 all the excluded systems. The other definitions arise from the use of the concept of a TNV circuit. The term 'excessive voltage' is no different from 'hazardous Figure 23.15 Hazard caused by disconnected earth voltage' in IEC 950, except that it allows a TNV circuit to be considered non-hazardous. Until the second edition of IEC 950, the definition of hazardous voltage conflicted with TNV circuit. can allow a through connection between mains and network via an a.c. line filter and surge suppressors connected to line. Subclause 23.6.3 TNV circuit design 4.4.2 has a test for this particular circumstance: if this configuration of components cannot be avoided, 4.4.2 ensures that the mains The maximum voltages on a TNV circuit must comply with one of current getting out to line will not exceed a safe value. In practice, three possibilities: steady state, a ringing signal or a telegraph to pass this test, the surge suppressors would preferably be of a type 23/8 Safety and EMC that did not conduct at all at 250V a.c. Even without this circuit 23.6.7 Restricted access area configuration, the green and yellow wire has been known to be connected to the mains pin by inexpert users when re-wiring a plug. Inventing the TNV circuit was an attempt to include in classical IEC Equipment that will be installed by experts (including equipment terms equipment that had traditionally been designed to different with a hard wired or heavy duty pluggable mains connection) does principles by the telecom industry. This convergence is seen as not have this requirement. Equipment is also exempt if it needs an increasingly necessary, because a box of hardware used for data earth connection to enable it to function, provided that it is market processing could equally well be used for switching telecom mess- to say SO. However it is not clear in the standard if the earth ages. Such a box should not have to conform with different safety connection of an interference filter is there 'to enable it to function'. standards depending on its applicat