Power
frequency EMF measurements
John Swanson, Technology. &
Science Labs., National Grid Transco Research. & Development. Centre,
1
General principles
Measurements of electric and magnetic fields can be
made for various purposes. Depending on
the purpose, different techniques will be appropriate. There is no single “correct” procedure; the
correct procedure will depend on the reason the measurements are being made and
the use that will be made of the results.
When embarking on a measurement programme,
therefore, it is important first to identify clearly the objective. This objective must be used to decide on the
procedures to be followed. This chapter
discusses the choices and issues involved, and gives
suggestions for appropriate procedures, but is not a substitute for intelligent,
informed choice on the part of the person specifying or performing the
measurement.
The main international standard relating to
measurements at power frequencies is a 1998 IEC standard
IEC-61786, “Measurement of low-frequency magnetic
and electric fields with regard to exposure of human beings – Special
requirements for instruments and guidance for measurements”. The provisions of this standard are referred
to where appropriate in the following material.
2
Types of measurement
The following are some of the main reasons for
performing measurements. This is not,
however, an exhaustive list.
Simple characterisation of the field in a building
The objective is to characterise the field in a
home, a work location, or similar, by a single number. Depending on resources and time available,
this can be done by a single measurement at a single location; a single
measurement at each of a series of locations; a sequence of measurements over
time at a single location; or a sequence of measurements over time at multiple
locations.
The objective is to make measurements of the field
at a location, specifically how it varies over space or time, to enable the
source of the field to be identified.
Comprehensive characterisation of the field in a building
The objective is to collect more data on the field
in a building than just a single number, so as to permit extraction of desired
information at a later date.
The objective is to perform measurements that
relate to a particular source of field (eg a power
line, an item of equipment) rather than to the field in a particular place, so
as to characterise that source.
The objective is to measure the exposure of a
person over a period of time during which they are exposed to fields from various
sources.
Compliance with
exposure limits
The objective is to assess whether a given set of
EMF exposure limits are exceeded. This
can be done either by assessing the fields in an area in which people will be
present, or by monitoring the exposure of the people, or a combination.
Laboratory
measurements
The objective is to measure the field within
experimental apparatus in a laboratory, eg the field
produced by coils used to expose an experimental system to magnetic fields.
3
Choice of instrument for magnetic
field measurements
3.1
Sensor technology
There are three technologies for measuring
magnetic fields: fluxgate magnetometers, Hall effect
devices, and search coils.
3.1.1.
Fluxgate magnetometers
These comprise a ferromagnetic core which is driven
in and out of magnetic saturation in opposite directions by a high-frequency
current generated within the instrument.
The addition of an external field creates an asymmetry, and the
instrument uses this asymmetry to measure the field. Fluxgate magnetometers are sensitive to all
external fields up to a certain frequency, static fields as well as alternating
fields. The static and alternating
components of the field can be separated and recorded separately by the
instrument circuits.
Fluxgate magnetometers are usually the preferred
choice when it is desired to measure static fields as well as alternating
fields. The fluxgate sensor itself can
be made reasonable small and is usually remote from the bulkier remainder of
the instrument, so they can have uses where it is necessary to probe the field
in difficult locations. Other than these
specific applications, their use is limited, as they are more expensive and
have higher battery consumption than search coil instruments. Care should be taken if using them in laboratory
settings, as they produce a finite high-frequency field themselves as part of
their operation which could perturb an experimental setup.
3.1.2. Hall effect devices
These devices pass a current through a suitable
semiconductor, and detect the field via the voltage produced across the element
perpendicular to the current. Like
fluxgate magnetometers, they detect the instantaneous total field, the sum of
the static and any alternating fields.
They are often used in other applications because they can measure high
fields at the Tesla level and above.
However, they are usually not very sensitive to low fields and suffer
from zero-point drift, and the probes are often fragile, which means they have
few applications in EMF dosimetry.
3.1.3. Search coils
Search coils are simply coils of wire. An alternating magnetic field induces a
voltage in the coil. Except for the
specialised instances where fluxgate magnetometers are appropriate, search
coils are the preferred technology for EMF dosimetry
where it is not desired to measure the static field as well.
Characteristics of
search-coil instruments
·
Size of
coil
To obtain a large enough signal, search-coil
instruments tend either to have relatively large coils (10-20 cm), or small
coils (less than 1 cm) with ferrous cores.
Larger coils are perfectly acceptable as long as the field itself does
not vary significantly over the area of the coil. Thus they will often be acceptable under
power lines or in the middle of rooms, but will be less appropriate close to
conductors or to equipment. IEC
specifies that the coil should have area 0.01 m2 or less.
It is sometimes recommended to use large coils so
as to measure the average field over an area comparable to part of the human
body. Usually, however, where the field
varies over space this rapidly, it is preferable to measure the variation of
the field with a small probe and then to apply any desired averaging to the
results of those measurements.
Small, ferrous-cored coils are often preferred and
produce a smaller, more versatile instrument.
The one disadvantage is that the ferrous core produces non-linearities at high fields, greater than 1 mT. For most
purposes this is irrelevant as fields are rarely that high, but care should be
exercised if high fields are to be measured.
·
Number
of coils
Each coil measures the component of field in one
direction. Instruments have either one
or three orthogonal coils. The choice
depends on the purpose of the measurement.
When identifying and investigating different sources of field, a single
coil can be useful, as the extra information on the direction of the field can
assist in identifying the source. For
most other applications, however, three coils are more useful.
The resultant field from multiple sources at the
same frequency always traces out an ellipse.
There are two alternative ways of quantifying an elliptically polarised
field, shown in figure 1.
Figure 1 Alternative
measures of an elliptically polarised field. 1:
“maximum”
field. 2: “resultant” field
Vector 1 gives the rms of
the field along the major axis of the ellipse, which is the direction of
maximum field. This is known as the
“maximum” of the field. Vector 2 gives
the actual rms of the field, known as the “resultant
field”.
The “maximum” field, vector 1, can be measured by
rotating a single coil until the maximum value is obtained. The “resultant” field, vector 2, can be
measured using three orthogonal coils. The
rms of the signals from the three coils are combined as root-sum-of-squares to give the resultant
field. This applies regardless of the
orientation of the coils relative to the field.
Three orthogonal air-cored coils can be arranged so
that, to a good approximation, their centres are coincident and therefore they
measure the field at the same point in space. Figure 2a shows an example. Ferrous-cored coils are nearly always physically
separate. This still gives satisfactory
results as long as the field does not vary over the distance scale of the separation
of the coils. As this can be small (of
the order of a cm) this is usually acceptable.
Figure 2b shows three such ferrous-cored coils.
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Figure 2 Examples of instruments using |
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(a) three
orthogonal coils arranged to be centred on a single point |
(b) three
separate orthogonal coils |
If only a single-axis instrument is available, the resultant
(total) field can be measured by performing three successive orthogonal
measurements. This can have limited
accuracy, however, if the field varies over the time taken to perform these
measurements. It can also be difficult
to locate the sensor at the same point each time; if accuracy is required, a
jig to locate the instrument should be used.
3.2
Type of display
Increasingly, most displays are digital. The main occasion when an analogue display is
more useful is when investigating a source, when variations in the field as the
meter is moved need to be readily observed, but even here a digital display can
be used.
3.3
Range and resolution
Average background fields in homes (ie fields in the general volume of the home, not close to
equipment) range from a few tens of nanotesla in some
European countries to over a hundred nanotesla
in
The desired range of the instrument depends on the
maximum field likely to be encountered.
Background fields in homes are rarely greater than 1 µT; fields in
industrial settings or near domestic equipment can be 1 mT
or more. Occupational exposure limits
are typically of the order of a mT,
so a range of at least this is necessary to assess compliance with such limits.
3.4
Storage abilities
If using measurements of field to investigate a
source, no storage ability is necessary; the changes in field as the instrument
is moved are observed in real time. For
some other purposes, storage within the instrument is not needed; the reading
can be written down or entered into a database.
For many purposes, however, it is helpful or necessary to store results
within the instrument. The choice of
measurement interval and total duration is a compromise limited by the total
storage capacity available in the instrument.
3.5
Frequency response
The output of a search coil is proportional to
frequency. Most instruments adapt this
frequency response within the instrument to give a final output which is either
broadly flat between a lower cut-off (typically 20-30 Hz) and an upper cut-off
(typically 500 Hz – 5 kHz), or is sensitive only to the power frequency, 50 or
60 Hz.
The choice of frequency response is determined by the
characteristics of the field to be measured and the purpose of the measurements. In many cases, the source of the field is the
power system. Then, the main component
of field will be at 50 or 60 Hz, with smaller harmonics at multiples of this,
principally three times or “third harmonic”, 150 or 180 Hz. Most flat responses will be adequate for
this. However, if there is particular
interest in harmonics, or if the sources is such that higher harmonics (eg from some ac-dc power conversion processes) or lower
frequencies (eg from some railways at one-third the
power frequency) are present, then an instrument with a particular frequency
response may be appropriate.
Note that search coils produce a signal when
rotated in a static field. To avoid this
being erroneously recorded as a power-frequency signal,
many instruments have filters to reduce the response at low frequencies. If an instrument sensitive to lower
frequencies is used (eg because railways are to be
measured), extra care must be taken to avoid this interference.
Exactly how flat a frequency response is depends on
the sophistication of the instrument.
However, achieving a flatter frequency response also tends to increase
the power consumption and the weight of the instrument. Therefore, instruments designed to be small,
light and with long battery life may have poorer frequency responses. This is part of the choice that has to be
made, but in many instances, the flatness of the frequency response will not
affect the result much.
3.6
rms and other measures
Increasingly, instruments are made with true rms detection of the field, and this is regarded as
preferable for most purposes.
Alternatives are rectified average and peak
detection. There may be occasions when each
of these is desirable. However, in
general, these are a legacy of less developed instrument technologies, and
should be avoided. They cause particular
problems when harmonics are present.
3.7
Accuracy
The accuracy required of an instrument should, in
principle, be determined by the measurement purpose. However, IEC specifies an overall uncertainty
in the measurement of a uniform field of ±(10% of
reading +20 nT), and most commercial instruments can
be regarded as accurate to at least this 10% value.
3.8
Other factors
IEC specifies that an instrument should function
over a temperature range of 0 to 45 °C and
from 5% to 95% relative humidity.
If measurements are to be done in the high electric
fields close to high-voltage equipment, the instrument should be immune to
these fields. IEC specifies that fields
of 20 kV m-1 should produce no more than 20 nT change in the reading. Most instruments will have satisfactory EMC
performance; IEC specifies immunity to 3 V m-1 from 150 kHz to 1 GHz
assessed in accordance with standard IEC 61000 (IEC 1995).
4
Choice of instruments for
electric-field measurements
Most practical electric-field measurements are made
by measuring the voltage (or current) between two parallel plates perpendicular
to the electric field. Other technologies
are available, but have few advantages.
Such a meter is a “free body” meter; the alternative is a “ground
reference” meter, which measures the voltage between a single electrode and
ground, but this is less common.
Parallel-plate sensors for electric fields tend to
be physically larger than magnetic-field sensors. Therefore, electric-field instruments are
more often single-axis. However,
three-axis meters are available, with the three separate parallel-plate
assemblies either on three orthogonal faces of a cube or spread over the
surface of a sphere.
One common measurement scenario is near or under
power lines. In this instance, the
electric field is essentially vertical for the first metre or two at least
above ground level. Therefore, in this
instance, a single-axis meter, aligned to measure the vertical field, is perfectly satisfactory.
Many of the issues discussed under magnetic fields
apply to measurements of electric fields as well: frequency response, storage,
type of display, etc. There is, however,
one major issue with electric fields that does not apply to magnetic fields.
4.1
Perturbed and unperturbed
electric fields
Electric fields are perturbed by any conducting
object. This includes the person making
the measurement and any conducting supports for the sensor.
Electric-field measurements should be either of the
unperturbed field or of the perturbed field and it should be clear which is
being measured. Measurements made by a
meter worn by a person are unavoidably of the perturbed field. In most other cases, however, it is
preferable to attempt to measure the unperturbed field. This can sometimes be an ambiguous
definition, where the field is perturbed by an object not connected with the
measurement process. The usual objective
is to measure the field that would be present in the absence of the person, but
including any perturbation caused by fixed objects that would still be present
when the person is not.
To obtain an unperturbed measurement, the
instrument sensors must be supported in a way that does not perturb the field,
and the person performing the measurement must be sufficiently distant.
The instrument can be supported either on an
insulating pole held horizontally, or on a vertical insulating support (a tripod
or similar). In both cases, if readings
in real time (as opposed to stored and read only later) are desired, the
sensors are connected to the rest of the instrument by a radio link or a
fibre-optic, or the instrument is self-contained with a display large enough to
be read from a distance.
A vertical tripod or similar support is often
mechanically easier and makes it easier for the operator to be distant. However, in humid conditions, moisture can
settle on both a tripod or a pole, making them less
good insulators and perturbing the reading.
As most electric fields are vertical near ground level, the vertical
support, which is parallel to the field lines, leads to greater perturbation
than the horizontal pole, which runs perpendicular to the field lines, along an
equipotential, and therefore is less sensitive to
small amounts of moisture. This is a
reason for preferring horizontal poles when some degree of perturbation is
regarded as inevitable.
The person making the measurements should be 2 m
distant from the instrument to ensure that perturbation is negligible. Measurements at 1 m may be acceptable but
less accurate. Hand-held measurements
should be avoided.
The perturbation caused by the person making the
measurement is, simplistically, to increase the field towards the top of their
body, and to reduce it towards the bottom of their body. There is therefore a height of measurement, around
1.5 m above ground and therefore higher than the 1 m above ground which is
recommended as a suitable measurement height for other reasons, where the
perturbing effect of the person is neutral.
This means in practice, the distance of the person from the instrument
may not be so critical. However, this
should not be relied on, and for good measurements, the person should always be
2 m distant.
Measurements of the perturbed electric field are
made by attaching a meter to a person’s body.
However, the reading obtained is extremely dependent on the amount of
perturbation, which in turn is extremely dependent on the exact location of the
meter on the person’s body. For certain
simple geometries and for certain locations on the body (eg
the top of the head of a person standing upright in a vertical field, where the
factor is approximately 20) it is possible to calculate the conversion factor
from the perturbed to the unperturbed field.
In most situations, however, this conversion factor cannot be
calculated, and varies as the person moves anyway. Therefore, such perturbed measurements have
limited use, and should never be compared directly to unperturbed fields.
The need to measure unperturbed fields undoubtedly
adds considerably to the effort and cost of making electric-field measurements,
but in most situations, it is the unperturbed rather than the perturbed field
which is the most useful.
5
Calibrations
It is necessary to be sure that any instrument used
is giving an acceptably accurate reading; this is achieved by calibration. IEC requires the overall uncertainty of the
calibration process to be no greater than ±(5% +10 nT).
Most instruments will be supplied calibrated by the
manufacturer. Periodic re-calibration
will be needed. This can be performed
either by the user or by sending the meter to a specialist calibration service
or to the manufacturer. Calibration
systems are not simple, and probably only users with
multiple instruments will be able to justify maintaining their own calibration
systems.
Magnetic fields are calibrated by placing them in a
known field produced by a coil system of known geometry. It is common to refer to these coils as a Helmholtz arrangement.
In fact, a Helmholtz pair, two equal coaxial
circular coils separated axially by one radius, is a legacy of the time when
fields had to be calculated analytically; its main advantage is the ability to
express analytically the field and its first few derivatives at the
centre. There are alternative systems
which are more efficient at producing a uniform field (for a review, see Kirschvink 1992). The simplest system is a single square coil;
the best results are achieved from three or four coaxial coils, of carefully
chosen numbers of turns and axial separation.
The size of the coil is determined by the requirement that the field at
the centre must be reasonably uniform over the area of the search coil in the
instrument being calibrated. IEC
requires less than 1% (or 1.5% for larger probes under certain circumstances) departure
of the field anywhere over the area of the search coil from the field at the
centre. For a 0.1 m diameter search
coil, this could be achieved by a single square coil of side 1 m, or a Merrit four-coil system of side 0.4 m.
IEC requires the field in the calibration coil to
be known to ±3%. In most practical
situations, the geometry of the coil system is known to rather better than
this, so the limit on the accuracy of the calibration system is the accuracy of
the measurement of the current flowing in the coils.
Electric fields are calibrated by placing them
between two parallel plates across which a known voltage is applied. The uniformity of the field is improved by
grading rings round the edges of the system, but to avoid perturbation, the
system of plates should still be placed a minimum distance away from objects
that might perturb the field. The limit
on the size of the plates is then proximity effects between the meter and the
plates. IEC specifies that a meter of
maximum dimension 0.23 m requires plates of 1.5 m square separated by 0.75 m.
Calibration at high electric fields may involve
applying voltages to the calibration plates sufficiently high such that corona
becomes an issue and corona guard rings become necessary.
For both electric and magnetic fields, when a
calibration is performed, there are three alternatives. One is simple to accept the instrument as
passing the calibration provided it is within a certain margin of the correct
field. A second is to adjust the
instrument so it reads the correct value, and a third is to record a correction
factor which should be applied to any readings taken to give the correct
reading. IEC allows all three, and the
choice is determined by the ease of adjusting the meter and the accuracy
required. The option of recording and
subsequently applying a correction factor gives the highest accuracy, but is
best avoided in many practical measurement situations where it increases the
potential for mistakes.
All calibrations should be performed as part of a
quality-controlled calibration system with traceable records. Exactly how this is done and what is required
depends on the quality regime in operation.
Calibrations should be performed at set
intervals. It is common to make this
yearly, but if documented experience shows that the meter retains its
calibration acceptably for longer than this, as will often be the case with
commercial instruments, then a longer calibration interval is acceptable.
6
Sample measurement procedures
This chapter has emphasised that there are no
universal rights and wrongs in EMF measurements; the choices made depend on the
purpose of the measurements. The
following are therefore suggestions for the particular scenarios identified at
the start of this chapter and should not be regarded as definitive.
·
Simple
characterisation of the field in a building
The objective is to characterise the field in a
home, a work location, or similar, by a single number. Depending on resources and time available,
this can be done by a single measurement at a single location; a single
measurement at each of a series of locations; a sequence of measurements over
time at a single location; or a sequence of measurements over time at multiple
locations.
Probable measurement procedure: use a
battery-powered instrument with logging facilities and three ferrous-cored
coils. Leave it in a standard location
for 24 hours or longer. The location
should be clearly specified in the study protocol, eg
“1 m above floor level at the middle of the side of the child’s bed”, and
should be distant (at least 1 m) from any items of electrical equipment. This usually means the centre of a room, or
as near it as is compatible with the occupants’ use of the home. Standardisation may be improved by providing
a stand to hold the meter. The logging interval will be determined by the need
for the logging capacity to last the required time. An instrument with 1 nT resolution will be preferable in homes but 10 nT resolution may be acceptable.
Similar measurements for electric fields are
possible but will be more problematical because of perturbations by people
walking near to the meter.
The objective is to make measurements of the field
at a location, specifically how it varies over space or time, to enable the
source of the field to be identified.
Probable measurement procedure: use a hand-held, battery-powered
instrument with a clear read out (analogue will be easier but digital is
acceptable as well). Walk round the
location observing the reading so as to identify areas of high field, and track
these to their source. It may be helpful
to have a single-axis readout as the direction of the
field helps determine the direction of the current producing it.
·
Comprehensive
characterisation of the field in a building
The objective is to collect more data on the field
in a building than just a single number, so as to permit extraction of desired
information at a later date.
Probable measurement procedure: use several
instruments to measure further characteristics of the field. For example: to characterise the field at
more than one location over a house, one approach is to leave several
instruments logging for 24 hours at locations spread over the house. Another approach is to measure a profile of
field (using a single instrument but making the measurements as close together
as possible) from one side of the house to the other.
The objective is to perform measurements that
relate to a particular source of field (eg a power
line, an item of equipment) rather than to the field in a particular place, so
as to characterise that source.
Probable measurement procedure to characterise a
power line: use a three-coil magnetic field meter. The size of the coils is less important. Choose one or a few standardised locations, eg directly under the centreline of the power line at
mid-span, and possible some standard distances perpendicular to the line, eg 10 m, 25 m, 50 m.
Perform measurements at 1 m above ground. For electric fields, use a single-axis meter,
aligned to measure the vertical field, held on a horizontal insulating pole at
1 m above ground, or a clean, dry vertical insulating stand. If extra detail is desired, measure the field
at more than one height: ground level, 1 m and 2 m above ground. Measurements at one point in time obviously
give the field only at that time. If the
load on the line is available, the measured field at one load can be scaled to
other loads; or the measurements can be logging measurements over an extended
period of time.
The objective is to measure the exposure of a
person over a period of time during which they are exposed to fields from
various sources.
Probable measurement procedure: use a small
three-axis meter. The priority is to
prolong battery life and to reduce the bulk, so factors such as the flatness of
the frequency response may be sacrificed.
Get the subjects to wear it at a standard position on their body,
possibly in a pouch on a belt, or for children, in a child-friendly back-pack
or similar. Preferably arrange for any
display to be blanked to reduce the temptation for the subject to experiment
with approaching high-field sources.
Give clear instructions on what to do at night time, ie
where to place the meter when it is taken off.
There are some very specific issues related to personal
exposure measurement (dosimetry) for the assessment
of occupational exposures to magnetic fields.
They are discussed here.
References
Diplacido J, Shih C H and ware B J 1978 Analysis of
the proximity effects in electric field measurements IEEE Transactions on Power
Apparatus and Systems PAS-97
2167-2177
IEC 1995 Electromagnetic
compatibility (EMC) IEC 61000
IEC 1998 Measurement of
low-frequency magnetic and electric fields with regard to exposure of human
beings – special requirements for instruments and guidance for measurements IEC
61786
Kirschvink J L 1992 Uniform magnetic fields and double-wrapped
coil systems: improved techniques for the design of bioelectromagnetic
experiments Bioelectromagnetics 13 401-411