A simple push-fit option, with 3 positions
Open - Close - Locked

Step 1: Locate the manhole chamber, & carefully lift the lid, using any PPE required. Note these can be heavy, 2 persons may be required.
Step 2: Next, determine the correct pipe(s) to be defended. Test the flow (flush toilet) & double check the pipe. This is important as NRV's can only be fitted to chamber inlets. Placing a NRV in an outlet, will block the flow.
Step 3: Using PPE, clean the pipe entry ready to house the NRV.
Step 4: Push your valve(s) into position, you may need multiple for multiple inlets. Check the position, with the flap pointed down & the hinge pivot horizontal. Locking position to be downward for normal use.
Step 5: Check the water flow once more. If satisfied, carefully place the chamber lid back.