The TISP4A250H3BJ is an asymmetrical bidirectional overvoltage protector. It is designed to limit the peak voltages on the Ring line terminal of the LCAS (Line Card Access Switch) such as Le75181, Le75183 and Le75282. The TISP4A250H3BJ must be connected with bar-indexed terminal 1 to the protective Ground, and terminal 2 to the Ring conductor. The TISP4A250H3BJ voltages are chosen to give adequate LCAS ring line terminal protection for all switch conditions. The most potentially stressful condition is low level power cross when the LCAS switches are closed. Under this condition, the TISP4A250H3BJ limits the voltage and corresponding LCAS dissipation until the LCAS thermal trip operates and opens the switches. Under open-circuit ringing conditions, the line Ring conductor will have high peak voltages. For battery backed ringing, the Ring conductor will have a larger peak negative voltage than positive, i.e. the peak voltages are asymmetric. The TISP4A250H3BJ has a similar voltage asymmetry and will allow the maximum possible ringing voltage, while giving the most effective protection. On a connected line, the Tip conductor will have much smaller voltage levels than the open-circuit Ring conductor values. Here a TISP4xxxH3BJ series symmetrical voltage protector gives adequate protection. Overvoltages are initially clipped by breakdown clamping. If sufficient current is available from the overvoltage, the breakdown voltage will rise to the breakover level, which causes the device to switch into a low-voltage on-state condition. This switching action removes the high voltage stress from the following circuitry and causes the current resulting from the overvoltage to be safely diverted through the protector. The high holding (switch off) current prevents d.c. latchup as the diverted current subsides.