10 can tilt up to open a larger gap creating a staged additional venting area for the gases generated by the explosion so as to avoid structural damage by the development of high pressures in the manhole cavity fig4 shows internal details of the lock body assembly 20 the lock body 34 is pivoted at one end on the pivot pin 36 received between vertical ribs 11 to allow limited rotation down from the cover 10 when the primary shear pin 32 also received in the ribs 11 is sheared off by the forces acting through the slide portion 28 the latch slide 26 is slidably received in a bore 38 formed in the lock body 34 a keeper blade 40 is received in a slot 42 in the latch slide 26 to prevent rotation of the latch slide 26 within the bore 38 a spring 44 interposed between keeper blade 40 and an end wall 46 of the slot 42 urges the latch slide 26 to the left to tend to retract the portion 28 radially inwardly the actuator bolt 30 has a rounded end 48 which engages a sloping cam surface 50 on the top of the latch slide 26 which forces the latch slide 26 to the right when the bolt 30 is rotated to be advanced until the fully advanced position is reached as seen in fig4 the bolt can be turned using an anti tamper special wrench tool 52 mating with a correspondingly specially shaped bolt head 51 to.