Page 189 - Electrical Safety of Low Voltage Systems

172 Chapter Te n

FIGURE 10.1
Ripple-free voltage.

3
interposition of double, or reinforced, insulation is required between
them. To facilitate this separation, extra-low-voltage circuits often
have dedicated conduits and junction boxes. Extra-low-voltage sys-
tems may be insulated from each other via the basic insulation. For
SELV systems, basic insulation between live parts and the earth is also
required.

10.2 Separated Extra-Low-Voltage (SELV) Systems

10.2.1 Protection Against Indirect Contact
Protection against indirect contact in SELV systems is fulfilled by a
harmless extra-low-voltage supply. This solution is acceptable only
if an effective electrical separation from higher voltages during both
normal operations and fault conditions is in place. Acceptable sources
to supply SELV systems, which guarantee sufficient electrical insula-
tion from other non-SELV circuits, are listed as follows:

A safety isolating transformer with no intentional connection

to earth and secondary voltage not exceeding 50 V.
A source providing the same degree of protection as the safety

isolating transformer (e.g., electric motor-driven generator).
An electrochemical source (e.g., a battery) or another inde-

pendent source (e.g., an engine-driven generator).

FIGURE 10.2
Nonsinusoidal
ripple-free voltage.