PD 6519-1:1995 Guide to effects of current on human beings and livestock - Part 1. General aspects
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For a given current path through the human body, the danger to persons depends mainly on the magnitude and duration of the current flow. However, the time/current zones specified in the following clauses are, in many cases, not directly applicable in practice for designing protection against electrical shock, the necessary criterion being the admissible limit of touch voltage (i.e.the product of the current through the body and the body impedance) as a function of time. The relationship between current and voltage is not linear because the impedance of the human body varies with the touch voltage, and data on this relationship is therefore required. The different parts of the human body — such as the skin, blood, muscles, other tissues and joints — present to the electric current a certain impedance composed of resistive and capacitive components.
The values of these impedances depend on a number of factors and, in particular, on the current path, on the touch voltage, the duration of the current flow, the frequency, the degree of moisture of the skin, the surface area of contact, the pressure exerted and on the temperature.
The impedance values indicated in this Technical Report result from a close examination of the experimental results available from measurements carried out principally on corpses and on some living persons.
Clause 3 is primarily based on the findings related to the effects of current at frequencies of50Hz or60Hz which are the most common in electrical installations. The values given are, however, deemed applicable over the frequency range from15Hz to100Hz, threshold values at the limits of this range being higher than those at50Hz or60Hz. It is considered principally the risk of ventricular fibrillation which is the main cause of fatal accidents in that range of frequencies.
Accidents with direct current are much less frequent than would be expected from the number of d.c. applications, and fatal accidents occur only under very unfavourable conditions, for example, in mines. This is partly due to the fact that with direct current, the let-go of parts gripped is less difficult and that for shock durations longer than the period of the cardiac cycle, the threshold of ventricular fibrillation remains considerably higher than for alternating current.
The main differences between the effects of a.c. and d.c. on the human body result from the fact that excitatory actions of the current (stimulation of nerves and muscles, induction of cardiac atrial or ventricular fibrillation) are linked to the changes of the current magnitude especially when making and breaking the current. To produce the same excitatory effects the magnitude of direct current flow of constant strength is two to four times greater than that of alternating current.
ICS 13.260 Protection against electric shock. Live working
ICS 29.020 Electrical engineering in general
ISBN 0580240665
Language:
English
Subject Keywords:
Electric shocks; Electrical resistance; Skin (body); Human body; Physiological effects (human body); Livestock; Direct current; Safety; Bibliography; Time; Electric current; Test equipment; Heart
