Chapter A General rules of electrical installation design

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F
F - Protection against electric shock
Protection against indirect contact hazards
can be achieved by automatic disconnection of
the supply if the exposed-conductive-parts of
equipment are properly earthed
(1) Touch voltage Uc is the voltage existing (as the result of
insulation failure) between an exposed-conductive-part and
any conductive element within reach which is at a different
(generally earth) potential.
3 Protection against indirect
contact
Exposed-conductive-parts used in the manufacturing process of an electrical
equipment is separated from the live parts of the equipment by the “basic insulation”.
Failure of the basic insulation will result in the exposed-conductive-parts being alive.
Touching a normally dead part of an electrical equipment which has become live due
to the failure of its insulation, is referred to as an indirect contact.
3.1 Measures of protection: two levels
Two levels of protective measures exist:
b 1 st level: The earthing of all exposed-conductive-parts of electrical equipment in the
installation and the constitution of an equipotential bonding network (see chapter G
section 6).
b 2 sd level: Automatic disconnection of the supply of the section of the installation
concerned, in such a way that the touch-voltage/time safety requirements are
respected for any level of touch voltage Uc (1) (see Fig. F7).
Earth
connection
Fig. F7 : Illustration of the dangerous touch voltage Uc
The greater the value of Uc, the greater the rapidity of supply disconnection required
to provide protection (see Fig. F8). The highest value of Uc that can be tolerated
indefinitely without danger to human beings is 50 V a.c.
Reminder of the theoretical disconnecting-time limits
Uo (V) 50 < Uo y 120 120 < Uo y 230 230 < Uo y 400 Uo > 400
System TN or IT 0.8 0.4 0.2 0.1
TT 0.3 0.2 0.07 0.04
Fig. F8 : Maximum safe duration of the assumed values of AC touch voltage (in seconds)
Schneider Electric - Electrical installation guide 2008
Uc