19 | Underground Cable

An underground cable is a cable that is buried below the ground. They distribute electrical power or telecommunications. There are many advantages with an armoured cable including superior crush resistance, better cold impact and cold bend performance, and a broader range of acceptable installation practices. Range of cable voltage are 1kV, 3kV, 11kV, 33kV up to max 500kV.

Underground Cable Construction

Conductors are made from electrolytic grade pure copper or aluminium.

Metallic Sheath: It provides protection to the cable from moisture and other chemicals (acids or alkalies) present in the environment or soil. It’s usually made up of Aluminium or Lead. It also provides a path for fault and leakage currents as the sheath is earthed at one cable end.
Bedding: It is a low grade insulator like Jute or Hessian which protects the metallic sheath from corrosion and from mechanical injury due to armouring.
Armouring: It provides mechanical protection from various stresses the cable may get exposed to during its installation and operational life. It’s usually a steel tape wound around the Bedding layer.
Paper Insulation: It provide absorption of vapor or moisture from underground. Most common type of paper is known as PILC (Paper Insulated Lead Covering)

For reference only

Classification 3 Phase Underground Cables

1. Belted cables: As the name suggests, it has an additional layer of oil-impregnated paper which is wound around the insulated conductors. Such an arrangement is useful for low and medium voltage levels up to 11 kV.

2. Screened cables: Used only in particular applications with specialised construction, these Underground cables can be further divided as H-type and S.L-type cables.

3. Pressure cables: These are used when the voltage requirement exceeds 66kV and solid cables can't be used. Either pressurized gas or pressurized oil is used in these cables.

Protection on underground cable

Non-armored cable

The meaning of non-armored is without mechanical protection from impact. This cable is best for signal or control cables applications.

Armour cable

This cables have a strong protection against mechanical impact. This cable in outer layer can be either galvanized wire or steel wire. The standard power cable are mostly identified as AWA (Aluminium Wire Armor). SWA refers to (Steel Wired Armor) and this underground cable is best for power cable applications. Armoured cables and cables having an earthed metal sheath suitable for use as a protective conductor may be buried directly in the ground without further protection, except against corrosion, which may be negated by the provision of a plastic covering. The armor provides additional protection where mechanical stress has the potential to cause damage to the cable, such as direct burial, outdoors or underground. The armor also enables the cable to withstand higher pulling loads. It should be noted, however, that the armour provides no protection for climatic conditions.

Types of underground cable

1. Cross link polythene or (XLPE)

This type of cable can width stand high voltage up to maximum of 500kV. The insulation material is a thermal setting compound which design to handle high temperature during operation. Commonly this cable is used on switch gear to power transformer.

These cable manufactured various sized to meet design requirement. The minimum size are 100mm² up to 1000mm² .

Temperature range for

Picture on the left showing the XLPE cable.

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2. Multi core armor

Single core cables are very high ambient wires, these cables are a good conductor of electricity as well as heat. These cannot melt or burn easily.

Multi core cables are cables, which contains more than one core in the cable. These cables can be used for analog and digital signals as well as power distribution.

This cable are known for signal cable used in the industries.

Example of multi-core applications.

traffic lights
distribute control on branch of circuit in factory machinery

The picture on the above showing a multi-core amour cable commonly used for signal circuit applications.

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3. PVC Armour Cable

Some underground cable are manufactured together with two(2) or more conduction.

Many factories using this type of underground cable for example :

perimeter lighting for night (security).
supply to other machineries
supply lines along side road.

Example on right showing a three core PVC amour cable commonly used for signal and power circuit applications.

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Mechanical Protection

Steel wire armoured (SWA)

Commonly abbreviated as SWA, these cable is a power and auxiliary control cable, designed for use in mains supply electricity.

The SWA does not need to be earthed as it is not an exposed conductive part.

These SWA cable needs to be buried at a sufficient depth to avoid damage due to disturbance of the ground (Regulation 522.8. 10).

While BS 7671 does not state a minimum depth, the generally recognised depth of a buried cable to prevent disturbance is 600 mm.

Refer picture above, various size of armor cable.

For reference only

Aluminium Wired Armoured (AWA)

Aluminium Wire Armour (AWA) is used in single core cable because it is non-magnetic. When an electric current passes through a cable it produces a magnetic field (the higher the voltage the bigger the field).

This is a hard-wearing power cable specifically designed for the supply of the mains cable, the armour provides mechanical protection allowing the cable to withstand higher.

Typically, the construction of an aluminium or steel wire armored cable comprises six components:

Conductor: plain stranded copper or aluminium
Insulation: materials such as cross-linked polyethylene (XLPE) provide high temperature resistance and excellent dielectric strength-providing enhanced electrical properties.
Bedding: a layer to create a protective barrier between the insulation and the armour.
Armour: a steel or aluminium armour provides mechanical protection to allow the cable to withstand the mechanical stresses to which it is exposed.
Sheath: the constituent parts of the cable are held together by a sheath offering a further level of protection. Black sheaths can be carbon-loaded for UV stability.
Voltage: voltage ratings of 600/1000V, 6.35/11kV and 19/33kV.

Underground Cable Installation

Depth of Underground Cable: Sarawak

LV Cable (415v) : 3 feet (D) x 2 feet (W) or 900mm x 600mm

11 kV Cable : 4 feet (D) x 2 feet (W) or 1200mm x 600mm

33 kV Cable : 5 feet (D) x 2 feet (W) or 1500mm x 600mm

Construction Requirement

Refer to project requirement about underground cable installation specification to be install on site.

For installation of underground cable on roads, do get the information from JKR standards and requirement.

Method 1 | Under road

Refer to figure on right, this underground cable installation is suitable for outdoor. These are some of common cable installation on public road.

The brick or concrete slab acts as warning to any excavating works to remind the danger of power cable.

Sand helps to absorb moisture to prevent any underground water near these power cable.

For Reference only

Figure above is another underground cable installation.

For Reference only

Method 2 | Off road installation

Another example of underground cable installation is using steel or HPVC pipes. Refer to figure on right some underground cable is insert to a piping underneath the ground. This is an example of road side installation or any cable laying site. The piping helps protect the cable from any kind of damage that can happen on site.

For reference only

Method 3 | Premise building

Cable trenching is a method of laying cables into the ground by digging trenches. Cable trenching, also known as underground cabling, became a popular method of installing cables for building/factory electrical system connection between power transformer and switchboard.

These design can be identified commonly at electrical switch board room. These cable comes from power transformer of the building/factory premise.

For reference only

How to measure and check underground cable ?

Click to visit topic no. 37.

Sarawak Electricity Rules and Regulation Rule.22

Rule 22 : Underground mains and connections

(1) An underground supply line shall be continuously insulated and protected to the satisfaction of the Chief Electrical Inspector, and where it is attached to a bridge or culvert it shall be protected in such a manner as may be directed by the Chief Electrical Inspector.

(2) Subject to the State Roads Ordinance, 1994 [Cap. 9], and any other written law, the breaking up of a street, road or public place for the purpose of laying an underground main and the connections thereto and the refilling and making good thereof shall be carried out by the owner, management, licensee, or his servant or agent, as the case may be, in the manner as may be required by the relevant authority charged with the responsibility for matters relating to streets,roads or public places.

(3) Except for the purpose of making immediate repairs, no underground main shall be laid by the owner, management, licensee, or his servant or agent, as the case may be, in any road unless reasonable notice has been given to the relevant authority.