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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

9.2.3. Protective Isolation

Like the extra-low protective voltage, the protective isolation is a galvanic separation from the feeding supply network but the output voltage may be greater than 50 V. This protective measure is applicable, for instance in television repair shops, in work to be done in the open or in boiler houses.

In this case only one load may be connected to an output winding of a transformer. Fig. 9.3. shows an otherwise dangerous situation in the case of double body contact and connection of two devices to one winding. The using plug-type connectors, switches and a tool or the like with protective conductor connections (protective class 1), all protective conductor connections must be connected with each other. If work is to be done in narrow space on metallic floors and the like, then the isolating transformer should be provided with protective insulation and arranged outside of this space. In addition a potential equalisation line must be arranged between protective conductor connection and tool. In this way any danger due to a damaged lead is safely avoided.


Fig. 9.3. Impermissible connection of two devices to one winding of a protective isolating transformer

IF = fault current;
UB = contact voltage