YD/T 993 is a Chinese telecommunications industry standard that specifies the lightning protection requirements and test methods for wired telecommunication terminal equipment. The category covers the equipment installed at the customer end of the network: telephones, modems, terminal adapters and the other apparatus that connects a subscriber line to equipment in the building, and it occupies a position in which a fault can travel in either direction. A surge arriving over the subscriber line may damage the terminal and, if it is not contained, may also damage the building wiring or expose a user; conversely a fault in the building supply may drive current through the terminal into the network. The standard therefore specifies the test levels and waveforms applied to the line port and to the power port, the behaviour required between ports, and the criteria by which the equipment is judged to have withstood the test, and it distinguishes between equipment connected to overhead cable and equipment connected to buried cable.