Page 168 - Electrical Safety of Low Voltage Systems

Protective Multiple Earthing (TN-C-S Grounding System) 151

FIGURE 8.10 Interruption of the PEN conductor in PME.

In order to limit the risk of accidental loss of the PEN, the distribu-
tor must develop a redundancy by establishing a network of conduc-
tors instead of using a single conductor. In addition, the mechanical
strength of the PEN must be assured by using cables of cross-sectional
2
2
area of at least 10 mm (copper) or 16 mm (aluminum).
In some countries (e.g., the Netherlands), the utility cables are
equipped with insulated metal sheaths. The sheaths and the PEN are
linked together at the utility and at the customer’s service entrance.
With this arrangement, even if the PEN is lost, the sheaths of the intact
cables will act as a return path to the source, safeguarding the person’s
safety.
The installation of an intentional ground electrode at the user’s
service entrance, connected to the main ground bus, as mandated in
some countries (e.g., U.S.A.), would limit the prospective touch volt-
age to the potential difference across the user’s electrode in case of a
broken PEN (Fig. 8.11). However, the system would become now TT
without necessarily having the associated safety requirements (i.e.,
residual current devices), and the users would still be at risk of elec-
trocution.

8.4 Stray Currents
As already anticipated, in PME systems, unavoidable stray currents
continuously circulate through the actual earth. As they depend on
the supplied loads, they are likely to escalate over time. Stray currents
may produce interferences among electrical systems by transferring
2
potential rises to healthy systems, thereby exposing persons to touch
voltages; these may also trigger corrosion phenomena, involving un-
derground metalwork, if they have a d.c. component.
In the presence of ascertained high stray currents, a solution might
be to isolate the user by installing a transformer in the dwelling unit.