Page 142 - Electrical Safety of Low Voltage Systems

TN Grounding System 125

contact upon faults occurring in any other location will be accom-
plished.
Table 7.1, although generally applicable, has been conceived by
assuming standard operating conditions. Such conditions are based
on the following considerations:

1. The circulation of fault currents causes large voltage drops.
2. Persons within buildings are not subject to the whole ground
potential, thanks to the main equipotential bonding connec-
tion that ties together all the EXCPs.
3. The cross-sectional areas of protective and phase conductors
are the same (i.e., Z PE = Z ph ).

In the aforementioned conditions, IEC standards conventionally esti-
mate a reduction in the postfault driving voltage of 20% of the nominal
value of the system voltage.
By applying this reduced voltage in Eq. (7.1) and, for example,
assuming |V ph |= 230 V, we obtain for V ST the value of 92 V. As per the
time–voltage safety curve (Fig. 5.19), persons can withstand this touch
voltage for a maximum time of 0.4 s, which is the value listed in
Table 7.1.
Table 7.1 applies to final circuits not exceeding 32 A. In some
countries (e.g., China), the maximum disconnecting times stated in
Table 7.1 applies to final circuits supplying hand-held or portable
equipment. Mobile equipment (e.g., drills, hairdryers, or any piece
of electrical equipment that is required to be moved by persons dur-
ing its use), in fact, are considered to be more dangerous than sta-
tionary ones (e.g., light fixtures, air conditioners, etc.). This assump-
tion is justified by the greater mechanical stress mobile equipment
normally undergoes, which may expose its live parts. In these coun-
tries, a longer disconnection time of5sis permitted for stationary
loads.
According to IEC, a maximum disconnection time of5sis allowed
in distribution circuits as long as the fault potentials appearing on
them do not affect any final circuit supplied by the corresponding
distribution panel (see Sec. 7.6).
For phase conductors with cross-sectional areas exceeding
2
16 mm , IEC standards allow the reduction of the cross section of
the PE to half of the phase conductor. The protective conductor’s
impedance, therefore, double (i.e., Z PE = 2Z ph ); hence, V ST reaches the
value of 123 V, and the time–voltage safety curve is not fulfilled anymore.
In addition, outside of the building, where the benefits of the
equipotentialization hardly exist, the diminution in the driving volt-
age can no longer be applied and the disconnection time of 0.4 s may
be excessive.