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Wolfson Electrostatics reports: Modern buildings are inherent static generators and dry, fine weather often brings complaints of electric shocks from people touching radiators, door handles, filing cabinets and hand rails. Whether it is a new car, a new office or the interior of a department store, the question is often asked, can repeated shocks from static constitute a health hazard?
On the face of it one might expect static electricity to pose a lethal threat. The voltages encountered may be as high as 15,000 or even 20,000 volts, whereas mains supply voltages of only 240 volts are known to be fatal. Furthermore, the destructive nature of static electricity is vividly demonstrated in nature by a lightning strike, itself an extreme form of static discharge. In the office, car or store, however, we do not encounter lightning-like discharges and what determines the severity of the electrostatic shock, in most cases, is the amount of electrical charge that is generated and stored on the human body. When a person reaches for a door handle and experiences an electric shock it is the release of electrical energy developed on their body by actions or movement prior to touching the handle. The charge may be generated by a very simple action such as walking across a carpeted or tiled floor. The electrical shock that is experienced occurs when the body loses its stored energy very rapidly in the form of an electrostatic discharge (ESD). This ESD can be characterised in terms of parameters which include the discharge energy, electrical current and duration of current flow.

Electrical energy that can be stored on the human body and then released in an ESD is measured in milljoules (mJ) and is governed by the capacitance of the body (C) and the potential or voltage attained (V), according to the formula E = ½ CV2. The capacitance of a person standing will usually lie in the range 100-300 picofarads (pF) and human body potentials in excess of 20,000 volts are unlikely to occur. Taking the maximum in each case, yields an energy level of 60 mJ, this represents a relatively small amount of energy compared with say a household light bulb which consumes a 1,000 times more energy every second.
The electrical current flowing during an ESD can be measured on a fast response oscilloscope. This quantifies the other two important characteristics of an electrostatic discharge – a relatively high current which maybe several amperes dissipated within a fraction of a microsecond.

The effects of electrical current passing through the human body are covered at length in the International Electro Technical Commission document IEC 479-2:1987. In this document it indicates that a transient or capacitive discharge, as is the case with static electricity, requires energy in excess of 5 Joules (5000mJ) to produce a direct serious risk to health. Although it is practically impossible to store this level of electrostatic energy on the human body, case histories have shown a number of accidents due to shock reaction. Such accidents include falling from ladders, involuntary recoil and dropping or releasing loads.
Footnote: In some industrial processes, discharges of several Joules due to static electricity can occur under extreme conditions. In such cases the static charge accumulates not on the human body but on highly polarised materials where the effective capacitance is very high. These situations can give rise to extremely energetic discharges resulting in severe physiological effects.

Wolfson Electrostatics, Department of Electrical Engineering, University of Southampton, Southampton, SO17 1BJ. United Kingdom. Telephone: +44 (0) 2380 552266 / 592509 Fax: +44 (0) 2380 593015

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