Understanding clear openings in manhole covers

Clear opening refers specifically to the maximum circular space that fits entirely within the frame aperture of a manhole cover. It defines the usable access size for operatives and equipment entering or exiting an underground chamber.

In simple terms, a 600 mm clear opening must be a completely unobstructed circular area, no part of the frame, hinge, seating, or lug should project into this space. If any feature intrudes, the product no longer meets the dimensional definition set out in BS EN 124.

This definition forms a core part of product designation and marking under BS EN 124, ensuring consistency across all certified access covers. It’s not a nominal or approximate size, it’s the verified internal dimension that determines whether a cover truly provides the access claimed.

For installations on the UK’s strategic road network, CD 534 builds on this requirement by stipulating minimum acceptable clear openings for manhole covers used in highways. It mandates that any chamber top intended for personnel access must have at least a 600 mm clear opening for rectangular frames (or 700 mm for circular frames), ensuring safe, unhindered access in operational environments.

In short, the clear opening defines real, usable access space, a critical measure that underpins safety, compliance, and correct specification for every manhole cover installed on the highway network.

The clear opening requirement for manhole covers is defined and governed by British and European standards that ensure safety, accessibility, and compliance across all installations.

BS EN 124

The BS EN 124 sets out the fundamental definition and measurement of clear openings for manhole tops. It defines the clear opening as:

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

This means that if a product is designated as a 600 mm clear opening, the entire 600 mm circular area must be completely free of obstruction - no part of the frame, hinge, seating, or lug may project into it. The standard also uses this dimension as a key part of product designation, e.g.:

“clear opening (CO in mm), e.g. 600 indicates CO = 600 mm.”

Therefore, manufacturers must ensure that any stated clear opening reflects the actual, unobstructed internal dimension verified according to the EN 124-1:2015 definitions. Any projection that reduces the usable aperture invalidates the stated clear opening and renders the product non-compliant.

BS EN 124 forms the legal and performance baseline for ductile iron access covers, setting not only the geometric definitions but also the requirements for testing, marking, and product classification by load class.

CD 534

CD 534 builds upon EN 124 for installations on the UK’s strategic road network, adding further dimensional and performance requirements. It specifies the minimum clear opening sizes necessary for safe and compliant access. According to section 2.9 of CD 534 (v0.2.0):

“A chamber top permitting access shall conform to the following minimum clear opening requirements:

1. The minimum clear opening for a frame with a rectangular opening is not less than 600 mm, with a diagonal measurement not less than 700 mm.
2. The minimum clear opening for a frame with a circular opening is a diametric measurement of not less than 700 mm.”

CD 534 also references BS EN 124 directly, requiring that all chamber tops used on motorways and all-purpose trunk roads must comply with EN 124 as a base standard. For covers exceeding one metre in size, CD 534 stipulates compliance with BS 9124.

Safety of operatives

A reduced access space increases the risk when entering and exiting chambers, particularly with breathing apparatus, tripods or harnesses. An obstructed 600 mm opening can create snagging points, restrict emergency egress and heighten confined-space risk.

Compliance and liability

If any internal projection encroaches into the circular aperture, the product does not meet EN 124 dimensional requirements. Misstating a clear opening on certification, datasheets or markings exposes specifiers and asset owners to non-compliance and potential claims.

Whole-life performance

Correct geometry supports safe, repeatable maintenance and helps avoid ad-hoc site modifications that can accelerate damage to seating, bedding or surrounding pavement. Independent sector research and guidance link poor ironwork detail and installation quality to premature failures that contribute to carriageway defects and disruption.

Network availability and road user impact

Ironwork that is non-compliant or degrades early can drive reactive works, lane closures and additional reinstatements. Current industry guidance emphasises collaboration and good practice to reduce the impact of highway works on road users, which starts with selecting and installing fit-for-purpose, compliant ironwork.

1. Match the number with the geometry
Ensure the published 600 mm (or other) clear opening is a fully unobstructed inscribed circle inside the frame. No hinges, lugs, bolt seats or seat fillets may intrude.

2. Inspect frames for projections
Look for internal features that could narrow the circular space. If present, the stated clear opening is not valid to EN 124.

3. Ask for EN 124 measurement verification
Seek assurance that dimensions have been verified to EN 124 definitions and that product designation and marking reflect the true clear opening.

4. Check suitability for route and risk
For trunk roads and high-risk locations, confirm the selection aligns with CD 534 requirements, including skid resistance and in-service performance, in addition to geometry.

5. Consider the installation specification
Confirm the chamber construction, bedding and frame support meet CC 500 requirements to preserve the declared aperture and avoid movement that could compromise clearance.

We can review drawings, provide dimensional verification notes aligned to BS EN 124 and advise on product selection for highways, local roads and developments. If you need assistance auditing existing stock or comparing alternatives, our technical team is ready to help.