The United Kingdom (UK) is a relatively small country when compared to North America. The mainland is only around 600 miles from north to south and 300 miles at its widest point, yet it provides
a home for a population that approaches
65 million people – many of whom reside in the estimated 43,000 towns and cities that scatter the island.
A vast network of utilities has evolved over many decades to support the country’s need for electricity, gas, water, telecoms and other services. Many of these services are buried underground.
This creates many different challenges for utility service providers and contractors who have a need to install or maintain their assets.
How are underground power cables protected in the UK?
It’s estimated that there are approximately 200,000 km (125,000 miles) of high-voltage electricity
cables underground in the UK ranging in voltage from 11kV to 400,000kV. Regardless of the voltage, network operators have a need to protect their cables, both for integrity of supply and for health and safety.
Most distribution and network operators have adopted or followed National Grid’s technical standard for the protection of transmission voltage cables and the ENA’s (Energy Networks Association) technical standard for the protection of distribution voltage cables. These standards were developed in conjunction with research carried out by organizations such as CIGRE International Council on Large Electric Systems) as well as input from other international standards.
Physical protection devices are an important component of protection systems and various types of products are used to provide protection and identification of buried cables. These vary in accordance with the asset owners’ engineering standards and are often related to voltage levels. For example:
• Marker tape may be used for low voltage cables
• Tape tile for medium voltage cables
• Cable tiles for high and extra high voltage cables ‘EH V’
• Cable marker posts (often used in conjunction with previous methods)
Traditionally, buried electricity cables have been protected by reinforced concrete or clay tiles placed above the cables. Encasing cables in concrete is discouraged within the UK as the cables can easily be damaged when access to them is required.
In the late 1990s, a distribution operator recognized the benefits of plastic systems and started to adopt the use of plastic warning tiles for the protection of high voltage cables. During the intervening period, plastic systems have come to dominate the sector and are now standard protection for both transmission and distribution voltage cables. Their benefits of light weight, high visibility, excellent physical protection, and cost effectiveness has meant that all high voltage buried cables in the UK
now utilize these systems.
The cables themselves are also required to be compliant with engineering standards and are manufactured with a protective covering, normally metallic, which can also be used as an earth conductor. Color coding and embossed letters identifying the cable further raise awareness and reduce risk of damage or injury.
Network operators are required to provide and maintain utility plans to identify the various types of cable and their location underground within public areas.
Are cable strikes still a problem?
Cable strikes are a continuing problem for power companies and it is an issue that has challenged the industry for many years. Despite deploying a variety of cable avoidance techniques, cable strikes can and do still happen. Overall in the UK, according to the Utility Strike Avoidance Group (USAG), an estimated 60,000 underground utility strikes occur every year which can cause network outages, serious injury and death. They also result in many millions of dollars in associated damages and compensation.
Britain has a relatively high proportion of its utility services installed underground in roads, footways
and other designated areas. Factors such as urban congestion and the simple lack of space in Britain’s crowded streets further increase the risk of cable strikes. Due to the limited availability of appropriate ground space, it is often impossible to designate clear routes for specific utilities and, therefore, many routes are shared with other utilities, adding further risk to cable damage.
As utilities have been installed over many decades, plans and maps are of variable quality. Ground and surface features may have changed markedly over the life of the utility and often reference points are unclear. A recent (2016) case study by the USAG found that where teams studied plans prior to excavating, only 48% of the assets (uncovered) were shown on the plans. Of these, 84% were found to
be inaccurately recorded. This perhaps illustrates the challenge posed for excavators who have a requirement to dig in UK streets.
It is often perceived that low voltage cables do not need the same standard of protection as high
voltage cables, yet it is clear from USAG data (see graph at top of page) that many of the cable strikes happen with lower voltage cables, which can be just as hazardous as EHV cables.
The cost of installing or even maintaining an underground cable is many times that of taking it aboveground. The true costs of strikes to underground damaged cables are often unclear and probably significantly understated. Any costs recorded often relate to the direct repair time/costs involved and ignore compensation costs, H&S costs and network downtimes. It clearly makes sense to protect valuable assets, which are designed for a long service life, from network interruption and, more importantly, avoid possible health and safety consequences. With vast networks of unprotected buried services lines, it is recognized within the UK utility industry that damage prevention is a shared responsibility and it is unlikely that any single method will provide the total solution to the problem of utility strikes.
Going forward, we need to find a robust way to keep excavation personnel safe and reduce costly damage to critical assets, while at the same time avoiding time-consuming disruption at street level. A multi-faceted approach is increasingly being viewed as the path forward:
• Improved cable records (utility supplied drawings), sharing of data and ease of availability
• Collection of strike data to aid learning and best practice development
• Continual training with excavation crews
• Cable identification tools (CAT and Genny, Ground Penetrating Radar, etc.)
• Trial holing (hand excavation methods using specifically approved tools and methods)
• Physical cable protection (cable tiles and high visibility warning devices)
Richard Attwell is an EHV cable specialist. During his 35 years in the industry, Richard has worked for large contracting companies and cable manufacturers in the high voltage industry. In 2012, Richard joined National Grid and worked as the Cables