Measures of protection: two levels

From Electrical Installation Guide

Protection against indirect contact hazards can be achieved by automatic disconnection of the supply if the exposed-conductive-parts of equipment are properly earthed

Two levels of protective measures exist:

• 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).
• 2^nd 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).

(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.

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Fig. F7:  Illustration of the dangerous touch voltage Uc

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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 CA.

Reminder of the theoretical disconnecting-time limits

Uo (V)		50 < Uo ≤ 120	120 < Uo ≤ 230	230 < Uo ≤ 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)