Page 174 - Electrical Safety of Low Voltage Systems

IT Grounding System 157

The presence of the intentional ground lowers the prospective
touch voltage to the potential drop across R G , with evident benefit
for the safety against indirect contact. If V ST = R G I G ≤ 50 V, where
I G is the first-fault current to ground, the automatic disconnection of
supply is not necessary, as the ground fault does not cause any hazard
to persons and may persist within the ECP. However, for the reasons
explained later on, it is recommended to clear this fault as soon as
practically possible.
The most valuable aspect of the IT systems is the possibility to
maintain the supply to a circuit even in the presence of a first fault
caused by a live part in contact with enclosures or earth. This feature
is of paramount importance when the loss of the electrical service
can compromise the safety of persons or disrupt a costly industrial
process.
During an unresolved first fault to ground, a second fault involv-
ing a different phase might take place. In this case, the phase-to-phase
voltage drives the fault current, and an actual short circuit occurs
(Fig. 9.3).
In this situation, at least one of the protective devices safeguarding
the circuits will trip and disconnect the supply. Even the two faulty
circuits might be simultaneously tripped off and the safety of the in-
stallation, where the continuity of supply is essential, may be further
compromised.
On the other hand, the second fault exposes persons to risk of
electrocution in the time frame the overcurrent devices take to trip.
With reference to Fig. 9.3, let us calculate the prospective touch voltage

FIGURE 9.3 Second fault to ground driven by the phase-to-phase voltage
(ECPs are shown independently grounded).