The UK standard for structural inspection of street lighting columns using Severity-Extent defect grading and the Column Condition Index (CCI).

ILP GN22 fundamentally changed how UK highway authorities manage their lighting column stock by replacing the age-based replacement philosophy of TR22 with a condition-based risk model. Under TR22, columns were typically scheduled for replacement at a fixed age — often 25 or 30 years — regardless of their actual structural state. GN22 recognises that material type, environmental exposure, and maintenance history cause columns of the same age to deteriorate at vastly different rates, making blanket age thresholds both wasteful and unsafe.

The inspection follows a ground-up sequence. The inspector begins at the root and foundation, examining the interface between the column and the ground for corrosion, cracking, or soil erosion. They then move to the base compartment door, checking for missing doors, misfitting panels, corrosion at the aperture, and cracked corners. The shaft is assessed for general corrosion, swage joint deterioration, impact damage, lean, weld failure, and loss of section. The bracket or arm is checked for joint corrosion, loose connections, misalignment, and structural cracking. Finally, the luminaire is inspected for secure fixings and any attachments such as signs, CCTV cameras, flower baskets, banners, or festive lighting are recorded because they alter the wind loading on the column.

For every defect identified at each component, the inspector assigns two independent ratings: a Severity score from 1 to 5 indicating how bad the defect is, and an Extent code from A to E indicating how widespread it is. This dual-axis approach captures the critical distinction between a small but dangerous defect (high severity, low extent) and a cosmetically widespread but structurally insignificant issue (low severity, high extent). The raw field data is then processed by asset management software to calculate the Column Condition Index (CCI), which places each column into an action band — from "no action required" through "monitor" and "programmed repair" to "emergency make safe."

The standard is maintained by the Institution of Lighting Professionals (ILP) as the successor to the earlier Technical Report 22 (TR22).

The Severity scale is a five-point ordinal measure of how critical the defect is to structural integrity and public safety. A score of 1 (None/Negligible) means no significant defects are found and the component is functioning as intended. Score 2 (Minor) indicates visible defects such as surface corrosion or hairline cracks that do not significantly affect structural strength — the column should be monitored at subsequent inspection cycles. Score 3 (Moderate) means defects are clearly visible — pitting corrosion, concrete spalling, or minor denting — and structural performance may be starting to degrade, warranting maintenance or non-destructive testing. Score 4 (Severe) applies to major defects such as significant loss of section, deep cracks, or a loose base compartment door, where structural integrity is compromised and corrective action is required. Score 5 (Hazardous/Critical) signals that the component has failed or failure is imminent, creating an immediate risk to public safety — for example, a column leaning dangerously or exposed live wiring — demanding emergency action within 24 hours.

The Extent scale uses five alphabetic codes to quantify what proportion of the component surface area is affected by the defect. Code A (Isolated) means less than 5% is affected — a single localised instance. Code B (Minor) covers 5% to 20%. Code C (Moderate) covers 20% to 50%. Code D (Significant) covers more than 50%. Code E (Widespread) indicates that the defect affects the entire component or constitutes total failure. The interaction between severity and extent is crucial: a Severity 4, Extent A defect (a severe but localised crack at the root) may warrant immediate NDT testing, while a Severity 2, Extent D defect (widespread surface corrosion) may only require programmed repainting.

This dual-axis model mirrors the approach used in other UK infrastructure standards. The CS 450 bridge inspection standard under the Design Manual for Roads and Bridges (DMRB) uses the same Severity 1-5 and Extent A-E framework for highway structures. GN22 adapted this proven methodology specifically for lighting columns, with defect types and component categories tailored to the unique failure modes of slender steel, aluminium, concrete, and cast iron columns.

A comparable multi-dimensional defect rating approach is used in the CS 450 bridge inspection, which uses the same Severity 1-5 and Extent A-E framework for highway structures. The NEN 2767 condition assessment extends this approach with a third dimension (intensity).

The component-level approach is what distinguishes GN22 from simpler inspection regimes. Rather than assigning a single condition grade to an entire column, the inspector must evaluate five discrete zones — each with its own defect types, severity rating, and extent code. This granularity is essential because column failures follow material-specific and location-specific patterns. A steel tubular column may have a perfectly sound shaft but severe pitting corrosion at the root where it meets the ground. A concrete column may show no corrosion at all but exhibit dangerous spalling that exposes reinforcement bars. By capturing condition data per component, the CCI algorithm can weight the most structurally critical zones appropriately.

The Root and Base zone covers the interface between the column and the ground — the most common failure point for steel columns. Inspectors look for surface corrosion, pitting and scaling corrosion, concrete spalling, impact damage from mowing equipment or vehicles, cracking, and soil erosion or ground movement that may undermine the foundation. The Door and Aperture zone assesses the base compartment where electrical connections terminate. Common defects include missing doors, misfitting or loose panels, broken locks, corrosion around the door opening, cracks at aperture corners (stress concentrations from the cut-out), and missing backboards that should insulate conductors from the column wall.

The Shaft zone covers the main vertical structural member — the primary load-bearing element. Defect types include general surface corrosion, corrosion at swage joints (a critical failure point where stepped columns change diameter and water pools in the joint), impact damage or denting from vehicles, lean or loss of verticality, seam weld failure along the longitudinal weld, and loss of section where corrosion has consumed wall thickness. The Bracket zone inspects the projection holding the luminaire, checking for joint corrosion, loose connections, misalignment, and structural cracking. The Luminaire zone verifies that the lantern is securely fixed, with checks for loose fixings, missing bowls or visors, and damaged canopies. Any attachments on the column — signs, flower baskets, CCTV cameras, banners, festive lighting — are also recorded here because they add wind loading that the original column design may not have accounted for.

For more information on highway infrastructure inspection methodologies, browse the standards directory. The Highway Electrical Association (HEA) provides complementary guidance on practical implementation.

The Column Condition Index (CCI) is the primary output of a GN22 inspection. It is not calculated by the inspector in the field — the inspector collects the raw severity and extent scores for each component, and the CCI is computed by the back-office asset management system using a weighted algorithm. The weighting reflects the structural criticality of each zone: a severe root defect scores higher than an equivalent defect at the bracket because root failure leads to total column collapse, while a bracket failure typically results only in a dropped luminaire.

The CCI places each column into one of several action bands. A "Very Good" rating indicates no action is required. "Good" means the column should be monitored and retested in 3 to 5 years. "Fair" triggers programmed repair planning. "Poor" demands structural testing using non-destructive testing (NDT) methods such as Relative Loss of Section (RLS) ultrasonic measurement. "Very Poor / Unsafe" requires immediate make-safe action — which may mean cutting the column down within 24 hours if it poses an imminent risk to highway users.

Material type is a critical input to the CCI calculation because it determines the failure modes that apply. Steel tubular and steel folded (octagonal) columns are primarily susceptible to corrosion, particularly at the root, swage joints, and door apertures. Aluminium extruded columns resist corrosion but may suffer fatigue cracking at welded joints. Cast aluminium bases are prone to impact damage. Concrete columns develop spalling and reinforcement corrosion. Cast iron columns fracture rather than deform. Timber columns rot from the inside out. Composite and GRP columns are relatively new and have different degradation profiles. In the digital form, the inspector selects the column material from eight options — Steel (Tubular), Steel (Folded/Octagonal), Aluminium (Extruded), Aluminium (Cast Base), Concrete, Cast Iron, Timber, and Composite/GRP — and this selection contextualises the defect grading downstream.