical research direction is also related of the persons which presents hypersensitivity to electromagnetic fields [29-33]. And this especially in the context of future smart grids that involve, at final user, the presence of a large number of devices that communicate via radio frequency fields. On the other hand, a real identification of this sensitivity and not self-declared could allow a correct recruitment of the personnel able to work in the electric power installations. VIII. CONCLUSIONS The measurements campaign of low frequency fields in some Romanian substations and transformer substations belonging to the power distribution system (with rated voltage up to 110 kV) led unequivocally to the conclusion that the limit values, established by regulations relating to occupational exposure, were not exceeded. Moreover, in the large majority of measuring points not even the limits for public exposure are exceeded. Regarding the magnetic field, a maximum strength value of 4.5 A/m was measured on the access paths of the six 110 kV monitored substations, at the moment of measuring, in real operation condition. In the pessimistic scenario (maximum current load), the estimated values by calculation, in the same places, will not exceed 11 A/m, far from admitted value of 80 A/m for public exposure. Under the 20 kV bus bars, the recorded values of magnetic field strength were of few tens A/m. We found that the allowable level for public exposure has been not exceeded than in areas of low voltage circuits (in transformer substations) and in their immediate vicinity (0.2 m) (where the public hasn't access). Even in these areas, exposure levels are 4 times lower than the limit for occupational exposure at power frequency magnetic field. Regarding the electric field, strictly localized, in only two points (from about 1200 measured values) and at 1.5 m above ground, values slightly exceeding 5 kV/m were measured, but the public exposure level in these points (average of the values measured at three different heights) was not exceeded. More extensive studies on electric and magnetic field exposure and its medical short and long term effects on professionals and the public are needed in the near future to determine the necessary adjustments of acceptable limits to be considered in the design and operation rules of electric power installations. REFERENCES [1]

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