ABB DS 941 Montage- Und Betriebsanleitung Seite 15

Earthing
Earthing installation should be carried out according to D.P.R.
27/4/1995 n° 547 and CEI 64-8/IEC 364.
All metal frames of electrical equipment should be connected to this
earthing system.
The earth wire should have a section not lower than that of the
power supply wire.
Note: a widespread habit, although not a correct one, in order to
protect user equipments housings from contact voltages, is to
connect the middle wire directly to the metal housings of the
fix equipment and to the contacts or to the plug type sockets
holes intended for connection to the earth wire. This type of
connection, in the event of insulation malfunction, would direct
the dispersion currents of the malfunctioning equipment back
onto the middle wire connected to one terminal of the residual
current breaker, and would not cause, therefore, the current
unbalance necessary to trigger the circuit breaker. Connection
to the middle wire of metal housings, contacts or plug-type
sockets holes should, therefore, be avoided.
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