The TISP8250D is a P-gate reverse-blocking thyristor (SCR) designed for the protection of telecommunications equipment against overvoltages and overcurrents on the telephone line caused by lightning, a.c. power contact and induction. The fixed voltage and current triggered modes make the TISP8250D particularly suitable for the protection of ungrounded customer premise equipment. Connected across the d.c. side of a telephone set polarity bridge, in fixed voltage mode these devices can protect the ringer in the on-hook condition. In an off-hook condition, either the fixed voltage or current triggered modes can protect the following telephone electronics. Without external gate activation, the TISP8250D is a fixed voltage protector. The maximum working voltage without clipping is 250 V and the protection voltage is 340 V. Lower values of protection voltage may be set by connecting an avalanche breakdown diode of less than 250 V between the TISP8250D gate and anode (see Figure 2.) By connecting a small value resistor in series with the line conductor and connecting the TISP8250D gate cathode terminals in parallel with the resistor, conductor overcurrents can gate trigger the TISP8250D into conduction. 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. Overcurrents develop sufficient voltage across the external gate-cathode resistor to trigger the device into a low-voltage on state. This low-voltage on state causes the current resulting from the overstress to be safely diverted through the device. The high crowbar holding current prevents d.c. latchup as the diverted current subsides.