Page 40 - Electrical Safety of Low Voltage Systems

Fundamentals of Electrical Safety 23

FIGURE 2.13 Earth-free local equipotential bonding between Class I
equipment.

in fact, makes the location a Faraday cage, which becomes equipoten-
tially energized upon faults (Fig. 2.13).
In the above conditions, persons cannot undergo any potential
gradients, even if different ECPs are within reach.
Similar to nonconducting locations, in the case of faults the sup-
ply is not disconnected and the equipment may stay energized for an
unknown period of time. This is why the local equipotential bonding
mustnotbelinkedtothegroundingsystemofthebuilding,whichmay
be present as a protective measure in other areas protected by discon-
nection of supply. Such a link, in fact, could energize the grounding
system and thereby transfer unresolved faults that occurred in the
earth-free area to grounded ECPs elsewhere in the same structure.
A major hazard of this protection, suitable only in strictly super-
vised installations, is at the interface with adjacent rooms, whose floor
is earthed. A person standing over both floors at the same time is ex-
posed to potential differences upon faults in the earth-free location. A
solution to this hazard is the interposition between the two locations
of a sufficiently wide insulated floor section.

2.3.2.4 Protection by Electrical Separation
As per IEC, electrically separated systems use isolating transform-
ers, with the same value of primary and secondary voltages not ex-
ceeding 500 V. Such transformers isolate persons from ground, and
fromothercircuits,therebypreventingthecirculationofearthcurrents
upon faults. In this arrangement, the ECPs must not be grounded.