Page 182 - Electrical Safety of Low Voltage Systems

IT Grounding System 165

FIGURE 9.9 ECPs earthed collectively to a single grounding system (neutral
not distributed).

9.5.2 ECPs Earthed Collectively to a Single
Grounding System
The ECPs may be earthed collectively, that is, connected to one single
grounding system. If the neutral is not distributed, at the occurrence
of a second fault, involving a different live conductor, the system be-
comes TN (Fig. 9.9).
In this case, the voltage between the line conductors, equal to 1.732
times the voltage between line conductor and neutral, drives the fault
current. The second fault may randomly occur in a different circuit,
which, for example, supplies an ECP remotely located with respect to
the location of the first fault. Thus, the fault impedance may be due
to the contributions of line conductors and protective conductors of
different cross-sectional areas. This possibility renders extremely chal-
lenging the prediction of the total fault-loop impedance in IT systems
“evolved” into a TN. As seen in Chap. 7, in TN systems created as
such in industrial facilities, the fault-loop exclusively comprises one
circuit at the time.
To take into account the second fault in a different circuit, the
protection against indirect contact of persons touching one faulty en-
closure is effective if the following condition is fulfilled:
√
3V ph
≥ I a (9.23)
2Z S
or equivalently, by solving for Z S ,
√
3V ph
Z S ≤ (9.24)
2I a