The Circuit Designer's Companion - diagramas.diagram...

292 The Circuit Designer’s CompanionChapter 9General product design9.1 SafetyAny electronic equipment must be designed for safe operation. Most countries havesome form of product liability legislation which puts the onus on the manufacturer toensure that his product is safe. The responsibility devolves onto the product designengineer, to take reasonable care over the safety of the design. This includes ensuringthat the equipment is safe when used properly, that adequate information is provided toenable its safe use, and that adequate research has been carried out to discover,eliminate or minimise risks due to the equipment.There are various standards relating to safety requirements for different productsectors. In some cases, compliance with these standards is mandatory. In the EuropeanCommunity, the Low Voltage Directive (73/23/EEC) applies to all electrical equipmentwith a voltage rating between 50 and 1000Vac or 75 and 1500Vdc, with a fewexceptions, and requires member states to take all appropriate measures“to ensure that electrical equipment may be placed on the market only if, having been constructed inaccordance with good engineering practice in safety matters in force in the Community, it does notendanger the safety of persons, domestic animals or property when properly installed andmaintained and used in applications for which it was made.”If the equipment conforms to a harmonised CENELEC or internationally-agreedstandard then it is deemed to comply with the Directive. Examples of harmonisedstandards are EN 60065:1994, “Safety requirements for mains-operated electronic andrelated apparatus for household and similar general use”, which is largely equivalent toIEC Publication 60065 of the same title; or EN 60950-1:2002, “Information technologyequipment. Safety. General requirements”, equivalent to IEC 60950-1. Proof ofcompliance can be by a Mark or Certificate of Compliance from a recognisedlaboratory, or by the manufacturer’s own declaration of conformity. The Directiveincludes no requirement for compulsory approval for electrical safety.The hazards of electricityThe chief dangers (but by no means the only ones, see Table 9.1) of electricalequipment are the risk of electric shock, and the risk of a fire hazard. The threat to lifefrom electric shock depends on the current which can flow in the body. For ac, currentsless than 0.5mA are harmless, whilst those greater than 50–500mA (depending onduration) can be fatal. † Protection against shock can be achieved simply by limiting thecurrent to a safe level, irrespective of the voltage. There is an old saying, “it’s the voltsthat jolts, but the mils that kills”. If the current is not limited, then the voltage level in† IEC Publication 60479 gives further information.