Varistors provide reliable and economical protection against high voltage transients and surges which may be produced, for example, by lightning, switching or electrical noise on AC or DC power lines. The varistor is placed in parallel to the circuit being protected. The typical varistor current vs. voltage curve illustrates how the varistor protects the circuit. At rated voltage, the varistor is “high resistance” with small leakage current. When an overvoltage situation occurs, the varistor becomes conductive in a response time of < 20ns. In the protection zone of the curve the voltage is clamped: exponential change in current causes only a small increase in voltage across the varistor. In this state, the varistor is absorbing the transient energy and shunting surge currents to protect the circuit. When the transient is dissipated, the varistor resets to “high resistance”. Varistors have no polarity and can suppress positive and negative transients.