FAQ on Transformers

1. I live in France in the vicinity of a high voltage transformer station. My land is adjacent to this transformer and the living room and the bedrooms are 10 meters away. The electric company tells me that it transforms 20 kV in 220 V for the power supply of dwellings. Every day, my children play in the garden and I’d like to ascertain the danger of this transformer. Do you have any value of EM field close to this kind of transformer?

It is very difficult to estimate the field, measurements are always welcome.

We already took measurements around a transformer in Belgium. With a primary at 15kV and a secondary at 400V (it is three phases voltage: 400 V is distributed to customers who receive 230 V between the phase and neutral). A typical charge from such a transformer is from 200 to 400 kVA (kiloVolt Ampere), which means more or less 360 Amperes at 250 kVA (at the low voltage side).

In such a case, we measure the following (this depends, of course, on the configuration, but the order of magnitude must be right) :

  • maximal value (near « low voltage » output) : 75 microTesla next to the transformer and 1 microTesla at 5 meters ;
  • in other sides (as near “high voltage” output) : 3 microTesla next to the transformer and 0.2 microTesla at 5 meters.

As to potential health risks from exposure to magnetic fields, we refer you to “Effects on health?” page for further information.

2. I’d like to buy a plot of land to build a house. A transformer is adjacent to this land (40 m): it transforms medium voltage (22 kV) to low voltage (220 V/ 380 V). The tranformator power is 400 kVA, the medium voltage line is composed of 3 cables. From which distance is the magnetic field intensity under the 0,4 µT epidemiological threshold?

This kind of medium voltage transformer is common. We have measured fields in the vicinity of such transfomers and we can say the following for the cases measured: 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 µTunless you are just above the underground cable that distributes power.

At 40 m, we can objectively say that the transformer no longer has any direct influence on the ambient field, the existing field arising from other influences (overhead lines, underground cables, household appliances, etc.).

3. Specialised in environmental architecture projects, we are very aware of the effects of fields. By default, we follow a precautionary principle and we endeavour to stay below the epidemiological threshold of 0.4 microtesla when dealing with a HV line. But what about a HV cabin? Are the precautions the same? Does the housing (masonry) play a role? What are the distance rules?

Our measurements on site have shown that at around 5m from a cabin, the epidemiological threshold is no longer exceeded. But there can be specific configurations that change the minimum distance: only a measurement can then quantify the value.

A masonry wall has no effect on a magnetic induction field.

4. Is it harmful to place a 20kV transformer in a school playground? How far from the transformer do we have to move away to be out of the 0.2 – 0.4 µT zone considered in epidemiological studies?

At first, it is important to keep in mind that 0.2 – 0.4 µT values are 24h mean values. Thus, these values can be exceeded at some moments of the day.

We think that it is not advisable to place a transformer in a school playground. Beyond the risk of potential effects of magnetic fields, actually fire risks have to be considered, as well as the availability of the transformer in case of an intervention. A 20 kV/400V transformer that supplies a village in electricity can undoubtedly be placed elsewhere than in a playground for these reasons.

If the location cannot be modified, a distance of several meters from the transformer would be necessary… this will, however, reduce the play area for the children and will increase installation costs for the operator.

Other precautionary measures could also be taken:

  • to adopt trefoil configuration for both input and output from the transformer station ;
  • to position the transformer station so as to have a low level voltage output in a protected area where very few children are likely to go ;
  • to choose a cabin with an aluminium sheet (at least 3mm thick) to partly shield it ;
  • to pass the cables underground as deep as possible under the access areas.

Moreover, it could be useful to impose an overhaul on the operator, expenses paid by him, of your entire electrical installation/wiring, with an adequate grounding, differential protection… because it is not impossible that the presence of the transformer might further the presence of leak current that could induced contact voltage on every metallic structure, not of foremost danger, but which involves international research for their harmful side effects.

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