This section considers some of the main requirements of EN 41003, strictions reflect the long standing unwillingness of PTTs to rely on using the clause numbers of the 1990/1991 first edition. The require- earth for the safety of their personnel. ments of IEC 950 second edition are very similar but with different clause numbers. 23.6.4 Earthing 23.6.1 Insulation PTT concerns about earthing have led to a number of restrictions in addition to not relying on it, as noted above. At the subscriber's end, line wires and functional earth connections must be separated from Much of EN 41003 is about TNV circuits and their insulation from earth, because they are generally earthed at the exchange end. other circuits, SO it needs to specify certain types of insulation and Earthing at both ends could lead to electrical noise, malfunction, minimum spacings. However EN 41003 does not contain detailed over heating and even hazardous touch voltages due to earth poten- specifications for these; it refers to the specifications of SELV tial differences. Also, the green and yellow wire (protective conduc- circuits and insulation that are given very fully in IEC 950 (and EN tor) can too often become disconnected or even acquire a hazardous 60950). voltage. It can easily be seen from Figure 23.15 how a disconnected earth 23.6.2 Definitions Mains EN 41003 uses the definitions in EN 60950 plus some new ones. Equipment Network Clause 3 of EN 41003 contains a new definition for 'telecommuni- cation network', a term which is quite difficult to define. This definition, unfortunately, can be read in different ways. However, for the current uses of the standard, which is invoked for permission Surge to connect equipment to a PTT network, this lack of precision does filter suppressor not matter. The exclusions in this definition do not mean that there capacitor is no standard for mains signalling, TV distribution systems etc.; it means that they were not considered when EN 41003 was written. IEC 950, second edition, can be used as it stands for