Patent ID: 7210497

Claim:
A pressure relief device for mounting on an electrical equipment housing over a vent opening for discharging pressurized fluid from the housing comprising: a multiple stage valve having a valve body with a throat alighnable with the vent opening for discharging fluid from the housing; a flange supporting the valve body for mounting the valve body on the housing over the vent opening; the flange having an annular working area between inner and outer diameters circumscribing the intended location of fastening elements for attaching the flange to the housing; a first-stage valve opening formed on the valve body for regulating the discharge of fluid from the throat of the valve body; a first-stage seal sealing the first-stage valve opening; a second-stage valve opening formed on the valve body for regulating the discharge of fluid from the first-stage valve opening; a second-stage seal sealing the second-stage valve opening; the second-stage valve opening having an effective diameter larger than an effective diameter of the first-stage valve opening by a predetermined ratio; the second-stage valve opening being spaced along the throat of the valve body from the flange; the effective diameter of the second-stage valve opening exceeding the inner diameter of the working area of the flange for enabling the effective diameter of the first-stage valve to be optimized for discharging fluid from the housing; fluid being discharged from the housing when a pressure applied by contents of the housing exceeds the pressure on a side of the first-stage seal opposite the housing, such that, in order, the first-stage seal becomes unseated, allowing flow of the fluid through the first-stage valve opening, and the second-stage seal becomes unseated, allowing flow of the fluid through the second-stage valve opening; and a spacing of the second-stage valve opening from the flange providing clearance along the valve body to access the intended location of the fastening elements for attaching the flange to the housing.