Surge suppressors, single phase, are used to protect equipment from transient overvoltages present on AC single-phase power lines. They are not plug-in style devices.  Surge suppressors may operate by absorbing the surge (the shunt type suppressor) or by blocking the surge from flowing (the series type suppressor) or by a combination of the two. The shunt type suppressor has a characteristic clamping voltage that is normally chosen to be around the maximum safe operating voltage of the circuit. The performance of a surge suppressor is measured by applying a known test surge (such as described by the domestic IEEE 587 standard or the international IEC 61643-1 standard) and measuring the maximum voltage that is let through to the device to be protected.