It is necessary to clarify the electrical requirements for SPDs/SPCs in order to realize both resistibility and safety of telecommunication systems. Recommendation ITU-T K.143 analyses the influence on human safety of lightning measures bridging across insulation in equipment by surge suppressors. Recommendation ITU-T K.143 provides guidance for design of lightning protection and requirements on surge suppressor in equipment from the human safety standpoint. Requirements for SPDs/SPCs in multiservice surge protective devices (MSPDs) external to the equipment and SPDs installed on lines in a building lie outside the scope of this Recommendation.