Types of protection against electric shock
The objective is to ensure that hazardous-live-parts shall not be accessible and accessible conductive parts shall not be hazardous. Different protective provisions must be implemented. Protective measures result from a suitable combination of them.
Different parameters must be taken into account: ambient temperature, climatic conditions, presence of water, mechanical stresses, capability of persons and area of contact of persons.
Basic protection includes one or more provisions that, under normal conditions, prevent contact with live parts. Particularly:
Protection by the insulation of live parts
This protection consists of an insulation which complies with the relevant standards (see Fig. F4). Paints, lacquers and varnishes do not provide an adequate protection.
Protection by means of barriers or enclosures
This measure is in widespread use, since many components and materials are installed in cabinets, assemblies, control panels and distribution boards (see Fig. F5).
To be considered as providing effective protection against direct contact hazards, these equipment must possess a degree of protection equal to at least IP 2X or IP XXB (see Protection provided for enclosed equipment: codes IP and IK).
Moreover, an opening in an enclosure (door, front panel, drawer, etc.) must only be removable, open or withdrawn:
- By means of a key or tool provided for this purpose, or
- After complete isolation of the live parts in the enclosure, or
- With the automatic interposition of another screen removable only with a key or a tool. The metal enclosure and all metal removable screen must be bonded to the protective earthing conductor of the installation.
Other measures of protection
- Protection by means of obstacles, or by placing out of arm’s reach.
- This protection is reserved to locations to which only skilled or instructed persons have access. The erection of this protective measure is detailed in IEC 60364-4-41. See Out-of-arm’s reach or interposition of obstacles.
- Protection by use of Extra-Low Voltage (ELV) or by limitation of the energy of discharge.
- These measures are used only in low-power circuits, and in particular circumstances as described in Extra Low Voltage (ELV).
Fault protection 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:
- The earthing of all exposed-conductive-parts of electrical equipment in the installation and the constitution of an equipotential bonding network (see Protective earthing conductor (PE) )
- 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 (see Fig. F6)
The higher the value of Uc, the higher the rapidity of supply disconnection required to provide protection (see Fig. F7). The highest value of Uc that can be tolerated indefinitely without danger to human beings is 50 V AC.
In DC the highest value of Uc that can be tolerated indefinitely without danger is 120 V.
Reminder of the theoretical disconnecting-time limits (IEC 60364-4-41)
|Uo (V AC)||50 < Uo ≤ 120||120 < Uo ≤ 230||230 < Uo ≤ 400||Uo > 400|
- in TN systems, a disconnection time not exceeding 5 s is permitted for distribution circuits, and for circuits not covered by Fig. F7
- in TT systems, a disconnection time not exceeding 1 s is permitted for distribution circuits and for circuits not covered by Fig. F7
- The 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.