Page 181 - Electrical Safety of Low Voltage Systems

164 Chapter Nine

A similar situation also occurs in the case of first fault to ground
caused by the failure of the basic insulation of an ECP. The RCD cannot
protect against indirect contact, as the circulation of the fault current
is, once again, entirely allowed through the toroid’s windings by the
system distributed impedance.
In sum, in IT systems RCDs neither can function nor can be blamed
for not intervening as a protection for both direct and indirect contacts.
The nature of the fault-loop, in fact, prevents their proper operation
and renders their installation ineffective.

9.5 Protection Against Indirect Contact in the Event of a
Second Fault to Ground
After the occurrence of the first fault to earth, the IT system is no
longer ungrounded, because of the accidental connection of the faulty
phase to earth. In the event of a second fault involving a different
phase, the IT system “evolves” into TT or TN according to the earthing
arrangement of ECPs (i.e., individually or collectively).

9.5.1 ECPs Earthed Individually or in Groups
If ECPs are earthed individually, or in groups, in the event of a second
fault, the system becomes TT and we are in the case in Fig. 9.3. Pro-
tection against indirect contact is achieved if the following condition,
applied to the generic ith ECP, is fulfilled:

R Gi I a ≤ 50 V (9.22)

where I a is the current causing the automatic operation of the dis-
connection device within the maximum permissible time specified in
Table 6.1 for TT systems for final circuits, or in a time not exceeding 1 s
for distribution circuits. As we have already substantiated in Chap. 6,
the optimum protection against indirect contact in TT systems is
carried out by RCDs. The fault current circulating through the
earth, due to the second fault may in fact be too low to operate
promptly the overcurrent devices. RCDs can clear the fault within
the safe time required by Table 6.1, generally in correspondence
of a ground current of at least five times their residual operating
currents.
The additional costs due to the necessity of RCDs, effective under
second fault conditions in the previous arrangement, and due to the
installation of individual ground electrodes, usually induce designers
to collectively earth the ECPs to a single ground electrode.