|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|
A protective measure which can be realised, easily and at low expediture is the connection to the neutral; it provides a good protective effect. All conductive parts and units not belonging to the service circuit are connected with the protective conductor (PE) which is connected with the earthed neutral conductor (N). Protective conductor and neutral conductor may in this case be formed by a common conductor (PEN) (protective conductor carrying current in operation) or they may be installed separately (protective conductor not carrying current in operation).
In the case of a breakage of the PEN conductor at the supply side, a high contact voltage may occur at the protective conductor under unfavourable conditions (danger of Life!). Therefore, additional comprehensive regulations must be observed for the installation of the protective conductor carrying current in operations, especially in overhead local transmission lines. Further, a connection of the PEN conductor to the earth bus at the feeding point and sometimes also in the network spurs is required. An effective potential equalisation must be provided, in the customer installation.
If, for the connection of safety plugs, a two-core lead is used (protective conductor carrying current in operation), the lead (also known as supply line) must be connected to the protective contact first and then it must be brached off to the current carrying connection (see Fig. 9.6.). The fault-current circuit for body contact of L1 with the motor casing shows that the fault current flows through the fuse. For the breaking current the equation 9.2. also is applicable in this case.
Under certain conditions, monitoring of the voltage at PEN to reference earth may be required; in this case an all-pole cut-out is effected when a maximum permissible voltage is exceeded.
Fig. 9.6. Connection to the neutral (also known as multiple protective earthing)
1 - Protective contact socket
2 - Pipes and other conductive parts in buildings
IF = fault current
RB = operational earthing resistance