Outdoor ELF-EMF exposure
Electrical energy has an inherent problem: it is not storable (except in batteries). To mitigate this difficulty, the only solution is to directly connect consumers to electricity production sites and produce electricity on demand.
Transmission and distribution networks convey electricity from the production stations (nuclear, hydraulic, thermal, wind) to supply areas. In order to minimise energy losses, electricity transport over long distances requires high-tension power lines. Distribution to consumers is carried out with low-tension power lines. Transformers are used to modify voltage from high to low tension.
The electrical grid (i.e. lines, cables, transformer stations) transmits, divides, and distributes the energy provided by power stations to users. The network was designed for an AC current of 50/60 Hz using a three-phase current (see further information in Electrical concepts).
EMF exposure around 2 kind of powerlines (380kV and 150kV)
The electric field and the magnetic field generated by an overhead power line diminish rapidly as one moves away from the power line.
Source: Report Deworme Commission (1990)
It is interesting to note that exposure varies according to the charge of the line: here below an example of the magnetic field intensity under a 380 kV powerline in a 5-week period.
Exposures in the vicinity of an overhead powerline and an underground cable
Underground cables are often seen as a solution to protect landscapes. However, the installation of these cables has various disadvantages.
- Compared with overhead lines, the cost per kilometre of cable is appreciably higher (1 km of cable can cost up to 10 times more than 1 km of line according to the tension level, ground profile etc.) (Bastard P., 2000).
- Cables are mainly installed in roadway systems and risk being damaged during work on the road network.
- Cables are also less accessible for repairs, and technology has not yet solved all the problems arising from very high voltage transmission.
External to an underground cable, there is no electric field. However, for the same current intensity, the cable generates a magnetic field higher than that for the overhead line. It decreases more quickly with cable depth (usually between 70 cm and 1.5 m) and lateral distance.
EMF exposure around a distribution transformer
Transformers make it possible to increase or decrease voltage.
- At the delivery point of the power station, the tension ranges from 10 kV to 30 kV. A transformer will increase the voltage to 380 kV, 150 kV or 220 kV.
- To allow distribution, other transformers will decrease the voltage gradually to the one necessary for the consumer (400 V between phases).
Measured values around a middle to low voltage transformer are:
For a 22 kV - 230 V/400 V transformer station (power=400 kVA) : the field value against the transformer is higher on the low voltage side (max value measured = 10 µT), the field being linked to current intensity and not to voltage value.
Beyond 5 m from the transformer, ambient fields at 1,5 m from the ground decrease below 0.4 µT unless you are just above the underground cable that distributes power.
EMF exposure around train lines and HVDC power transmission lines
Les CE statiques qui peuvent être mesurés sous des lignes de transport de l’électricité ou des lignes de train sont respectivement de l’ordre de 20 à 30 kV/m et de 600 V/m. Il faut savoir que ce sont des valeurs maximales, mesurées en l’absence de tout obstacle car le CE est facilement atténué.
A proximité des installations électriques en courant continu, on peut mesurer des CM statiques de l’ordre de quelques dizaines de µT sous les lignes de transport d’électricité et de l’ordre de 200 µT sous une ligne de train. Quand on s’éloigne de 5 mètres, on obtient des valeurs inférieures à 10 µT pour les lignes de transport de l’électricité et de l’ordre de 100 µT pour les lignes de train (further information on these values in our FAQ).
Documents & Links
Other topics that might interest you ...
- FAQ: Measurement & Exposure values
- FAQ: Transformers
- FAQ on static fields
- Informations on health effects of static fields
- Electrical concepts - Electrical concepts interesting to have in mind in order to tackle the subject of electromagnetism and electric and magnetic fields without apprehension.
- Electricity network - How does the electrical grid work? All you would like to know about the transmission of electricity from power plant to the outlet on our walls.