Protection for Safety

In the following text, a direct shock is from contact with a live part which is intentionally live. An indirect shock is from contact with an exposed conductive part or an extraneous conductive part made live from a fault. 

Section 411 Protection against both direct and indirect shock 

The intention of SELV is to minimise the shock hazard by voltage limitation. Subsection 411-02 sets out the parameters, which include a safe source of supply such as a safety transformer to BS 3535 and other precautions to avoid the SELV circuit becoming live at a higher voltage. If the SELV does not exceed 25 V a.c. or 60 V ripple-free d.c., i.e. not more than 10% ripple, the direct contact shock risk is regarded as negligible and exposed live parts are allowed except in locations of enhanced shock risk, such as most of those in Part 6 where the conductors have to be protected against direct contact by a barrier, enclosure or insulation. The SELV circuit is not earthed and the cables are not metal-sheathed. As conductive parts of the installation are not deliberately or fortuitously earthed, it will usually be more practicable to use insulated rather than metallic conduit and ducts. 

Where it is not possible to meet all the requirements in subsection 411-02 for SELV, but the voltage does not exceed extra-low voltage, the protection is called protective extra low voltage (PELV). The difference between a SELV and PELV system is that the latter is connected to earth but in other respects meets the SELV requirements. Other extra-low voltage systems are called functional extra-low voltage (FELV). The relevant precautions are detailed in section 471-14. The socket outlets in SELV, PELV and FELV systems 

Protection by limitation of energy is in subsections 411-04 and 471-03. The intention is not to prevent the shock sensation but to limit the shock current and/or its duration so as to avoid injury to persons and animals. Common examples of equipment complying with this requirement are electric fence energisers, electrostatic paint and powder sprayers, and tungsten inert gas (TIG) welding electrodes. 

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For example, in considering workers in contaminated areas of the site who work on scaffolds, the OSHA Scaffolding Standards are more protective for safety hazards resulting from working on scaffolds. HAZWOPER is more protective for health hazards resulting from the contamination. The applicable provisions of both standards would apply to the work. 

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