erefore that the circuit between the Before looking at detailed requirements and tests, it is worth con- two boxes would never attain a hazardous voltage even in the sidering what has been a difficult matter in the development of the presence of a single fault in the PC (an SELV circuit in IEC 950 telecom safety standards. A single box (for example a mains oper- terminology). ated feature phone) can be designed with confidence to satisfy the However, a PC is often not just one box but several, and it would safety requirements, in particular to present no safety hazard to the not be practicable to test every PC add-on, including some not yet network. Electrical barriers are used to isolate the network from the designed, when approving the modem. So BS 6301 is content with mains and from any other hazardous voltages (see Figure 23.12). a warning label about what should be connected to a port, unless the EN 41003 23/7 port is barriered. Either way adds significantly to the cost of the signal. The formulae for ringing signals seem complex and daunting equipment. but are not difficult to use. This, and the telegraph signal, are those In EN 41003, however, there is no such requirement. The view of existing technology where the operating voltage is some tens of was taken that if the PC and the modem are designed to co-operate volts; modern digital circuits in ISDN, and SCVF telegraphy, are with one another, they must use the same operational interface SELV circuits operating at a few volts. specification (RS 232 or V.24) and it is reasonable to assume that In the classical safety standards, where there are only two kinds both boxes will be designed according to the appropriate safety of circuit, those that are safe to touch and those that are not, the rules specification at this interface. In the example of a modem and a PC, for keeping these apart are fairly simple. With the addition of TNV the appropriate safety specification on the interface between them circuits, which are somewhere in between, the separation and ac