This part of IEC 61000 sets high-altitude electromagnetic pulse (HEMP) immunity requirements for electrical and electronic equipment intended for use indoors. The indoor HEMP environment depends on the electromagnetic shielding quality of a facility and the level of protection against the conducted environment. This standard is intended for all facility types including residential, commercial, light industrial, hospitals, heavy industrial, power substations and power generation facilities. This standard includes indoor equipment intended to be connected to a low-voltage (1 kV or less) power network, to the telecom network and/or to external antennas. Immunity test values are based on the 90 % severity levels in IEC 61000-4-25. All telecom lines are assumed to have gas-tube protectors at the point where they enter the building and insulation flashover on low-voltage lines are assumed at three times that of lightning. For signal ports connected to internal cables, severity test levels are based on cables that have a total length of 10 m and a procedure is provided for longer cables. This part of IEC 61000 does not specify safety requirements for equipment, such as protection against shock, insulation coordination, and related dielectric tests. Nevertheless, the immunity tests described in this part of IEC 61000 involve hazardous voltages. High-voltage precautions are necessary to protect the health and safety of test personnel.