here to protect may be damaged. However, it can be clearly does not apply But whether an interface should be an SELV difficult to know whether or not such an arrangement passes the test, circuit or a TNV circuit, to match its neighbouring box, may not be the criterion being breakdown of insulation. A technique is de- clear. For EN 41003, this information is given by ETSI (European scribed in an annex for observing the characteristic waveforms Telecommunications Standards Institute) because EN 41003 was across insulation (with or without a surge suppressor in parallel), to written for certain ETSI publications (those that become NETs or distinguish between proper operation of a suppressor and break- CTRs) that have mandatory effect. down of insulation. ETSI is in the course of publishing a technical report, ETR 12, Annex F of EN 41003 contains out of date guidance on insulation which summarises this information for a large number of possible spacings for 4.5 and should be ignored. It is not required to provide interfaces on subscribers' terminal equipment. It also states, for each any particular spacings, only to withstand the voltage tests. interface, whether or not 4.4.2 and 4.5 apply and whether or not It will be noted that separation between the line and other parts SELV circuits as specified in 2.3.6 of IEC 950 can be used. (De- may have to meet more than one requirement of the standard for signers of any kind of equipment would be well advised never to use different purposes, applying to the same physical piece of insula- 2.3.6 or 2.3.7, and not even 2.3.5 if it can be avoided). Additionally, tion. Of these, the requirements of 4.5 are probably least onerous ETSI has identified a possible safety hazard that arises on the ISDN and the most onerous must prevail; this should not be regarded as a 'passive bus' and has written tests and requirements that do not yet contradiction within the standard. appear in EN 41003 (see ETSI ETS 00046 and ETS 300 047). This Documentation 23/9 hazard is the possibility of too m