Page 126 - Electrical Safety of Low Voltage Systems

TT Grounding System 109

FIGURE 6.16 Accidental interruption of the neutral conductor in three-phase
TT systems.

The absence of the neutral as the return path to the sources causes
the phase-to-phase voltage to supply the two ECPs, which will divide
across them according to their own impedances. This condition may
permanently damage the insulation of the ECPs and provoke ground
currents (i.e., order of 500 mA). These ground currents, if not promptly
interrupted, may trigger fires.

6.9.1 Resistance of the Utility Neutral in TT Systems
As previously substantiated, safety against indirect contact in TT sys-
tems is achieved through prompt disconnection of supply upon the
first ground fault. The RCD needs to sense the ground-fault current
in order to operate within the limits established by the time–voltage
safety curve. The amount of earth current depends on both the ground
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resistances R G and R N .
The certain disconnection of the faulty circuit will occur only if

V ph
≥ I dn (6.6)
R N + R G
Let us replace in Eq. (6.6) the expression of R G as per Eq. (6.5), and
solve for R N . We obtain

V ph − 50
≥ R N (6.7)
I dn
For example, if we conservatively assume I dn equal to 1 A at the user
and V ph equal to 230 V, the maximum value that R N should assume
to guarantee the positive tripping of the RCD is 180 . Thus, utilities
must keep the resistance of their grounding electrode systems, which
earths the neutral point of the supply, below the above-calculated
threshold. If the condition expressed in Eq. (6.7) is not fulfilled, the
ground fault cannot be cleared by the customer, and the grounding