Page 223 - Electrical Safety of Low Voltage Systems
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206 Chapter Twelve
FIGURE 12.5 Neutral conductor grounded independently of substation’s earth
in TT systems.
12.2.3 High-Voltage Ground Faults in IT Systems
As explained in Chap. 9, in IT systems the power source, that is, the
secondary side of the supply transformer, is isolated from the earth,
while the ECPs must be independently grounded from the source—
individually, in groups, or collectively. The transformer, though, may
besubjecttoprimaryvoltagefaults,andthereforeitsenclosuremustbe
earthed to guarantee a clear return path to the fault currents. The ECPs
of the low-voltage system are permitted to be linked to this grounding
system (Fig. 12.6), providing that the resulting touch voltage V T is
cleared in a time compatible with the chart in Fig. 12.2.
=
FIGURE 12.6 High-voltage fault in IT system where high- and low-voltage
equipment share the same earthing system.
