These devices are designed to limit overvoltages on the telephone line. Overvoltages are normally caused by a.c. power system or lightning flash disturbances which are induced or conducted on to the telephone line. A single device provides 2-point protection and is typically used for the protection of 2-wire telecommunication equipment (e.g. between the Ring and Tip wires for telephones and modems). Combinations of devices can be used for multi-point protection (e.g. 3-point protection between Ring, Tip and Ground). The protector consists of a symmetrical voltage-triggered bidirectional thyristor. Overvoltages are initially clipped by breakdown clamping until the voltage rises to the breakover level, which causes the device to crowbar into a low-voltage on state. This low-voltage on state causes the current resulting from the overvoltage to be safely diverted through the device. The high crowbar holding current prevents d.c. latchup as the diverted current subsides.These protectors are guaranteed to voltage limit and withstand the listed lightning surges in both polarities. After a Type A surge the equipment can be faulty, provided that the fault mode causes the equipment to be unusable. The high current Type A surges (10/160, 200 A and 10/560, 100 A), will cause the TISP4xxxL3BJ to fail short circuit, giving a non-operational equipment pass to Type A surges. After a Type B surge the equipment must be operational. As the TISP4xxxL3BJ has a current rating of 40 A, will survive both Type B surges, metallic (differential mode 25 A, 9/720) and longitudinal (common mode 37.5 A, 9/720), giving an operational pass to Type B surges. For metallic protection, the TISP4350L3BJ is connected between the Ring and Tip conductors. For longitudinal protection two TISP4350L3BJ protectors are used; one between the Ring conductor to ground and the other between the Tip conductor to ground. The B type ringer has voltages of 56.5 V d.c. and up to