How access covers are measured:
Access cover dimensions are always referred to by their ‘clear opening size’ and sometimes also by their frame depth (see diagram).
Clear Opening:
The clear opening is the inside frame dimensions and these should normally be marked on the cover. If these are missing or have become illegible over time then it will be necessary to remove the cover and physically measure these dimensions. The clear opening size of the access cover frame must always be equal to or greater than the size of the chamber it is covering to ensure the frame is adequately supported.
Base Opening:
These are the most important dimensions when establishing the correct size of frame intended to fit on a given chamber or gully-top opening. i.e. The central opening at the base of a frame must never be smaller than the opening at the chamber top so that the frame flange is always completely supported by the chamber top, usually via bedding materials. This harmonisation of component dimensions is vital in achieving the most effective traffic-load transfer possible to an underlying chamber structure.
Clear Area:
The Clear Area dimensions are the two gross measurements that are taken perpendicular to each other and a product’s permanent frame walls or supports and coincide with the narrowest points at that location. In view of this, it is common for one of these dimensions to be the same as the Clear Opening.
A frame product’s Clear Area often disregards the presence of cover seating or locking-feature encroachment in the corners of the frame. i.e. Such features are generally ignored as their presence is generally not considered detrimental to unrestricted chamber access unless the equipment in a given chamber requires the full use of a frame product’s Clear Area.