Where the electrical installation designer considers it inappropriate to connect external conductive parts of an installation in the greenhouse to the earthing terminal of the PME source, or where the electrical distributor's guidance notes do not allow this, a number of alternatives may be provided in the supply. The electricity needed by the greenhouse is available:
TT System -
The electrical installation in the greenhouse has the main earth terminal (MET) to which the exposed conductive parts of the installation are connected and the external conductive parts are connected to it as the main equipotential. The MET is connected to the ground by a mounting ground electrode.
TN-CS System (PNB)-
The power supply transformer is dedicated to supplying only one factor, the designer may adopt a protective neutral connection (PNB) form of the TN-CS system where the neutral conductor connected to the power supply is grounded. Where there is no transformer, the approval of the distribution network operator must be obtained before using this option.
System-Local TN-S.
Where the supply transformer supplies a number of consumer installations, some of which use a PME feeder earth connection, a local TN-S system may be derived from the supply via a suitable isolation transformer.< br>
Extra protection for sockets
In supplying electricity to the greenhouse, each final circuit must have additional protection using an RCD. In addition, in the final circuits of the supplier, sockets with a rated current of more than 32 A, an RCD must be supplied with a rated operating current of more than 100 mA.