Page 41 - Electrical Safety of Low Voltage Systems

24 Chapter Tw o

FIGURE 2.14
Person in contact
with the primary
voltage in
separated
systems.

Between the primary and secondary windings of an isolating
transformer is a double or reinforced insulation or a grounded metal-
lic screen or sheath. The double insulation lowers the probability of
persons coming into contact with the primary voltage, which is dan-
gerous as it is ground-referenced (Fig. 2.14).
Persons are exposed to the danger of indirect contact if three con-
current faults, not necessarily simultaneous, occur: failure of the first
layer of the double insulation, failure of the second layer of the double
insulation, and failure of the basic insulation of the ECP. Thus, three
levels of protection must fail to determine a hazardous situation.
However, according to international standards, 12 the isolating
transformer, with the double insulation, is no longer required, and
an ordinary transformer, that is, with only basic insulation between
the windings, may be used in separated systems. With an ordinary
transformer, persons are in danger if two concurrent faults take place:
failure of the basic insulations of the transformer and of the appliance.
This solution is consistent with the other standard protective measures
against indirect contact, which, as already said, are characterized by
two “layers” of protection.
As anticipated, owing to the high impedance to ground of the sep-
aratedsystem,inthecaseoffailureofthebasicinsulationofanECP,the
ground current cannot flow. Thus, even if the ECP is energized, per-
sons are not in danger, as the fault-loop cannot be established through
the earth.
In reality, no electrical system is truly isolated from ground, even
when there is no intentional earthing connection of the source. Ev-
ery circuit is, in fact, “coupled” to earth through a “virtual” capacitor,
whose two armatures are the circuit wires and the actual earth; the di-
electric is the means interposed between the armatures (e.g., the air).