Power Problems: Sources of spikes

Spikes cause untold damage on modern electronic equipment. This page addresses some of the sources of spikes and their expected size.

Human beings

We have all experienced the effect of combing our hair using a plastic comb. The hair can become charged and because like charges repel the hair tends to `stand on end'. If the hair is vigourously combed in the dark under ideal conditions significant sparks will be seen.

Similar effects can be felt when walking over plastic carpet and then touching a grounded object such as a door handle or lift button.

This effect is caused by the generation and retention of static charge on the body. As far as we know it is harmless to human beings if a little surprising occasionally.

Because our sister company Arbra Instruments supplies the mining industry we are aware of some useful technical data which relates to the build up of static charge on human beings. To avoid standard explosive detonators from being triggered by static charges a standard test is performed. A 300pF capacitor is charge to 15kV. The capacitor is then discharged into the detonator 10 separate times. The detonator must reliably NOT fire!

These figures give us a useful baseline for the expected energy we can get from a human with plastic clothes or environment.

Lightning

The UK experiences about 200 000 lightning strikes per annum. This is about 2 per square mile on average. However telecomms aerials and electricity distribution pylons probably collect far more than an average share.

A typical 200kV strike will result in a slow 3kV spike (when more than 1/2 mile from point of contact) travelling down copper wires towards sensitive equipment.

In Europe isolating transformers are usually homologated to 4kV.

In an indoor 230Vac environment the wiring will flash over at about 6kV. This effectively limits the amplitude of spikes in an office or household wiring system. In outdoor situations this may increase to 10 or even 20kV.

Applicable standards

European test standards are defined in IEC 61000 and USA ones in IEEE587. The USA also uses ANSI/IEEE C62.41 which is similar to BS6651.