Page 221 - Electrical Safety of Low Voltage Systems

204 Chapter Twelve

FIGURE 12.3 Low and high sides of substation independently earthed in TN
systems.
The curve in Fig. 12.2 relates the maximum duration of faults
allowable by the high-voltage protective devices to the permissible
touch voltage on ECPs supplied by the secondary side of the trans-
former.
When Eq. (12.1) is fulfilled, both the neutral and protective con-
ductors of the low-voltage system may be connected to the same earth
electrode of the substation.
In TN systems sharing the same grounding electrode, the pro-
tective conductor (PE) equalizes the potential between the electrode
itself and the low-voltage ECPs. Therefore, even in the presence of a
fault potential V G across R N , the potential difference across the basic
insulation of low-voltage equipment (i.e., V S1 and V S2 ) will be equal
to V ph and no overvoltage will be caused.
If Eq. (12.1) is not fulfilled, the neutral and the protective conduc-
tors of the low-voltage system must be earthed independently of the
substation’s grounding system (Fig. 12.3).
In high-voltage fault conditions, the low-voltage ECPs remain at
zero potential (i.e., V ECP = 0 and V S2 = V ph ), while the transformer’s
enclosure in the substation reaches the fault potential V G = R H I G .
A potential difference V S1 , whose magnitude may be as large as
V ph + V G , appears across the insulation separating the transformer’s
enclosure and the secondary windings, or any low-voltage system
bonded to R N . This temporary overvoltage stresses the low-voltage
insulation and might compromise its integrity, if its dielectric strength
is exceeded.

12.2.2 High-Voltage Ground Faults in TT Systems
In TT systems, the neutral conductor serving the low-voltage cus-
tomer’s installation may be connected to same electrode that earths
the high-voltage ECPs of the substation (Fig. 12.4).