Underground utility colour codes are used to differentiate and identify underground utilities to protect it from damage during excavation. There are different types of utilities and in order to tell them apart coloured lines, flags or sometimes both are used. They help mark the location and indicate the type of utility that is buried underground.
Underground Utility Colour Codes - Gwydion M. Williams

Utility systems are buried underground due to the nature of their function, convenience and aesthetics. Excavators are required to check and locate these lines before any digging takes place. Before any excavation work begins, they are liable to contact the concerned companies and organisations and notify them about the proposed excavation activity so that the existing underground utilities can be determined and marked.

Why are Underground Utility Colour Codes Important?

The Code of Practice introduced by the Health and Safety Authority states that..

‘The aim of the Code of Practice is to improve the level of safety with which excavation work is carried out. In particular, it aims to reduce the incidence of damage to underground services and in doing so to minimise risk to personnel who are involved in this work. The COP is intended to provide practical guidance to utility/service providers, clients, designers, planners, project supervisors (both design process and construction stage), contractors, safety representatives and any personnel who are involved in work where there is a risk from underground services.’

Different detection and location methods are used due to the fact that utilities are made of different types of materials. For metal pipes and similar cables, electromagnetic equipment with a transmitter and receiver is used. For plastics and concrete, radiolocation and ground penetrating radars are used. Utilities such as petroleum products are permanently marked using posts and bollards due to the nature of their content. There are maps to locate these utilities however, in order to have a precise and accurate location certain equipment’s are required.

A specific type of spray paint, which is often in a fluorescent colour, is used to mark lines. For flags, a type of logo representing the company or municipal utility is used to identify the ownership of the lines. Flags are also used as a sort of advertisement by companies installing lines for irrigation purposes for lawns and gardens in which case the head of sprinklers are usually marked so that landscapers will leave the head open to avoid damage from tractors, construction equipment’s used for digging holes and fencepost.

The United Kingdom uses a colour code similar to the US for marking underground utilities like water, gas, electric, telephone and cables. The system is based on convention with no written standard. Contractors by painting onto the pavement following the colour code mark areas with underground utilities.

Underground Utility Colour Codes - Diagram
The national colour coding system for underground utilities are;

Red – Electric power lines, conduit and cables.

Orange – Telecommunication, alarm or signal lines.

Yellow – Gas, oil, steam, petroleum, or other flammable material.

Green – Sewage and drain lines.

Blue – Drinking water.

Purple – Reclaimed water, irrigation, and slurry lines.

Pink – Temporary survey markings, unknown/unidentified facilities.

White – Proposed excavation limits or route.

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