Page 165 - Electrical Safety of Low Voltage Systems

148 Chapter Eight

From Eq. (8.1) we can derive the condition R N must comply with
to keep the PEN potential rise below 50 V:

R N 50
≤ (8.2)
R E V ph − 50

In some countries (e.g., Germany), fulfilling Eq. (8.2) is a compulsory
requirement for the supply network operators.

8.2.2 Ground Fault on the Medium-Voltage Utility
Distribution System
The enclosure of the utility transformer, being an ECP, needs to be
earthed and therefore may be connected to the system ground R N .
Low- and medium-voltage systems, then, share the same earth termi-
nal where the PEN conductor originates.
At the occurrence of a ground fault at the transformer primary,
the fault current by circulating through the earth and reclosing
toward the upstream source of the supply network energizes the sys-
tem ground. R N reaches the potential V N and so does the PEN con-
ductor (Fig. 8.6).
Consequently,personsincontactwithlow-voltageECPs,remotely
supplied by the “live” PEN, are exposed to the whole earth potential
V N during the utility fault-clearing time.
In PME, utilities, in order to ensure that the neutral potential due
to primary faults is not dangerous, must accordingly lower the neutral
resistance R N . When it is impossible to decrease R N , the distributor
must alternatively separate the service PEN conductor from the trans-
former enclosure’s earth by creating two distinct grounds (Fig. 8.7).

FIGURE 8.6 Ground fault at the transformer primary.