Page 188 - Electrical Safety of Low Voltage Systems

CHAPTER10

Extra-Low-Voltage

Systems

Despite our best efforts to achieve complete
electrical safety, against stupidity, ignorance,
and negligence, there is no defense.
m. mitolo

10.1 Introduction
Protection by extra-low voltage, realized by supplying electrical sys-
tems with nondangerous voltages, is a measure against direct and in-
direct contact, suitable in all situations, but especially indicated in wet
1
locations or in restrictive conductive locations. The extra-low voltage
must not exceed 50 V a.c. or 120 V ripple-free d.c. between conductors
2
or between any conductor and the earth. The aforementioned values
are deemed not hazardous to persons in standard conditions; there-
fore, persons cannot undergo electric shocks even if in contact with
live parts. Typical applications of extra-low voltages may be lighting
systems in particular locations and electrical equipment of machines.
Extra-low-voltage systems are grouped in three different cate-
gories: separated extra-low voltage (SELV), protective extra-low voltage
(PELV), and functional extra-low voltage (FELV).
A constant voltage to which may be superimposed a sinusoidal
ripple, whose r.m.s. value does not exceed 10% of the d.c. voltage itself
is conventionally defined as ripple-free (Fig. 10.1).
If the ripple is not a sine wave, the maximum peak of the total
voltage must be less than 140 and 70 V, respectively, for a nominal 120
and 60 V ripple-free d.c. system (Fig. 10.2).
To prevent neighboring electrical systems at higher voltages from
accidentally coming in contact with extra-low-voltage circuits, the

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