3.4.2 -  Resistance of the shock path

In simple terms the human body can be considered as a circuit through which an applied potential difference will drive a current. As we know from Ohm's Law, the current flowing will depend on the voltage applied and the resistance of the current path. Of course, we should try to prevent or to limit shock by aiming to stop a dangerous potential difference from being applied across the body. However, we have to accept that there are times when this is impossible, so the important factor becomes the resistance of the current path.

The human body is composed largely of water, and has very low resistance. The skin, however, has very high resistance, the value depending on its nature, on the possible presence of water, and on whether it has become burned. Thus, most of the resistance to the passage of current through the human body is at the points of entry and exit through the skin. A person with naturally hard and dry skin will offer much higher resistance to shock current than one with soft and moist skin; the skin resistance becomes very low if it has been burned, because of the presence of conducting particles of carbon.

In fact, the current is limited by the impedance of the human body, which includes self capacitance as well as resistance. The impedance values are very difficult to predict, since they depend on a variety of factors including applied voltage, current level and duration, the area of contact with the live system, the pressure of the contact, the condition of the skin, the ambient and the body temperatures, and so on.

Fig 3.6 Path of electric shock current

Figure 3.6 is a simplified representation of the shock path through the body, with an equivalent circuit which indicates the components of the resistance concerned. It must be appreciated that the diagram is very approximate; the flow of current through the body will, for example, cause the victim to sweat, reducing the resistance of the skin very quickly after the shock commences. Fortunately, people using electrical installations rarely have bare feet, and so the resistance of the footwear, as well as of the floor coverings, will often increase overall shock path resistance and reduce shock current to a safer level.

Guidance Note 7 (Special Locations) provides data on the impedance of the human body. However, the figures are complicated by the fact that values differ significantly from person to person; it would be sensible to assume a worst case possibility which suggests that the impedance of the human body from hand to foot is as low as 500 Ohms. Since this calculates to a body current of 460 mA when the body has 230 V applied, we are considering a fatal shock situation.

There are few reliable figures for shock current effects, because they differ from person to person, and for a particular person, with time. However, we know that something over one milliampere of current in the body produces the sensation of shock, and that one hundred milliamperes is likely quickly to prove fatal, particularly if it passes through the heart.

If a shock persists, its effects are likely to prove to be more dangerous. For example, a shock current of 500 mA may have no lasting ill effects if its duration is less than 20 ms, but 50 mA for 10 s could well prove to be fatal. The effects of the shock will vary, but the most dangerous results are ventricular fibrillation (where the heart beat sequence is disrupted) and compression of the chest, resulting in a failure to breathe.

The resistance of the shock path is of crucial importance. The Regulations insist on special measures where shock hazard is increased by a reduction in body resistance and good contact of the body with earth potential. Such situations include locations containing bath tubs or showers, swimming pools, saunas and so on. The Regulations applying to these special installations are considered in {Chapter 7}.

Another important factor to limit the severity of electric shock is the limitation of earth fault loop impedance. Whilst this impedance adds to that of the body to reduce shock current, the real purpose of the requirement is to allow enough current to flow to operate the protective device and thus to cut off the shock current altogether quickly enough to prevent death from shock.

How quickly this must take place depends on the level of body resistance expected. Where sockets are concerned, the portable appliances fed by them are likely to be grasped firmly by the user so that the contact resistance is lower. Thus, disconnection within 0.4 s is required. In the case of circuits feeding fixed equipment, where contact resistance is likely to he higher, the supply must be removed within 5 s. For situations where earth contact is likely to be good, such as farms and construction sites, disconnection is required within 0.2 s. Earth fault loop impedance is considered more fully in {Chapter 5}.