9.2.6. Connection to the Neutral
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
= operational earthing