This underground inspection chamber restrictor ring (sometimes known as a reducer ring) is designed to reduce/restrict the access on 450mm inspection chambers (including B5494 & B5696) from 450mm diameter to 350mm diameter. This reducing ring is easy to install and can be installed for any 450mm chamber base regardless of the depth/invert, however commonly used where the chamber depth/invert is greater than 1 metre (1000mm).