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 to 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 (200 A, 10/160, and 100 A, 10/560) the equipment is allowed to be non-operational or operational. For an operational pass, series resistance must be added to reduce the Type A currents to within the TISP4xxxL3LM ratings (50 A, 10/160 and 30 A, 10/560). Alternatively, a series fuse with an I2t greater than 0.45 A 2s and less than 5.6 A2s could be used to give a non-operational pass. After a Type B surge the equipment must be operational. As the TISP4xxxL3LM has a current rating of 40 A, will survive both Type B surges, metallic (25 A, 9/720) and longitudinal (37.5 A, 9/720), giving an operational pass to Type B surges. For metallic protection, the TISP4350L3LM is connected between the Ring and Tip conductors. For longitudinal protection two TISP4350L3LM protectors are used; one between the Ring conductor to ground and the other between the Tip conductor to ground. The FCC Part 68 B type ringer has voltages of 56.5 V d.c. and up to 150 V rms a.c., giving a peak voltage of 269 V. The TISP4350L3LM will not clip the B type ringing voltage as it has a high impedance up to 275 V. The TISP4070L3LM should be connected after the hook switch to protect the following electronics. As the TISP4070L3LM has a high impedance up to 58 V, it will switch off after a surge and not be triggered by the normal exchange battery voltage.