This section deals with the danger posed by static electricity to electronic components such as those found in computer systems. As described previously, static charge can be extremely damaging and steps should be taken to avoid and minimise its risks.
Static electricity can be defined as electrical charge at rest. For example, in walking over a carpet we commonly build up a static charge relative to the surroundings. It is not so much this build-up of static electricity, or charge, which causes the problem but rather when the charge is dissipated. The charge builds up until an object is touched with a different charge and then charge will be transferred from one object to the other. In the case of a person who has built up a charge, this is normally dissipated to a 'grounded' object such as a doorknob or car door. If the charge built up is sufficiently high then you will feel a 'shock'. This transfer of charge is called electrostatic discharge (ESD) - in electrical terms we refer to ESD damage and ESD precautions, etc.
If the discharge is felt then the voltage was probably over 2000 volts (for comparison mains electricity is 230 volts). If you hear the discharge then it would be around 2000-7000 volts. If you see a small blue spark then the voltage was probably over 10 000 volts.
Many of the semiconductor chips (integrated circuits) and other components in a computer system operate at 2-5 volts and can be damaged by as little as 40 volts ESD. For us to feel the discharge requires much higher voltages than those which can damage the sensitive components and so we might cause damage without realising it. Another well-known example of ESD is a lightning strike. In this case the clouds gather a charge relative to the Earth below. When the charge builds up sufficiently there is a discharge between the clouds and the ground involving many hundreds of thousands of volts.
Note that some of the voltages mentioned above are extremely high but they involve low current discharges (apart from lightning). Therefore the discharges are not harmful to humans but do represent a great risk to electrical components.