Three phase surge suppressors or surge protection devices (SPD) protect equipment from transient overvoltages present on AC power lines. 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.