Shielding from household electric fields

Power frequency electric fields

Electric fields (EF) are a physical risk factor that are present everywhere where electricity is used. Power frequency electric fields are created when there is a fluctuation in voltage. The power comes from the electric grid and is distributed throughout the household or a business building. Power frequency electric fields are present everywhere where electricity is installed. These electric fields may cause negligent or high exposure levels, depending on many factors, including

• type of electrical wiring – two prong electric cables and sockets provide no grounding,
• placement of electrical wiring – the further the wiring, the less exposure,
• grounded metal construction elements – ungrounded metal within the building will re-emanate EF,
• type of construction materials – conductive and semi-counductive materials may attract and re-emanate EF making large surfaces as radiators of EF,
• abundance of electrical wiring over large ares – the more electrical cables are distributed over the building, the more EF is created,
• type of electric appliances – grounded and shielded appliances produce negligent EF levels,
• seasonal and environmental conditions – higher air humidity may turn construction materials into semi-conductors, which then may attract and re-emanate EF,
• the distance to the sources – the closer the observer to the source, the creater the exposure.

Electric power fluctuates 50 or 60 times in a second, meaning a frequency at 50 or 60 Hz (50 Hz in Europe and elsewhere in the worls, 60 Hz in America). Because of the frequency, power electric fields are different from static electric fields, such as from clothes and from radiofrequency electric fields, such as from mobile phones and FM-radio and TV transmitters.

Electric field sources

In a household, the sources of power frequency electric fields are for example, electric wires, sockets, switches, plugged in electrical appliances and all other devices and installations that have been connected to the power grid. As a rule, electric appliances radiate electric fields when these are plugged in to the power socket.

Often electric fields also radiate from walls, floors and ceilings, because electric cables run thorugh these and the electric field penetrates the construction materials. Many materials, such as dry walls (made out of gypsum boards) are easily penetrated and do little to attenuate the field. Sometimes building walls radiate electric fields even when there’s no electric cables running through these. This indicates a conductive or semiconductive material within the wall that collects the field from elsewhere and then retransmits it. Metal support frames within the wall are an example of such perfectly conductive construction materials that retransmit electric fields.

Building materials may sometimes become semi-conductive due to environmental conditions. In the winter, when indoor heating is used, the indoor air is dry and so are the walls – this may be enough to remove any electric fields from the walls. In the summer, when indoor humidity is high (RH>50%), the walls may become semi-conductive. When electric cable is nearby, the wall may retransmit the electric field and become itself a source of electric field. This applies also to the floors and ceilings.

Shielding electric fields

All conductive and partially semiconductive materials will re-emanate electric fields, when ungrounded. On the other hand, grounding conductive construction elements would stop the electric field and direct it into the ground via the least resistive grounding path. Grounding also works for most semi-conductive materials.

The afore mentioned conductive construction elements are required to be grounded in many countries. Besides electrical safety, there are also other resons for grounding, such as preventing excessive corrosion.

When the conductive construction elements have not been grounded during the construction of the building, some options remain. Grounded shields are the most used option to remove unwanted electric fields. A designated area (e.g. workstation, resting area) would would need to be determined and the respective sources of electric fields identified. Next, one of such investigations is depicted by illustrations.

Case study – blocking electric fields from the floor and walls

In a case study we measured electric fields emanating from the floor and from the dry wall. There were no electric cables running withing the walls nor the floor, but as it was a summer period, the humidity had turned the walls into semi-conductive material, which enabled nearby electric fields to latch onto these, so the walls become itself radiators of electric field. A metallic shield was used to capture the electric field and direct into the ground. One kitchen aluminum foil roll was used to construct the shield sheets, where the narrow foil sheets were taped together, paying also attention that the foil sheet would overlap and create a single conductive area). The resulting test solution ended with satisfactory result of ~5 V/m.

DISCLAIMER: This post describes an episode of a scientific research and should not be tried by anyone without a proper training.