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IT Grounding System 163
FIGURE 9.7 IT system earthed via a neutral grounding resistor R s .
In order for |V NG | not to exceed |V 1N |, R s must fulfill the following
condition:
1
R s ≤ (9.21)
3 C 0
Equation (9.21) constitutes the criterion to size the grounding resistor
in IT systems, once C 0 is estimated. However, it is important to note
that R s must be sufficiently high to prevent first fault currents from
tripping protective devices. As long as the system is high-resistance
grounded, the benefits of the continuity of the service can be enjoyed
without the risk of overvoltages.
9.4 Protection Against Direct and Indirect Contact by
Using RCDs in IT Systems
The direct contact can cause the circulation of dangerous current
through the human body even in IT systems. The additional protec-
tion, normally provided by RCDs in TT and TN, is not effective in IT
systems. The fault current, in fact, cannot activate the RCD, because
it flows entirely back through its toroid via the system distributed
impedance. The RCD does not sense any unbalance and, therefore,
cannot intervene (Fig. 9.8).
FIGURE 9.8 Direct contact in IT systems and RCDs.
