Surge suppressors, plugin style are plugged into wall outlets to protect equipment from transient overvoltages.  Surge suppressors or surge protection devices (SPD) are devices used to protect equipment from transient overvoltages present on AC power, data, or telephone circuits. 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. Shunt type suppressors have a characteristic clamping voltage that is normally chosen to be around the maximum safe operating voltage of the circuit. The performance of surge suppressors 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. To prevent equipment damage from both internal and external surges, point-of-use SPDs can be used. The equipment's power plugs (or data lines) can plug directly into the protected outlet.