Page 152 - Electrical Safety of Low Voltage Systems

TN Grounding System 135

Safety-related systems (e.g., fire alarm, fire pumps, etc.), that is,
systems whose correct operation is necessary for ensuring or main-
taining safety, may be supplied ahead of the main protective device. If
the corresponding loads are not balanced among the phases, neutral
current will flow. The neutral conductor may result, therefore, ener-
gized, even if the main breaker has been opened. In an emergency
situation, which requires the safety-related systems to be running,
persons (e.g., first responders) may be exposed to the risk of electro-
cution if in contact, directly or indirectly, with the neutral wire, even if
the main breaker is in the open position. In this case, the simultaneous
switching off of neutral and line conductors is a safety requirement.

7.10 The Touch Voltage in TN Systems
As already anticipated in Chap. 4, in electrical systems exceeding 1 kV,
defined as high-voltage systems, international standards convention-
ally define the touch voltage as the potential difference between faulty
equipment touched with one hand and the two feet of the person. 6
Ground faults occurring on the primary side of substations cause cir-
culation of currents through the earth and therefore also step poten-
tials. In reality, part of the fault current I F may be prevented from
circulating through the ground, with benefits for the safety. Upon per-
mission of the local utility, in fact, the metallic sheaths of their incom-
ing power cables, or the overhead ground wires of their transmission
lines, may be used to drain off part of the fault current provided that
the sheaths are adequately sized to withstand such current (Fig. 7.21).

FIGURE 7.21 Earth current due to primary side faults.