These protector pairs have been formulated to limit the peak voltages on the line terminals of the `7581/2/3 LCAS (Line Card Access Switches) type devices. An LCAS may also be referred to as a Solid State Relay, SSR, i.e. a replacement of the conventional electro-mechanical relay. Overvoltages are normally caused by a.c. power system or lightning flash disturbances which are induced or conducted on to the telephone line. These overvoltages are initially clipped by protector 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. For negative surges, the high crowbar holding current prevents d.c. latchup with the SLIC current, as the surge current subsides.