|Introduction to Electrical Engineering - Basic vocational knowledge (Institut für Berufliche Entwicklung, 213 p.)|
|9. Protective Measures in Electrical Installations|
|9.2. Measures for the Protection of Man from Electric Shock|
The measures described in 9.2.2. and 9.2.3. ensure a high protective effect against electric shock but they are very expensive. When in a room many electrical appliances are put into operation at the same time and when, for reasons of safety, it should be avoided that, in the case of body contact of one of the devices, the whole installation is switched off, then the protective wire system is the adequate protective measure. These conditions may occur, for example, in an operation theatre.
No conductor of the service circuit must be earted, not even the neutral conductor-that is a condition of the protective wire system. On the other hand, all of the conductive parts not belonging to the service circuit and all conductive parts of the building (water pipes, other pipe lines, metallic structures of the building) must be connected together in any case. There is a puncture cut-out between neutral conductor (H) and earth. In the case of a simple body contact, there is no danger of an occurrence of a dangerous contact voltage. However, in the case of a double body contact, there is the danger of a dangerous occurrence of contact voltage.
If in an installation - as has been mentioned above -, with an suddenly occurring body contact no danger to man shall occur and no switching off is to be effected, then a switching monitoring continuously the isolation resistance to earth of the service circuit (e.g. 25 W/V) is employed, releasing a fault signal (Fig. 9.4.). This type of monitoring will reliably indicate a first body contact. When signalling is not desired, care must be taken that, in case of a double body contact, the breaking current Ia will flow and the installation is reliably and quickly switched off. The conditions for the breaking current Ia will be explained in Section 9.2.5.
Fig. 9.4. Protective wire system with monitoring of the insulation resistance
1 - Monitoring relay for insulation resistance to earth with signalling the fault by a horn
2 - Water pipes
3 - Conductive parts of the building
4 - Other pipe lines