A surge arrester is a device to protect electrical equipment from overvoltage transients caused by external (lightning) or internal(switching) events. Also called a surge protection device (SPD) or transient voltage surge suppressor (TVSS), this class of device is used to protect equipment in power transmission and distribution systems. (For consumer equipment protection, different products called surge protectors are used.) The energy criterion for various insulation material can be compared by impulse ratio. A surge arrester should have a low impulse ratio, so that a surge incident on the surge arrester may be bypassed to the ground instead of passing through the apparatus.