Page 192 - Electrical Safety of Low Voltage Systems
P. 192
Extra-Low-V oltage Systems 175
FIGURE 10.5 Fault to
ground of an output
terminal and
consequent touch
voltage.
10.3 Protective Extra-Low-Voltage (PELV) Systems
Like SELV systems, PELV circuits must be supplied by safety isolat-
ing transformers whose secondary output does not exceed 50 V. The
distinct difference with SELV consists of the permitted connection to
earth of a point of the system (Fig. 10.6).
The connection to earth may be necessary due to safety reasons,
as explained in Sec. 10.3.1, and be achieved through a link to the
main earth terminal, as well as to a grounded ECP. PELV systems
offer a greater risk of electrocution than SELV. In fact, during ground
faults occurring in the higher voltage system, the output voltage of the
safety transformer may exceed the nominal voltage of the PELV by the
ground potential rise V G . In the worst-case scenario, persons standing
in a zero potential area would be exposed to the touch voltage V G + 50.
For this reason, to lower the risk of electrocution it is important to
R
G
FIGURE 10.6 The PELV system.
