Page 169 - Electrical Safety of Low Voltage Systems

152 Chapter Eight

FIGURE 8.11 Interruption of the PEN conductor in PME with intentional user’s
ground.

Such a transformer, with turn-ratio of 1:1, must be grounded at the
secondary winding, preferably at the midpoint (Fig. 8.12).
This transformer breaks the electrical system into independent
3
“islands” and confines both return and fault current in a metallic
return path (i.e., the PE) by creating a TN-S system within the PME.

8.5 Stray Voltages
The term stray voltage indicates permanent neutral-to-ground po-
tentials and not temporary potentials due to fault conditions at the
customer’s location. The neutral conductor, although earthed at the
substation, may, in fact, be energized above ground, where it enters
the user’s premises.
The reason being that supply cables employed to power up
the customers, composed of phase and PEN conductors, have fi-
nite impedance, which may cause voltage drops along their runs
(Fig. 8.13).
The voltage drop along the PEN increases with its length and the
contributions from the customers; therefore, V NG at User B is greater

FIGURE 8.12
Transformer inside
of the dwelling unit.