16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.

chapter 5

chapter 6

Installation control and protection
  3.1 - Introduction 3.5 - High temperature protection
  3.2 - Switching 3.6 - Overload currents
  3.3 - Isolation 3.7 - Protection from faults
3.4 - Electric shock protection

3.8 - Short circuit and overload
------- protection

3.4.1 - The nature of electric shock

The nervous system of the human body controls all its movements, both conscious and unconscious. The system carries electrical signals between the brain and the muscles, which are thus stimulated into action

The signals are electro-chemical in nature, with levels of a few millivolts, so when the human body becomes part of a much more powerful external circuit, its normal operations are swamped by the outside signals. The current forced through the nervous system of the body by external voltage is electric shock.

All the muscles affected receive much stronger signals than those they normally get and operate very much more violently as a result. This causes uncontrolled movements and pain. Even a patient who is still conscious is usually quite unable to counter the effects of the shock, because the signals from his brain, which try to offset the effects of the shock currents, are lost in the strength of the imposed signals.

A good example is the 'no-let-go' effect. Here, a person touches a conductor which sends shock currents through his hand. The muscles respond by closing the fingers on the conductor, so it is tightly grasped. The person wants to release the conductor, which is causing pain, but the electrical signals from his brain are swamped by the shock current and he is unable to let go of the offending conductor.

The effects of an electric shock vary considerably depending on the current imposed on the nervous system, and the path taken through the body. The subject is very complex but it has become clear that the damage done to the human body depends on two factors:

1. - the value of shock current flowing, and
2. - the time for which it flows.

These two factors have governed the international movement towards making electrical installations safer.


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Extracted from The Electricians Guide Fifth Edition
by John Whitfield

Published by EPA Press Click Here to order your Copy.

Click here for list of abbreviations