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close this bookIntroduction to Electrical Engineering - Basic vocational knowledge (Institut für Berufliche Entwicklung, 213 p.)
close this folder9. Protective Measures in Electrical Installations
View the document9.1. Danger to Man by Electric Shock
Open this folder and view contents9.2. Measures for the Protection of Man from Electric Shock
View the document9.3. Checking the Protective Measures

9.1. Danger to Man by Electric Shock

The electrical current exerts effects on man (also see Chapter 1). A useful effect is produced, in a few electromedical therapies. On the other hand, hazards to man can be caused by an electric shock (inadvertent passage of current through the human body). The consequences of an electric shock are dependent on the intensity of the current passing through the human body and on the duration of action. The current path through the human body is also of particular importance. A great danger is given when the current path passes through the heart; this is given when the voltage acts from hand to hand or from hand to foot. Depending on intensity and duration of the passage of current through the body, muscle contraction, which may reader impossible the release of the touched electrode, will occur. Consequences are burns, unconsciousness and ventricular fibrillation which is an extreme danger of life.

To prevent accidents due to electrical current, comprehensive safety regulations have been en acted in all countries which must be observed in any case. The increasing exchange of goods necessitates in future a standardisation of these regulations on an international level. Efforts are made by the “International Electrotechnical Commission” (IEC), and the “Standing Commission for Standardisation” of the “Council for Mutual Economic Aid” (CMEA) to arrive at generally accepted rules.

The definition of a few physical quantities in connection with protective measures is illustrated in Fig. 9.1. Here, it is assumed that the phase conductor L1 has body contact (low-resistance connection of the phase conductor with the casing of the motor) and that the protective conductor with the function of a neutral conductor (PEN) is interrupted. The contact voltage UB is the voltage directly acting on man in the event of a fault while the failure voltage UF in the present case is given by the voltage between phase conductor and neutral conductor. The current flowing in case of a fault is called fault current Ip! The operational earthing resistance RB, is the resistance given when the neutral conductor at the supply side is connected with reference earth and the position transition resistance RSt is the resistance between the position (of the installation and the like) and reference earth. Depending on the condition of the position, RSt, can become very small (e.g. wet basement floor) while UB practically becomes equal to UF. The danger to man at a well conducting position is, therefore, particularly high.


Fig. 9.1. Example of the representation of contact voltage, failure voltage and fault current (leakage current)

IF

=

fault current

UF

=

failure voltage

RB

=

operational earthing resistance

RSt

=

position transition resistance

UB

=

contact voltage

As has been mentioned, above, the detrimental effects is dependent on the fault current and the time of its action. The fault current is determined by the contact voltage UB and the resistance of the human body. This resistance depends, among other things, on the skin surface and perspiration and may vary within wide limits. In the case of slight injuries of the skin surface, the resistance is considerably reduced. In order to acquire the effectiveness of protective measures by measuring techniques irrespective of the largely varying resistance of the human body, in regulations the maximum permissible contact voltage (e.g. 65 V, for alternating current the effective value is decisive) is specified. Another possibility is the statement of the maximum permissible contact voltage at a maximum permissible disconnection time. Table 9.1. contains a few values of an IEC Publication as an abstract.

Table 9.1. Maximum permissible contact voltage in dependence of the disconnection time

Contact voltage in V

Maximum permissible disconnection time in s

< 50

¥

50

5

75

1

110

0.2

220

0.05

280

0.03

In the past many different measures for the protection of man from electrical shock have been developed. Of the great variety of protective measures, one or several have to be selected in accordance with the given concrete conditions. For electrical devices, three protective classes have been specified. Devices with protective conductor connection belong to the protective class 1, devices with protective insulation belong to protective class 2 and devices for extra-low protective voltage to class 3.

For the protection of man from electric shock, protective measures in electrical devices and installations are necessary. The maximum permissible contact voltage continuously applied or occurring only during a specified period of time must not exceed the specified maximum value.

Questions:

1. Gather information about “first aid” in cases of accidents due to electrical current!

2. Which are the conditions under which the danger due to an electric shock is particularly high?

3. Explain the terms contact voltage, failure voltage and fault current!