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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
close this folder9.2. Measures for the Protection of Man from Electric Shock
View the document9.2.1. Protective Insulation
View the document9.2.2. Extra-low Protective Voltage
View the document9.2.3. Protective Isolation
View the document9.2.4. Protective Wire System
View the document9.2.5. Protective Earthing
View the document9.2.6. Connection to the Neutral
View the document9.2.7. Fault-current Protection

9.2.2. Extra-low Protective Voltage

If a safe protection against accidental contacts of the live parts in operation is not possible or if tools have to be used in an environment where particular hazards are given (e.g. in narrow boilers), an expedient protective measure is the use of extra-low protective voltage. In the no-load state, it must not be greater tan 50 V. For electrical toys for children lower values are specified.

When extra-low protective voltage is used, no live conductor must be earthed in operation. The extra-low protective voltage is generated in special transformers or by motor generators. The circuits of extra-low protective voltage have to be installed in such a way that they are safely separated from other circuits. For this purpose, only such plug-type connections or connectors have to be used which safely prevent an inadvertent connection of the tools and the like to other voltage sources. Like with protective insulation, conductive parts of the tools and the like, which do not belong to the service circuit, must not be connected with a protective conductor.