The TSPD or Thyristor Surge Protection Device are specialized silicon based overvoltage protectors， used to protect sensitive electronic circuits from damaging overvoltage transient surges caused by induced lightning and powercross conditions. The TSPD protects by switching to a low on state voltage when the specified protection voltage is exceeded. This is known as a “crowbar” effect. When an overvoltage occurs， the crowbar device changes from a high?impedance to a low?impedance state. This low?impedance state then offers a path to ground， shunting unwanted surges away from the sensitive circuits. This crowbar action defines the TSPD’s two states of functionality： Open Circuit and Short Circuit. Open Circuit – The TSPD must remain transparent during normal circuit operation. The device looks like an open across the two wire line. Short Circuit – When a transient surge fault exceeds the TSPD protection voltage threshold， the devices switches on， and shorts the transient to ground， safely protecting the circuit.

　　The electrical characteristics of the TSPD help the user to define the protection threshold for the circuit. During the open circuit condition the device must remain transparent; this is defined by the IDRM. The IDRM should be as low as possible. The typical value is less than 5 A.