Page 144 - Electrical Safety of Low Voltage Systems

TN Grounding System 127

By neglecting the source impedance, the resistance of the fault-
loop is given by the series of the resistances of phase and protective
conductors, as follows:

1.5 L 1.5 L S PE + S ph
R Loop = R ph + R PE = + = 1.5 L (7.7)
S ph S PE S PE S ph
where L and are, respectively, length and resistivity at 20 Cofca-
◦
bles at fault point. The multiplier 1.5 accounts for the 50% increase in
conductors’ resistance.
By combining Eqs. (7.6) and (7.7), we obtain

1.5 L (1 + m)
R Loop = (7.8)
S ph
Thus, by applying the Ohm’s law, the minimum ground-fault current
I G is

0.8V ph S ph k
0.8V ph
I G = = (7.9)
R Loop 1.5 L (1 + m)

7.5 Protection Against Indirect Contact in TN-S
System by Using RCDs
When Eq. (7.4) cannot be fulfilled through overcurrent devices (i.e.,
the loop impedance Z Loop is too high), or the user is not within the
equipotential area, RCDs may constitute the only way of protection
against indirect contact. However, in some particular circumstances,
residual current devices cannot protect persons.
For instance, let us consider a ground fault occurring on the pri-
marysideoftheuser’ssubstationinaTN-Ssystem,wheretheearthing
3
system is shared by high- and low-voltage ECPs (Fig. 7.10).
If the transformer’s enclosure is linked to the same system ground
as the low-voltage system, the neutral wire becomes energized at the
ground potential V G . The protective conductor conveys V G to ECPs,
and persons touching them will be exposed to dangerous touch po-
tentials. The RCD, installed on the low-voltage side of the supply sys-
tem, cannot trip, because it cannot sense the fault current, that does
not circulate through it.
In TN-C systems, the RCD cannot work at all, as the ground-
fault current is returned to the source by the PEN conductor, which
is encircled by the toroid as a neutral wire. This would cause no un-
balance in the case of a ground fault and the operation of the RCD is
prevented.