What is ESD?

However when a conduction path does exist a current will flow and the charge will be reduced. There is a time constant associated with the discharge. A high resistance will mean that a smaller current will flow for a longer time. A low resistance will give rise to a much faster discharge.

Obviously the levels of voltage and current which are produced depend of a large variety of factors. The size of the person, the level of activity, the object against which the discharge is made, and of course the humidity of the air. These all have a pronounced effect so it is almost impossible to predict the exact size of the discharges that will occur.

However one of the major factors that affects the voltages that are produced is the types of material that are being rubbed together. It is found that different materials give different voltages. The voltage produced is dependent upon the position of the two materials in a series known as the tribo-electric series.

Tribo-electric series

The further apart they are in the series, the greater the voltage. The one that is higher up the series will receive a positive charge, and the one lower down a negative charge. Looking at the tribo-electric series list below it can see that combing ones hair with a plastic comb will give rise to a positive charge on the hair, and the comb will become negatively charged.

There are many ways in which charges can be built up. Even walking across a carpet can give rise to some very large voltages. Typically this might give rise to potentials of 10 kV. In bad cases it could even lead to potentials of three times this value. Even the act of walking across a vinyl floor may lead to potentials of around 5 kV being generated. In fact any form of movement where surfaces are rubbing together will lead to the generation of static electricity. Someone working at a bench using electronic components could easily generate static potentials of 500 V or more.