Page 163 - Electrical Safety of Low Voltage Systems

146 Chapter Eight

the PE. It is, therefore essential to periodically inspect and maintain
such connection. Upon loss of this bond, the building would become
a TT system, wherein in the absence of RCDs, which are not strictly
necessary in PME, users are exposed to electric shock hazards.

8.1.1 Fault-Loop Impedance in PME Systems
As said, low fault-loop impedances guarantee the safe operation of the
user’s overcurrent protective device with respect to the permissible
times of Table 7.1.
In TN systems, Z Loop exclusively depends on parameters known
to the user, such as the impedances of circuits and transformers. In
PME, instead, the fault-loop impedance includes the impedance Z e of
the utility low-voltage distribution system, which is usually unknown
to the customer. Z e , which increases with the distance of the fault’s
location from the supply source, may also change in time without the
user knowing it because of modifications in the utility distribution
system. If the total fault-loop impedance Z Loop = Z e + Z user is exces-
sive, the ground fault current might be so low that Eq. (7.4) cannot be
fulfilled, and there would be no effective protection of persons against
indirect contact.
The installation of RCDs in dwelling houses, even in PME systems,
although redundant in the case of low value of Z Loop , can, indeed,
guarantee safety when Z Loop is too high.

8.2 Energization of the PEN Conductor in PME Systems
PME systems imply a considerable responsibility of the local utility,
since, together with the electric energy, the distributor provides the
users with an earth connection, which must ensure public safety.
In fault conditions, the utility PEN, although multiple grounded,
may assume a voltage, with respect to the earth, as is substantiated
later on. Such neutral-to-ground voltage can be transferred as a shock
potential to the users’ ECPs and EXCPs. If utilities cannot “certify”
the neutral potential as harmless to persons, a TT system should be
employed, instead.

8.2.1 Ground Fault on the Low-Voltage Utility
Distribution System
The PEN conductor may become live due to a ground fault occurring
along its distribution system, for example, as a result of the fall to earth
of overhead cables (Fig. 8.4) or of a contact of the line with an EXCP
not connected to a protective conductor.
R N represents the ground resistance of the utility’s earth electrode
system: the neutral conductor is earthed not only at intervals along its
run (e.g., at the transmission poles) but also at the customers’ dwelling