Page 123 - Electrical Safety of Low Voltage Systems

106 Chapter Six

FIGURE 6.12 ECPs that leak to ground in three-phase TT systems.

linked to one ground become EXCPs (extraneous-conductive-parts)
to the ECPs connected to the other ground and vice versa.

6.7 Leaking-to-Ground ECPs in Three-Phase TT Systems
As previously explained in Chap. 2, RCDs execute the vectorial sum-
mation of all alternating currents flowing through a circuit’s wires and
compare the result to their operating threshold.
Let us assume that two ECPs, supplied by different phases, are
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leaking to ground currents, whose vectorial summation is below the
RCD operating setting. If direct contact occurs with the healthy phase
L 3 (Fig. 6.12), the ground current I 3 will flow through the person.
The RCD will sense the vectorial summation of these currents,
whose magnitude may be less than the magnitude of I 3 and below its
operating threshold (Fig. 6.13). This may cause the RCD not to trip,
despite a potentially dangerous shock current I 3 circulating through
the person.

6.8 Electrical Interferences in TT Systems
Let us consider the case of dwelling units, as a part of a building or a
complex, sharing a common grounding system. At the occurrence of
a ground fault in one unit, the common grounding system becomes
live and reaches the voltage rise V G . Such fault potential is transferred
to other dwelling units’ healthy ECPs (Fig. 6.14).