3.9 Precautions to control the indirect RF radiation hazards
Indirect effects of RF radiation, such as ignition hazards to flammable substances, may occur at levels well below the “derived/investigation” levels particularly at MF/HF. This is because flammable substances may be stored on a site having associated conducting structures, such as pipe work, that could act as a fairly efficient receiving antenna. Actual risks are, however, rare, but may include industrial processing plants, fuel storage facilities and petrol filling stations. Detailed evaluation is, however, far from simple. The general procedure recommended below is, therefore, based on progressive elimination. The detailed precautions adopted will however need to take account of any national standards or legislation in the country concerned.
An initial assessment should be carried out, based on practical, worst case estimates, of the minimum separation needed between a particular type of transmitter and a conducting structure to avoid such a hazard. The first step in doing this is to determine the minimum field strength that might present an ignition hazard for the particular transmitter frequencies in use. This is a function of the type of flammable substance and the perimeter of any loop formed by metallic structures, usually pipe work, and can most easily be determined from tables or graphs. The vulnerable area should then be determined from this minimum field strength by calculation, mathematical modelling or from tables/graphs.
If the vulnerable area, as determined above, contains any such sites on which flammable substances are stored, or if any are being planned, a more detailed assessment should then be made. This should be based on the actual dimensions of any metallic structures, the gas category of the flammable substance(s) being stored and the measured field strength. This detailed assessment should be carried out by calculation of the extractable power from the metallic structure to determine whether this exceeds the minimum ignition energy of the flammable substance. Should this be the case, then the extractable power should be measured and any necessary modifications to the structure and/or other safeguards implemented.
In a similar category to ignition hazards, is the possible detonation of explosive materials. This will very rarely be encountered but detailed guidance is available from national standards, such as BS 6657 in the United Kingdom. Other indirect effects that should be considered include interference to the safety systems of vehicles, machines, cranes etc. close to, or within the boundaries of, transmitting stations. The immunity of these systems is covered by electromagnetic compatibility (EMC) regulations and CISPR.
Where necessary, precautions similar in principle to those described above may need to be applied.
3.10 Fieldstrength values to be determined
Preliminary, using data given by a number of international and national authorities concerned with the health aspects of EMFs, the range of electrical and magnetic field strengths are shown in Figs. 29 and 30, respectively.
These curves/graphs should not be used as a basis for an administration’s regulatory requirements. They represent a composite view of the limits currently depicted and are certain to evolve over time. As such, they are merely illustrative of the methodology that could be applied to develop useful standards within an administration.
Also, it must be recognized that results of independent studies of the subject are not entirely consistent and as a result the interpretation of the results by responsible authorities has in the past and will continue in the future to result in differing requirements in different countries.
The range of the electrical field strengths derived from the tables given by international
and national authorities concerned with the health aspects of EMFs
The curves “a” and “b” represent the upper and lower boundaries respectively of some known, existing recommendations for RF exposures levels (presented in this section, as example). All curves from authorities making such recommendation lie between these boundaries, and any curve between curves “a” and “b” should allow adequate broadcasting services.
authorities concerned with the health aspects of EMFs
The curves “a” and “b” represent the upper and lower boundaries respectively of some known, existing recommendations for RF exposures levels (presented here, as example). All curves from authorities making such recommendation lie between these boundaries, and any curve between curves “a” and “b” should allow adequate broadcasting services.
The differences between the suggested maximum levels at the same frequency (Figs. 29 and 30) depend on different conditions considered by the various sources suggesting the limits.
3.11 Additional evaluation methods
The application of dosimetric concepts enables the link to be established between external (i.e. outside the body) field strengths and internal quantities of electric field strength, induced current density and the energy absorption rate in tissues. The development of experimental and numerical dosimetry has been complementary. Both approaches necessitate approximations to the simulation of human exposure; however the development of tissue equivalent materials and minimally disturbing probes in the experimental domain and the use of anatomically realistic models for computational purposes have improved the understanding of the interaction of RF fields with the body.
Whereas current density is the quantity most clearly related to the biological effects at low frequencies, it is the specific energy absorption rate (SAR), which becomes the more significant quantity as frequencies increase towards wavelengths comparable to the human body dimensions.
In most exposure situations the SAR can only be inferred from measured field strengths in the environment using dosimetric models. At frequencies below 100 MHz non-invasive techniques have been used to measure induced current, and in extended uniform fields, external electric field strengths have been related to induced current as a function of frequency. In the body resonance region, exposures of practical significance arise in the reactive near field where coupling of the incident field with the body is difficult to establish owing to non-uniformity of the field and changing alignment between field and body. In addition, localized increases in current density and SAR may arise in parts of the body as a consequence of the restricted geometrical cross-section of the more conductive tissues.
Dosimetric quantities can be calculated by use of suitable numeric procedures and calculational models of the human body. On the other hand such quantities can be measured using suitable physical models (phantoms).
3.11.2 Specific Absorption Rate (SAR) measurement
The Specific Absorption Rate, SAR (W/kg), is the basic limit quantity of most RF exposure regulations and standards. SAR is a measure of the rate of electromagnetic energy dissipated per unit mass of tissue.
The Specific Absorption Rate (SAR) may be specified as the value normalized over the whole body mass (sometimes referred to as the “whole body averaged SAR”) or the localized value over a small volume of tissue (“localized SAR”).
SAR can be ascertained from the internal quantities in three ways, as indicated by the following equation:
SAR = = C =
E: value of the internal electric field strength in the body tissue (V m–1)
: conductivity of body tissue (S m-1)
: density of body tissue (kg m-3)
C: heat capacity of body tissue (J kg-1 C-1)
dT/dt: time derivative of temperature in body tissue (C s-1)
J: value of the induced current density in the body tissue (A m2).
The local SAR in an incremental mass (dm) is defined as the time derivative of the incremental absorbed energy (dW) divided by the mass:
SAR = d/dt (dW/dm)
This quantity value is important from two standpoints; the resulting non-uniform distribution of energy absorption when exposed to a uniform plane wave, and the localized energy absorption arising from non-uniform fields in close proximity to a source of exposure.
Exposure regulations or standards contain derived electric and magnetic field limits. The underlying dosimetric concept assures that compliance with the (external) derived levels will assure compliance with the basic SAR limits. However, external or internal SAR measurements can also be used to show compliance. For partial-body near-field exposure conditions, the external electromagnetic fields may be difficult to measure, or may exceed the derived limits although the local SAR is below the basic limits. In these cases internal SAR measurements in body models have to be conducted. The most important methods to measure SAR will be described below.
3.11.3 Electric field measurement
The SAR is also proportional to the squared RMS electric field strength E (V/m) inside the exposed tissue:
SAR = E2/
where (S/m) is the conductivity and (kg/m3) is the mass density of the tissue material at the position of interest. Using an isotropic electric field probe, the local SAR inside an irradiated body model can be determined. By moving the probe and repeating the electric field measurements in the whole body or in a part of the body, the SAR distribution and the whole body or partial-body averaged SAR values can be determined. A single electric field measurement takes only a few seconds, which means that three-dimensional SAR distributions can be determined with high spatial resolution and with a reasonable measurement time (typically less than an hour).
3.11.4 Temperature measurement
The SAR is proportional to initial rate of temperature rise dT/dt (C/s) in the tissue of an exposed object:
SAR = cT/t
where c is the specific heat capacity of the tissue material (J/kgC). Using certain temperature probes, the local SAR inside an irradiated body model can be determined. One or more probes are used to determine the temperature rise Tduring a short exposure time t (typically less than 30 s to prevent heat transfer). The initial rate of temperature rise is approximated by T/t, and the local SAR value is calculated for each measurement position. By repeating the temperature measurements in the whole body or in a part of the body, the SAR distribution and the whole-body or partial-body averaged SAR values can be determined.
Three-dimensional SAR-distribution measurements are very time consuming due to the large number of measurement points. To achieve a reasonable measurement time the number of points has to be limited. This means that it is very difficult to measure strongly non-uniform SAR distributions accurately. The accuracy of temperature measurements may also be affected by thermal conduction and convection during measurements, or between measurements.
3.11.5 Calorimetric measurement
The whole-body average SAR can be determined using calorimetric methods. In a normal calorimetric measurement, a full-size or scaled body model at thermal equilibrium is irradiated for a period of time. A calorimeter is then used to measure the heat flow from the body, until the model is at thermal equilibrium again. The obtained total absorbed energy is then divided by the exposure time and the mass of the body model, which gives the whole-body SAR. The calorimetric twin-well technique uses two calorimeters and two identical body models. One of the models is irradiated, and the other one is used as a thermal reference. This means that the measurement can be performed under less well-controlled thermal conditions than a normal calorimetric measurement.
Calorimetric measurements give rather accurate determinations of whole-body SAR, but do not give any information about the internal SAR distribution. To get accurate results a sufficient amount of energy deposition is required. The total time of a measurement, which is determined by the time to reach thermal equilibrium after exposure, may be up to several hours. Partial body SAR can be measured by using partial-body phantoms and small calorimeters.
3.11.6 Body current measurement
Measurement devices for body current may be carried out in two categories:
– Measurement devices for body to ground current.
– Measurement devices for contact current.
18.104.22.168 Induced body currents
Internal body currents are induced in persons occur from partial or whole-body exposure of the body to RF fields in the absence of contact with objects other than the ground.
The two principal techniques used for measuring body currents include clamp-on type (solenoidal) current transformers for measuring current flowing in the limbs, and parallel plate systems that permit the measurement of currents flowing to ground through the feet.
Clamp-on current transformer instruments have been developed that can be worn.
The meter unit is mounted either directly on the transformer or connected through a fibre-optic link to provide a display of the current flowing in a limb around which the current transformer is clamped. Current sensing in these units may be accomplished using either narrowband techniques, e.g., spectrum analysers or tuned receivers (which offer the advantage of being able to determine the frequency distribution of the induced current in multi-source environments, or broadband techniques using diode detection or thermal conversion.
Instruments have been designed to provide true r.m.s. indications in the presence of multiple frequencies and/or amplitude-modulated waveforms.
The upper frequency response of current transformers is usually limited to about 100 MHz however air cored transformers (as opposed to ferrite-cored), have been used to extend the upper frequency response of these instruments. Whilst air-cored transformers are lighter and therefore useful for longer term measurements, they are significantly less sensitive than ferrite cored devices.
An alternative to the clamp-on device is the parallel plate system. In this instrument, the body current flows through the feet to a conductive top plate, through some form of current sensor mounted between the plates, and thereby to ground. The current flowing between the top and bottom plates may be determined by measuring the RF voltage drop across a low impedance resistor. Alternatively, a small aperture RF current transformer or a vacuum thermocouple may be used to measure the current flowing through the conductor between the two plates.
Instruments with a flat frequency response between 3 kHz and 100 MHz are available.
There are several issues that should be considered when selecting an instrument for measuring induced current.
Firstly, stand-on meters are subject to the influence of electric-field induced displacement currents from fields terminating on the top plate. Investigations have shown that apparent errors arising in the absence of a person are not material to the operation of the meters when a person is present.
Secondly, the sum of both ankle currents measured with clamp-on type metres tends to be slightly greater than the corresponding value indicated with plate type meters. The magnitude of this effect, which is a function the RF frequency and meter geometry, is not likely to be material. Nonetheless, the more accurate method of assessing limb currents is the current transformer. The precise method of measurement may depend upon the requirements of protection guidelines against which compliance assessments are made.
Thirdly, the ability to measure induced currents in limbs under realistic grounding conditions such as found in practice need to be considered. In particular, the differing degree of electrical contact between the ground and bottom plate of the parallel plate system and the actual ground surface may affect the apparent current flowing to ground (Ref.).
Measurements can be made using antennas designed to be equivalent to a person. This enables a standardized approach to be used and permit current measurements to be made without the need for people to be exposed to potentially hazardous currents and fields.
3.11.7 Contact current measurement
The current measurement device has to be inserted between the hand of the person and the conductive object. The measurement technique may consist of a metallic probe (definite contact area) to be held by hand at one end of the probe while the other end is touched to the conductive object. A clamp-on current sensor (current transformer) can be used to measure the contact current which is flowing into the hand in contact with the conductive object.
Alternative methods are:
− the measurement of the potential difference (voltage drop) across a non-inductive resistor (resistance range of 5-10 Ω) connected in series between the object and the metallic probe holding in hand;
− a thermocouple milliammeter placed directly in series.
The wiring connections and the current meter must be set up in such way that interference and errors due to “pick-up” are minimized.
In the case where excessively high currents are expected an electrical network of resistors and capacitors can simulate the body’s equivalent impedance.
3.11.8 Touch voltage measurement
The touch voltage (no-load-voltage) is measured by means of a suitable voltmeter or oscilloscope for the frequency range under consideration. The measurement devices are connected between the conductive object charged by field induced voltage and reference potential (ground). The input impedance of the voltmeter must not be smaller than 10 k.