Measurement of electromagnetic fields Any system using electricity as an energy source generates electromagnetic radiation when it is in operation. Depending on the design of the system, the electromagnetic fields which it produces may be propagated in the space around it, extending significantly further than the external limits defined by its enclosure (casing) or the site where it is installed. This is the case for electrical machinery, motors, welding units, induction furnaces, high-voltage power lines, transformer stations, household electrical appliances and electronic instruments used for data processing, transmission, monitoring or measurement. These electromagnetic fields interact with matter, both inanimate (interference with nearby electrical devices) and animate (plants, animals, etc.). It is therefore important to be able to measure the values of the radiated magnetic and electric fields propagated around any electrical or electronic device: • to overcome the purely technical problems linked to the electromagnetic compatibility of instruments and machines, • but also to make sure that the people living and working near these electrical systems are not exposed to fields liable to cause lasting or even temporary negative effects on them.
1. THE ELECTROMAGNETIC WAVE
2. THE ELECTROMAGNETIC SPECTRUM
The electromagnetic wave is the radiated energy produced by an electrical load. It is characterized by oscillation of the electrical and magnetic fields. Each system generating or absorbing electrical energy is the source of electromagnetic waves in the form of variable electric fields and magnetic fields which are propagated in the air at the speed of light. Roughly speaking, an electromagnetic wave comprises: The electric field (E): generated by the difference in potential between two conductors subjected to an electrical voltage, this field depends on the voltage V. The magnetic field (H): as this field is generated by a current in a conductor, it depends on the current i.
The electromagnetic spectrum is the decomposition of the electromagnetic radiation into its different components in terms of wavelength. Some waves can be detected with the human eye, while others have much lower frequencies detectable using radio devices.
Wavelength (m)
10 2
10 3
1
Size of the wave
Common name of wave
10 -1
10 -3
10 -4
This dot
10 -5
10 -6
Cell
Radio waves
10 -7
10 -8
10 -9
Virus
Infrared
H
E
10 -2
Football
House
i
V
10 1
10 -10
10 -12
Water molecule
Ultraviolet
Microwaves
10 -11
"Hard" X-rays "Soft" X-rays
Gamma rays
Sources
AM radio
In the case of a sinusoidal alternating wave, the electric field E and the magnetic field H are sinusoidal and in phase. Their directions are perpendicular to one another and perpendicular to the direction of propagation.
Frequency (Hz)
10 6
FM radio
10 7
10 8
Microwave oven Mobile phone 10 9
Wire
10 10
Radar
10 11
Human body
10 12
10 13
Electric light bulb
10 14
10 15
Synchrotron
10 16
10 17
Radiography
10 18
10 19
Radioactive elements
10 20
Optical fibre
Wireless
E
3. INTERACTIONS WITH MATTER The effects of electric and magnetic fields on matter and tissues vary according to their frequency and their intensity. Low-frequency fields are liable to induce electric currents in matter and biological tissues. Effects described as "thermal" may follow. These thermal effects are the basis for the action of higher-frequency fields used in certain applications (cooking and drying with microwaves).
H V Representation of the three components of an electromagnetic wave
This wave is characterized by its frequency F in Hertz (Hz) or its wavelength in metres; these two quantities are linked by the following relation:
λ = Co / F where Co = the speed of light in m/s, i.e. 300,000 km/s = 3 x 108 m/s F = frequency in Hz λ = wavelength in m
4. OBLIGATIONS The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has defined exposure limits adopted in many countries. The exposure limits adopted by the European Community are based on a recommendation issued by the ICNIRP, including those in Directive 1999 / 519 / CE (public) and the recent directive 2013/35/UE of 26th June 2013 concerning workers' exposure to electromagnetic fields, which must be transposed into law in the member states by 1st July 2016. For the latter directive, the employer's role will be to assess the hazards and determine the exposure which can be measured in order to find out objectively whether the standard recommended thresholds have been exceeded or not.
Example: for a wave at 300 MHz, the wavelength is 1 metre.
Portable, on-site and laboratory test and measurement instruments z 2015 Catalogue
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