The purpose of this Recommendation is to provide: ?C guidance to manufacturers@ network operators and test houses on the classification of the type of interface@ i.e.@ external [subject to alternating current (a.c.) surges or conducted lightning surges] or internal (subject to short fall-time transients induced in building cabling); ?C information on the waveform of transients appearing at equipment interfaces for insulation coordination purposes. This Recommendation mainly applies to customer equipment due to safety being achieved in other ways for network equipment@ e.g.@ earthing or powering from a safe supply@ such as a supply voltage equal to or less than 60 V direct current (d.c.). NOTE ?C 60 V d.c. is a class 1 energy source (ES1) voltage. However@ the waveform of transients appearing at equipment interfaces is also provided for telecommunication centres and access network sites@ in case it is needed in the future. This Recommendation assumes that the telecommunication equipment is either mains powered or powered from the telecommunication network@ and is installed according to the earthing and bonding requirements relevant to each different environment (i.e.@ [ITU-T K.27]@ [ITU-T K.35] and [ITU-T K.66]). This Recommendation does not provide safety or resistibility requirements such as test procedures and test criteria. This Recommendation does not describe mitigation measures or the various approaches used to determine the need to provide additional protection external to the equipment (primary protection). This Recommendation does not address overvoltages caused by power contact with low-voltage or medium-voltage power lines.