Why static charge builds up on people

See also Static shocks, and how to avoid them

Charge can often build-up on people and reach levels that give uncomfortable shocks. Dry air humidity encourages static charge build-up, and under dry external conditions the atmosphere in a building can become even drier. Electrostatic charge build-up can be far worse under these conditions. In the UK the worst case is often in winter (January to March) when cold and dry external air is warmed and brought into the building leading to very dry internal atmospheric conditions.

There are a wide range of factors that can affect the amount of electrostatic charge that can build up as a voltage on personnel. Some common major factors include:

• The floor material and its electrical resistance, and resistance-to-ground

• Personnel footwear, especially the materials of the shoe sole and its electrical resistance

• Atmospheric humidity

• The manner in which a person walks, e.g. scuffing and friction of the shoes against the floor

• Actions of brushing against furniture, sitting and rising from seats

Static electric charges are separated whenever two materials make and break contact. This occurs with regularity between the shoe sole and floor material in walking action. Charge tries to dissipate and recombine where possible, but this cannot happen if it is prevented from moving by insulating materials. Charge builds up if the charge is generated faster than it can dissipate, and a high voltage can quickly result.

Shocks are felt by people when they touch a discharge path (e.g. metal frame or another person), if the voltage on their body exceeds about 4 kV (4000 V).

Charge generation is affected by the types of materials in contact. A guide to this is given in a triboelectric series.

If a material in the series (e.g. wool) is rubbed against a material lower in the series (e.g.rubber) the higher material (wool) normally charges positive wheras the lower material (rubber) charges negative. The further the materials are apart in the table, the stronger the charging action is expected to be. So polyurethane soled shoes (quite common!) walking on a glass floor (not unknown!) is a recipie for static problems! Unfortunately a shoe sole (e.g. PU walking on PP) which gives low generation on one floor material may give high generation with another (e.g PU walking on glass or nylon)

Some shoe sole materials are commonly materials such as rubber and polyurethane (PU) are much lower in the table than wool and nylon which are high in the table. If a PU shoe is worn walking on a wool/nylon mix carpet, high charge generation is expected. Conversely if a PU shoe sole is walked on a polymer carpet or tile (such as PET or PVC) much lower charge generation is likely to occur. However in both cases, charge could build up under certain conditions.

The floor resistance-to-ground (Rg) gives a measure of how easily charge can dissipate to earth. Resistance values above about 10⁹ Ω can be expected to promote electrostatic charge build-up on people and trolleys moving on the surface. Sometimes, if the floor material is identified as a major part of the problem, certain areas may be replaced or covered to provide lower charge generation and lower resistance to ground. In some cases a grounded mat can be provided. Unfortunately modification of the floor covering alone does not guarantee that charge build-up will be prevented, especially under dry atmospheric conditions. As previously explained this is due to many influential factors being outside control. If dry atmospheric conditions (less than 40% r.h.) are frequent it may be advisable to investigate humidification of the atmosphere. An optimum range is often found to be 40% - 60 % r.h.

Table 2 shows some floor materials that do, or dont, tend to give static problems in our experience. Please note that these general guidelines cannot be taken to apply to any specific material - if you want to know how your material really performs you have to test it under the conditions of usage! A lot may depend on whether the manufacturer has formulated the material with static electricity in mind - specially formulated polymer floors are used in static controlled areas in the electronics industry to reduce static electricity build-up.

The same factors of charge generation and dissipation control whether furnishing materials cause static charge build-up. Again, highly insulating man-made fiber materials are common factors, but natural materials such as wool also can be. A person wearing a wool sweater sitting on a man-made fiber chair may charge to a high voltage when they leave their seat!