Page 219 - Electrical Safety of Low Voltage Systems

202 Chapter Twelve

the damage and the extensive destruction of the insulation, unless the
supply of electric current is promptly interrupted.
Excessive voltage stress, therefore, can create hazards for persons
by triggering fires and explosions and/or compromising the conti-
nuity of service of critical apparatus. In addition, overvoltages are a
cause of damage to electronic equipment, which usually have lower
dielectric strength.

12.2 Temporary Overvoltages and Safety
Ground faults that occur in the primary side of substations, supplied
through distribution systems operating at medium/high voltages,
may cause circulation of high currents (i.e., tens of kiloampere) in their
earthing systems. The intensities of the ground-fault currents depend
on how the neutral of the primary side distribution system is “op-
erated.” Such neutral, defined as the common point of a polyphase
supply system, may be solidly grounded, isolated from ground or
grounded through impedances or resistances.
High and low sides of substations, though, may share the same
earthing system and, therefore, in primary fault conditions, the low-
voltage ECPs connected to it become energized. The duration of the
resulting prospective touch voltage in the low-voltage system, in the
order of thousand of volts, although temporary, may be excessive and,
therefore, unsafe for the low-voltage system. In fact, medium/high
voltage protective relays in charge of the protection of the primary
side of substations may be time-delayed to facilitate their coordination
with other devices.
The high-voltage fault may overstress the low-voltage equipment
by imposing an excessive voltage across its insulation and earth. The
stress voltages may breakdown the insulation and cause faults as well
as start fires.
In the following sections, we will examine the above issues as
occurring in typical earthing arrangements.

12.2.1 High-Voltage Ground Faults in TN Systems
In TN systems, the substation’s grounding system will connect to-
gether the transformer tank, the ECPs of high-voltage and low-voltage
equipment, and all the EXCPs eventually present. In Fig. 12.1, the
low side and high side of the transformer share the same earthing
system.
In the case of earth fault in the high-voltage system, all the afore-
mentioned metal parts become energized due to their connection to
the grounding system. This may expose persons to dangerous touch
voltages if in contact with low-voltage equipment.