Recommendation ITU-T K.75 provides guidance to manufacturers, network operators and test houses on the classification of the type of interface, i.e., external (subjected to a.c. surges or conducted lightning surges) or internal (subjected to short fall-time transients induced into building cabling). This Recommendation defines telecommunication overvoltage categories as a function of the inherent resistibility voltage of the equipment port or interface. For each category, the impulse withstand voltages between the telecommunication interface and the person or hazardous circuit have been defined (peak value and waveshape). These peak values and waveshapes should be suitable for insulation coordination purposes.