A pressure relief apparatus for a pressurized system comprising: a pressure relief device (30, 230, 330, 1030, 1130) comprising an activating portion (1033, 1131) configured to relieve pressure when pressure in the pressurized system exceeds a threshold, wherein the activating portion (1033, 1131) comprises a designed area of weakness (332, 432, 532, 732, 932, 1080, 1080b, 1132, 1136) and a pre-activation reactive region (32, 332, 432, 532, 732, 932, 1080, 1080b, 1132, 1135, 1136), wherein the pre-activation reactive region (32, 332, 432, 532, 732, 932, 1080, 1080b, 1132, 1135, 1136) preferably overlaps with the defined area of weakness or is defined adjacent to the defined area of weakness (332, 432, 532, 732, 932, 1080, 1080b, 1132, 1136); and a pressure relief device integrity sensor (5, 250, 305, 552, 652, 752, 852, 905, 1070, 1170) configured to sense the integrity of the pressure relief device before activation, wherein the integrity sensor is preferably positioned adjacent to the pre-activation reactive region (32, 332, 432, 532, 732, 932, 1080, 1080b, 1132, 1135, 1136); the pressure relief device integrity sensor comprising a normally closed circuit (10, 210, 310, 510, 610, 710, 810, 910) configured to transmit a signal; the apparatus further characterized in that the pressure relief device integrity sensor comprises: a sensor element (250, 305, 552, 652, 752, 852, 905, 1070, 1170) secured against the pressure relief device (30, 230, 330, 1030, 1130) with a limited freedom of movement relative to the device; and a tab; wherein the tab is configured to impinge upon the sensor element (250, 305, 552, 652, 752, 852, 905, 1070, 1170) in response to a non-activating change in at least one pre-activation reactive region (32, 332, 432, 532, 732, 932, 1080, 1080b, 1132, 1135, 1136), thereby causing the sensor element (250, 305, 552, 652, 752, 852, 905, 1070, 1170) to deform; and wherein deformation of the sensor element (250, 305, 552, 652, 752, 852, 905, 1070, 1170) causes a change in the signal. a pressure relief device (30, 230, 330, 1030, 1130) comprising an activating portion (1033, 1131) configured to relieve pressure when pressure in the pressurized system exceeds a threshold, wherein the activating portion (1033, 1131) comprises a designed area of weakness (332, 432, 532, 732, 932, 1080, 1080b, 1132, 1136) and a pre-activation reactive region (32, 332, 432, 532, 732, 932, 1080, 1080b, 1132, 1135, 1136), wherein the pre-activation reactive region (32, 332, 432, 532, 732, 932, 1080, 1080b, 1132, 1135, 1136) preferably overlaps with the defined area of weakness or is defined adjacent to the defined area of weakness (332, 432, 532, 732, 932, 1080, 1080b, 1132, 1136); and a pressure relief device integrity sensor (5, 250, 305, 552, 652, 752, 852, 905, 1070, 1170) configured to sense the integrity of the pressure relief device before activation, wherein the integrity sensor is preferably positioned adjacent to the pre-activation reactive region (32, 332, 432, 532, 732, 932, 1080, 1080b, 1132, 1135, 1136); the pressure relief device integrity sensor comprising a normally closed circuit (10, 210, 310, 510, 610, 710, 810, 910) configured to transmit a signal; a sensor element (250, 305, 552, 652, 752, 852, 905, 1070, 1170) secured against the pressure relief device (30, 230, 330, 1030, 1130) with a limited freedom of movement relative to the device; and a tab;