 a Guide is not precise enough for design or testing purposes. To provide a complementary 23.4.1 Single fault principle but more precise European document, CENELEC produced EN 41003 which can be applied in conjunction with a classical safety Most product safety standards are based on the principle of provid- standard SO as to cover all the safety requirements for a product that ing two levels of protection: the equipment must remain safe even is to be connected to a network, as shown in Figure 23.5. The in the presence of a single fault. The design must also make sure that Radio, Domestic Measuring Electro- equipment TV, Hifi appliances equipment medical EN41003 EN41003 EN41003 EN41003 EN41003 Figure 23.5 Addition of telecom safety section Telecom safety considerations 23/5 Radio, Domestic Measuring Electro- equipment TV, Hifi appliances equipment medical Edn 2 telecom EN41003 EN41003 EN41003 EN41003 Figure 23.6 Telecom safety in IEC 950 a fault is unlikely, so that the possibility of two faults occurring which supplementary insulation guards the basic. A plastic coffee simultaneously can be ignored. For example, basic insulation has to pot is class II equipment. be designed to certain minimum dimensions and to withstand a voltage test, so that breakdown is very unlikely. However, if that 23.4.2 Electric shock were to break down, there must be another level of protection still in place. This second level may, for example, be added to basic Safety is concerned, among other things, with fire, mechanical and insulation by earthing an accessible metal enclosure, such as a heat hazards but mainly with electric shock, and the telecom safety coffee pot, in conjunction with a fused supply; if either the basic requirements are concerned only with electric shock. insulation breaks down or the earth connection fails the equipment The criterion for what is an unacceptable shock is the voltage on is still safe. This is class I protection (see Figure 23.7). Class II parts that are accessible to touch, or the current that can b