Page 270 - Electrical Safety of Low Voltage Systems

Applications of Electrical Safety 253

constituted by the presence, within the RCL, of electrical equipment,
either fixed and/or hand-held, which may break down.
Extended contact with large earthed conductive surfaces greatly
reduces the person’s body resistance to ground. In these conditions,
the threshold of ventricular fibrillation is lowered and the restrictions
in movement makes more difficult to let-go of energized parts. Work-
ers are in a hazardous situation as they do not have the benefit of the
standard body resistance to ground, which would limit the flow of the
current through the person.
In order to ensure protection against indirect contacts, IEC 60364–
7-706 requires equipment used in RCL be supplied by isolating trans-
formers (i.e., electrically separated systems; see Chap. 2) or through
SELV systems (see Chap. 10). Class II hand-held pieces of equipment
are also advisable, although not required.

15.7 Electrical Safety in External Lighting Installations
As per IEC 60364–7-714, 10 external lighting installations comprise
lighting fixtures, along with their wiring and accessories, located out-
side buildings. Accessories may include transformers, breakers, re-
closers, switches, manholes, poles, and whatever is functional to the
performance of the system.
External lighting installations may expose the general public to
touch potentials caused by faults.
In TT systems, an important safety requirement is the prohibition
of earthing lighting poles, whose circuits are protected by the same
RCD by means of independent ground electrodes (e.g., one rod for
each pole).
To better understand the reasons behind this restriction, let us
consider Fig. 15.11.
Let us assume that the basic insulation of the neutral wire of pole
A fails and it comes in contact with the metal structure. This is a
fault situation; however, the residual current device cannot pick up
because the neutral conductor is not energized, and, therefore, there
is no current leakage to ground. If the phase conductor of pole B
also fails, the ground current I 2 will be impressed to earth, as the
fault-loop in TT system comprises it, and we expect the RCD to trip
promptly.
However, part of the fault current, indicated in Fig. 15.11 with
I 1 , will return to the source through the ground rod R G1 and the un-
due neutral-to-enclosure connection occurred at the first pole. I 1 will
flow also through the RCD, thereby, desensitizing it. The RCD, in fact,
will not sense the actual fault current I 2 , but only the portion I 3 not
circulating through it, whose magnitude may be lower than its resid-
ual operating current. The fault might not be cleared by overcurrent