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close this bookIntroduction to Electrical Engineering - Basic vocational knowledge (Institut für Berufliche Entwicklung, 213 p.)
View the document(introduction...)
View the documentPreface
View the document1. Importance of Electrical Engineering
close this folder2. Fundamental Quantities of Electrical Engineering
View the document2.1. Current
View the document2.2. Voltage
View the document2.3. Resistance and Conductance
close this folder3. Electric Circuits
View the document3.1. Basic Circuit
View the document3.2. Ohm’s Law
close this folder3.3. Branched and Unbranched Circuits
View the document3.3.1. Branched Circuits
View the document3.3.2. Unbranched Circuits
View the document3.3.3. Meshed Circuits
close this folder4. Electrical Energy
View the document4.1. Energy and Power
View the document4.2. Efficiency
View the document4.3. Conversion of Electrical Energy into Heat
View the document4.4. Conversion of Electrical Energy into Mechanical Energy
close this folder4.5. Conversion of Electrical Energy into Light
View the document4.5.1. Fundamentals of Illumination Engineering
View the document4.5.2. Light Sources
View the document4.5.3. Illuminating Engineering
View the document4.6. Conversion of Electrical Energy into Chemical Energy and Chemical Energy into Electrical Energy
close this folder5. Magnetic Field
View the document5.1. Magnetic Phenomena
View the document5.2. Force Actions in a Magnetic Field
close this folder5.3. Electromagnetic Induction
View the document5.3.1. The General Law of Induction
View the document5.3.2. Utilisation of the Phenomena of Induction
View the document5.3.3. Inductance
close this folder6. Electrical Field
View the document6.1. Electrical Phenomena in Non-conductors
close this folder6.2. Capacity
View the document6.2.1. Capacity and Capacitor
View the document6.2.2. Behaviour of a Capacitor in a Direct Current Circuit
View the document6.2.3. Types of Capacitors
close this folder7. Alternating Current
View the document7.1. Importance and Advantages of Alternating Current
View the document7.2. Characteristics of Alternating Current
View the document7.3. Resistances in an Alternating Current Circuit
View the document7.4. Power of Alternating Current
close this folder8. Three-phase Current
View the document8.1. Generation of Three-phase Current
View the document8.2. The Rotating Field
View the document8.3. Interlinking of the Three-phase Current
View the document8.4. Power of Three-phase Current
close this folder9. Protective Measures in Electrical Installations
View the document9.1. Danger to Man by Electric Shock
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
View the document9.3. Checking the Protective Measures

9.2.4. Protective Wire System

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