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6.7. Grounding the converter system
6.7.1. General requirements
A grounding system aims to provide the following functionalities:
– Potential limitation of any point of an installation in relation to ground potential. This
enables the entire installation to be kept on an equipotential surface, so that any
shielded cables can be grounded on both sides, without significant equalizing currents
flowing through the shields. This ensures low interference with the measuring signals.
– Defined management of the short-circuit current in the event of a fault in the system, so
that protective devices can actuate and isolate the defective circuit part.
– Ensuring that non-current conducting metal parts do not reach a hazardous potential in
the event of malfunctions.
– Paths for the protective ground must present a sufficiently high current-carrying
capacity to enable the dissipation of any fault currents that occur, so that no hazardous
voltages can result between two simultaneously accessible metal parts. According to
"Schaltanlagen, ABB Calor Emag Taschenbuch, 10th edition", chapter 5.3.3, the minimum
cross-section of a ground conductor should be defined as follows:
With 16.2 kA initial short circuit current and 0.5 s maximum breaker opening time, the
cross section of the grounding conductor has to be 95 mm
6.7.2. Protective earth (PE) and functional earth (FE)
Protective earth (PE) shall be separated from the functional earth (FE). PE conductors are
part of power supply cables. PE potential should be routed such that safe discharge of fault
current in case of a ground fault of the related component is ensured.
Functional earth (FE) includes ground structures needed for a proper function of the
converter system. Especially, the return path for the parasitic disturbance currents
originating from the switching semiconductors acts as functional earth. Due to EMC
reasons, it is therefore useful to consequently split PE from FE.
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