The importance of clear openings in manhole covers and frames

Clear opening is a critical compliance and safety factor in the design and specification of carriageway manhole covers certified under BS EN 124.

The BS EN 124 standard defines the clear opening as:

“The largest circular area that can be inscribed within the frame opening.”

In practical terms, this means that a 600 mm clear opening must be completely unobstructed – no part of the frame, hinge, seating, or lug should project into this circular space where personnel access is required.

However, it’s important to note that BS EN 124 allows any clear opening up to 1 metre, depending on the intended use. The standard only advises a minimum 600 mm clear opening for chambers that require personnel access. For chambers that do not require personnel entry, smaller clear openings are permitted, including those that take account of frame projections. In such cases, the clear opening is less critical to safety.

Where a chamber is designed for personnel access, any internal projections that restrict or reduce the clear opening below 600 mm present a serious hazard. Reduced openings limit space for operatives to enter or exit safely, particularly when wearing breathing apparatus or using confined space equipment.

In addition, CD 534, the national standard governing chamber tops used on the UK’s strategic road network, reinforces this requirement. It stipulates that any manhole cover intended for personnel access must have a minimum clear opening of 600 mm for rectangular frames, or 700 mm for circular frames, ensuring safe and unhindered entry during maintenance operations.

It has come to light that some products on the market may have been third-party certified without fully meeting this geometric requirement. While this is likely the result of oversight rather than intent, it underlines the importance of vigilance across the supply chain.

What to check

• Verify the stated clear opening matches the unobstructed internal dimension of the frame, particularly where personnel access is required.
• Inspect frames carefully for internal lugs, hinge housings or seat details that may project into the aperture.
• Seek assurance from manufacturers that the product has been measured and verified in accordance with BS EN 124-1:2015 definitions and meets the minimum access dimensions in CD 534.

True compliance safeguards both operatives and asset owners.

We fully support transparent testing, accurate dimensional verification, and honest product representation across the industry, ensuring every manhole cover performs safely, reliably, and in full accordance with the standards.

Read more about clear openings in manhole covers in our knowledge base