Page 39 - Electrical Safety of Low Voltage Systems

22 Chapter Tw o

FIGURE 2.12 Protection by nonconducting location.

terminal, must be used in nonconducting locations. In addition, ar-
rangementstoalsokeepinsulatedextraneous-conductive-parts(EXCPs),
such as radiators, must be in place.
Even though the failure of an ECP is not dangerous, a second fault
to ground involving a different, and simultaneously accessible, ECP
can expose persons to dangerous potential differences. All the ECPs,
thus, must be properly separated from each others by spacing not less
than 2.5 m. The interposition of obstacles between ECPs that cannot
be spaced by 2.5 m is a possible equivalent solution.
In the above conditions, indirect contact caused by breakdown
of the basic insulation of equipment is not dangerous, since, within
nonconducting locations, persons cannot be exposed to any potential
difference (Fig. 2.12).
Because of its delicate nature, protection by nonconducting loca-
tion is suitable only in installations strictly supervised by qualified
persons. 11 The key feature of this protection is, in fact, the absence
of any earth reference, which might be introduced into the premises
by unaware persons, via portable grounded equipment supplied by
extension cords and/or EXCPs.

2.3.2.3 Protection by Earth-Free Local Equipotential Bonding
In earth-free locations, the appearance of dangerous touch voltages
is prevented by means of local equipotential bonding conductors. In
the case of failure of the basic insulation, in fact, such conductors, by
connecting together both simultaneously accessible Class I equipment
supplied by different phase conductors and the floor (if conductive),
can prevent, or reduce, the appearance of dangerous potential differ-
ence in the installation being protected. The equipotential bonding,