Page 122 - Electrical Safety of Low Voltage Systems

TT Grounding System 105

FIGURE 6.10 Neutral-to-ground first fault in TT systems desensitizing the RCD.

At the occurrence of a ground fault on ECP 1, the faulting-to-
ground neutral on ECP 2 conduces current I 2 , which desensitizes the
RCD. The RCD will not sense, in fact, the total ground current I 1 , but
only the difference I 1 − I 2 and, therefore, might not trip. In addition, I 2 ,
by flowing through R G2 , energizes all the ECPs connected to it, even
if healthy, with risk for persons.
It is important to stress that the presence of two independent
grounds in the same facility is extremely dangerous, since the ECPs

FIGURE 6.11 Independently grounded ECPs in TT systems.