Cables and materials in the supply network

A cable looks simple from the outside but it is the result of many technical choices. Which material should conduct the current? How should it be insulated so it is both safe and lasts for decades? And should it be buried above or below ground at all? Material knowledge is also a fixed part of the curriculum already in the second half of the basic course.

§The conductors: copper and aluminum

The two most commonly used conductor materials in electrical cables are copper and aluminium. Copper conducts electricity better and can therefore handle the same load with a smaller cross-section but it is heavier and more expensive. Aluminium is lighter and often cheaper but conducts electricity worse and must therefore have a larger cross-section to safely carry the same amount of current.

• 01Copper: better conductivity, smaller cross-section, heavier and more expensive
• 02Aluminum: lighter and often cheaper, but requires larger cross-section
• 03The choice depends on span, load and economy in the specific installation

§Insulation: from oil and paper to modern plastic

Older cables in the electrical network were typically insulated with oil and paper — a technology that worked but required maintenance and was sensitive to damage. Today the vast majority of new cables are insulated with PEX, also called XLPE (cross-linked polyethylene), which is more robust, easier to work with and has a long expected lifespan.

§Air line or buried cable?

Through the so-called cable action plans, the state and Energinet have since the late 2000s worked systematically to lay large parts of the Danish power grid underground instead of on air lines and masts. This applies to both parts of the transmission network and the vast majority of the medium voltage network. The purpose is partly a more beautiful landscape without masts, partly fewer operational disruptions because buried cables are far less exposed to storms, ice and lightning than air lines.

§Cable marking and depth in the ground

A buried cable is not just laid at a random depth and forgotten. Cables are buried at a specified depth that protects them from ordinary excavation work with spade and small machines and they are often marked with a warning tape some distance above the cable itself so a digging person or machine gets clear warning before the shovel reaches the cable. Combined with checking the Utility Register (LER) before excavation begins it is part of significantly reducing the number of accidental cable damage.

Experience from real storm events underscores why this development has taken place. When storm Bodil hit Denmark in December 2013, flooding in several places caused grid operators to disconnect power as a safety measure, including around Isefjorden where rising water levels posed a risk of short-circuiting in transformer stations. Such events are part of the background for why more and more lines are being laid underground and secured better against water and weather.

§Material choice follows you through the entire education

Material knowledge is introduced in the basic course's second part but it is far from just theory you need to remember for a test. When you later in the main course work with actual cable work at 10-20 kV and construction of transformer stations it is the same knowledge of conductors insulation and cable types that underlies why a particular cable is chosen for a particular task — and why you must treat it in a particular way when it must be spliced stripped or terminated correctly.

“The cable you never see because it lies a meter under the sidewalk is often what keeps the power running when the storm rages over the land.”