An Electrostatic Discharge (ESD) clamp includes first power rail, a second power rail separate from the first power rail, and an ESD discharging circuit including a plurality of cascaded MOS transistors coupled between the second power rail and an electrical ground. A bias circuit is configured to turn on the ESD discharging circuit in response to an ESD event on the second power rail, and to turn off the ESD discharging circuit during a normal operation of the ESD clamp.