Low Voltage Cables

Low Voltage Cable 

1. Introduction:

The function of cables is to transfer energy in a proper way from the source to the use devices, and during the process of transferring energy, some phenomena occur such as the high temperature of the cable as a result of the defects, as well as the drop in voltage, the formation of electromagnetic fields around the cable, in addition to the mutual induction between the cable circuit and the adjacent circuits.

 2- Ingredients:

Cables are made with either single core  , two cores, or three cores , and perhaps more, and it can be said in general that the use of three-core cables leads to lower costs and reduce voltage drops, while single-core cables are more flexible and easier to install and connect, so the use of single-core cables is better inside commercial buildings due to the high exposure of the cable from bends as well as the many branches and connections on the cable.

1.           Connector

2.           Separation 

3.           Blanket 

4.           and external protection.

 The triplex cable consists of:

1.                  Connector

2.                  Separation

3.                  Filler Material

4.                  Belt and curtain    screen

5.                  Cover and Exterior Protection.

2-1 Heart (Conductor):

The core of the cable is made of a material with high electrical conductivity, and copper or aluminum is used in the manufacture of the conductor for the cable, and it is usually preferred to use copper conductors because of their better electrical, mechanical and chemical properties, while aluminum conductors are also widely used due to their cheap price and light weight for copper conductors, for the same value of current. In residential buildings, commercial and administrative facilities, copper conductors are used up to a maximum of (16) mm2 and stranded conductors are used for sectors above the In order to obtain flexibility, the International Electrotechnical Commission (IEC) has determined the  scientific measure of  resistance of annealed fermented copper  on the  basis that 1.724 μohm cm at 20 m is equivalent to 100% resistance, and aluminum conductors need 160% of the copper conductor sector to obtain the same electrical conductivity, and when using aluminum conductors, precautions must be taken from the surrounding environmental factors.

 2.2. Insulator:

Polymeric materials are now used in the industries of all cables used in commercial buildings with different efforts, and polymeric materials are materials extracted from the petrochemical industries, and there are two main types of these materials used in the manufacture of cable insulators:

1.                             Thermoplastics:

These are polymer types that soften with heat and harden with cold. The most important types of polymers are polyvinyl and PVC chloride,  and they are characterized by excellent electrical properties up to 3 kV: it is not suitable for higher voltages than that, except by using special types of them, and as we mentioned, it hardens with cold and softens with heat, and it is better not to expose it to temperatures below zero or more than 70 degrees Celsius continuously, and it is also characterized by the self-extinguishing property of flame, as it burns when it touches the flame directly, but it extinguishes as soon as the source of the flame is removed. Its combustion produces toxic gases and the temperature of the conductor must not exceed 160 degrees Celsius during periods of short circuit, otherwise the insulator will be damaged, which resists ozone well and is damaged by exposure to chlorine.

2.                              Thermosettings  :

These are materials that do not soften with heat until the temperature of their combustion or decomposition, and the most important types of them – interlocking polyethylene (XLPE), which is characterized by excellent electrical, physical and chemical properties and can be used at a continuous temperature of the conductor up to 90 degrees Celsius and at a temperature of 250 degrees Celsius in periods of short circuit, and its resistance is considered excellent to moisture and ozone gas, which escalates as a result of the corona phenomenon.Corona arises from an increase in the strength of the electric field of the insulator but it is not resistant to chlorine, which is a very hard non-flammable material that is usually used in cables with voltages higher than 3 kV, as its use in voltages below that is unjustified due to its high price.

Table (1) shows the most important polymeric materials used in the manufacture of cable insulators and their electrical and physical properties in general.

 

Table (1): Polymeric Insulation Materials Used in Cables

Common Name	Electrical Properties	Physical Properties
Thermosolids: Polyethylene Chaiki XLPE Drying Ethylene Propylene Rubber HEPR SR Silicone Rubber Neoprene	Excellent Excellent Good medium	Excellent Excellent Good Good
Thermoplastics: Polyvinyl Chloride (PVC) Polyethylene PE Nylon	Good Excellent medium	Good Good Excellent

 2-3- Metal Shell:

Local and international standards require the cable to be equipped with a Shielding (Metallic Sheath)  if the cable voltage exceeds a certain limit (1 kV according to IEC-502 and 2 kV according to NEC – 1993) and the metal sheath is made of either a conductive material (lead - lead alloy - aluminum alloy) or a semiconductor material or both, and the metal sheath is made in different forms, including: 

1.    A strip of copper or steel wounds on the insulator in a spiral way

2.    A copper zigzag strip is applied to the insulator in a longitudinal manner and fixed by longitudinal welding.

3.    Copper wires wound on the insulator in a spiral manner

4.    A solid lead shell that is adhered to the length of the insulator by extruded lead sheath.

Many types of cables have an inner metal sheath that is placed on or close to the conductor and an outer metal sheath, meaning that the insulator is surrounded by two sheaths inside and out.

The metal shell achieves many advantages, including:

1.    Limiting the electric field inside the cable

2.    Providing a path for the Ground Palace Current

3.    Reducing the electrical stresses on the insulator, especially the tangential areas of the insulating surface that caused its damage.

4.    Reduce noise on communication devices

5.    Reduce the risk of electric shock in case of good cable grounding

6.    Mechanical, chemical and natural protection of the insulation material

 

2-4- External Protection:

External protection or external sheath is used to protect the layers of the cable underneath it from environmental conditions and installation, and the choice of external protection material depends on the same factors as the selection of insulator, i.e. on electrical, mechanical, physical and chemical properties, and the cable can be provided with metal or non-metallic protection or both:

Non-metallic protection:

It is either in the form  of an extruded jacket on the metal shell of XLPE, PVC or other material, or in the  form of fiber braids  that are wrapped around the metal shell and are made of glass fibers, asbestos or others. All fibers need a saturated or submerged material to achieve some resistance against moisture, solvents, corrosion and weathering factors, and there are special types of materials used in external protection that have anti-temperature properties. and slow down the flame and resist the solvent oils.

Metal Protection:

The need for metal outer protection appears if the cable is exposed to high mechanical stresses, harsh chemicals, or high thermal stresses during the short circuit. Metal outer protection is available in different shapes and materials, including galvanized steel, aluminum, bronze, lead, and copper, and the outer protection layer is made in the form  of armouring reinforcement  in one of the following configurations:

1.                   Galvanized Steel Interlock Reinforced

2.                  Long welded zigzag metal casing on cable stretch

3.                  Lead, copper or aluminum metal feeder

4.                  Steel wires are screwed along the extension of the cable, and the technical saws of the cables should be consulted to identify the electrical and mechanical properties of all types of external protection.

3. Cable selection

The correct choice of cable depends on several factors, including:

3-1- Characteristics of pregnancy:

This is in terms of operating voltage, continuous load current, load cycle, duration of overload in emergency periods, allowable time period for overload, as well as limits of voltage change.

3-2- Voltage Regulator:

Two values should be specified for the cable voltage:

1. Operating voltage U is the rated voltage between the conductor and the ground during normal operation

2. The UO insulation voltage,  which is the maximum line-to-line voltage that  the cable can withstand when using cables in a grounded system, can be (U = √3 UOs), while in the case of ungrounded networks, the U value should increase  to 133% or 173% of the √  3U0 value, due to the high cable voltage during periods of ground shortage.

3-3- Connector section area:

The area of the connector section is selected according to several factors as follows:

1. Current Load Capacity

2. Overload in emergency periods

3. Allowable voltage drop limits

4. Characteristics and Effects of Palace Currents

5. Installation conditions in terms of installation method, connector temperature, adjacent cables, etc.

6.                     Cable End Connection Requirements

The area of the conductor section is first determined according to the current load capacity through the cable manufacturer's tables, then this section area must be corrected according to the installation conditions and its difference from the standard conditions corresponding to the tables, the derating factors should be used as known in the technical bulletins  of the cables so that the correct area of the conductor section can be determined.

When selecting a cable, it is necessary to know the amount of voltage drop between two ends when the rated current passes through in order to determine the amount of voltage regulation, and it is customary to give the drop caused by the passage of current in the cable on the basis of each conductor separately, and it is usually calculated in millivolts per amperage per meter of the length of the cable, and it can be calculated as follows:

For single-phase circuit:                     mv = 2 Z

For three-phase circuit:                              mV = √3 Z                  

Where (mV ) is the voltage drop in millivolts per amperage per meter of the cable length and (Z) the impedance per conductor per km of the cable length in ohms at the maximum operating temperature, and the value of (Z) for the two circuit conductors in the single-sided circuit and for the single phase in the three-phase circuit is calculated, and to find the percentage of voltage drop, the value of the voltage drop is divided by the phase voltage in the single-phase circuit and by the line voltage in the three-phase circuit.

Sometimes it happens that the determining factor for the selection of the conductor section area is the ability of the cable to carry short currents and not its ability to carry current under normal load conditions. Short currents that amount up to more than twenty times the rated load current create mechanical and thermal stresses according to which the maximum amount of time that the cable can withstand is determined in the presence of short current, and the cable insulator is the most affected part of these stresses, and the maximum allowable period of time for the short current changes inversely with the square of the short current, and the cable factories give the method of this change in the form of maps as shown in Figures (1) and (2).

 

Figure (1): Short Current Rationers for Insulation Cable XLPE	Figure (2): Short Current Rationers for Insulation Cable PVC

 

3-4 Installation Method:

The cable can be buried directly in the ground or inside a sewer, and it can also be placed on shelves or inside air pipes, and in all these cases, it is necessary to know the space allocated for the passage of the cables and the extent of their proximity to each other, especially if these cables work on different voltages.

3-5- Characteristics of Short-circuit cases and protection system:

The values of the short currents, the protection system on the cable, and the maximum short circuit period to which the cable can be exposed should be determined according to the characteristics and method of adjusting the protection devices.

3-6- The environment in which the cable passes:

The cable path may pass through areas with high temperatures, which requires special types of insulation, and the cable may pass through dangerous areas, exposed to fires or explosions, or contain incendiary chemicals, or exposed to high mechanical stresses, and in such cases, the appropriate cable must be chosen in terms of insulation materials, external protection, or mechanical reinforcement, and it may sometimes be necessary to choose a cable with high flexibility due to its path being exposed to repeated sharp bends.

 

4-  Specifications of Cables:

Once the appropriate selection of the cable is completed, writing its specifications becomes a routine work that includes:

1.                  Number of Connectors or Cores

2.                  Conductor material and sectional area

3.                  Insulation Type

4.                  Rated voltage (phase operating voltage and insulation voltage).

5.                  Method of protection by metal shell

6.                  External Protection

7.                  Cable Ability to Withstand Short Currents

8.                  Any other equipment required

 

5- Installation reserves:

The process of installing the cable needs special precautions to avoid damage to it, summarized as follows:

1.    Cables with PVC material in their components should not be installed  in very cold weather, where the insulation or outer protection layer is very brittle and easy to be exposed to cracks.

2.    One of the main disadvantages of aluminum connectors is the formation of a thin solid layer of oxide on the surface of the conductor, and although this layer provides protection against corrosion of the conductor, it causes many problems, especially during the processes of welding, connecting and fixing the ends of the cable, so the technical bulletins for the installation of aluminum cables must be followed accurately, and these bulletins can be obtained from cable factories.

3.    Lack of sharp corners as they may cause damage to the cable while it is being pulled inside the duct.

4.    Do not expose the cable to more tensile forces than allowed during the process of pulling it

5.    Tighten the ends of the cable to prevent water or moisture from entering it and reaching its core, and this point is even more important if the cable is placed in an environment exposed to water or moisture.

6.    The bending radius of the cable should not be less than the permissible limit

 

6. Test:

Although most of the tests for cables are done in the factory, some tests must be performed upon receipt and after installation, and the most important of these tests are:

1. Dimensions: The diameter of the conductor, the thickness of the insulation, the sheath and the rest of the components of the cable are measured with great care upon receipt and a special micrometer is used in this regard. It must be ensured that it conforms to the specifications given by the manufacturer, and special attention must be paid to the thickness of the insulation and conform to the standard specifications

2. Insulator Resistance and Capacity: The insulator's resistance and capacity are measured using ordinary measuring devices and methods, and this test can be easily performed to measure the resistance between each conductor and the casing, between each conductor and the ground, and between each conductors, and this measurement can be done after installation and then after operation at periodic intervals.

3. High Voltage Test: This test is done by highlighting an electrical voltage on the cable and then raising this voltage up to four times the rated insulation voltage for a period of 15 minutes, either in one phase or several stages, and this test can be performed using a constant voltage or an alternating voltage, and it is preferable to use a constant voltage, especially after the cable installation process.

Low Voltage Cable Standards in Saudi Arabia

In Saudi Arabia, the design, selection, and installation of low voltage cables are primarily governed by:

• Saudi Building Code (SBC 401 - Electrical Requirements): This code adopts and references international standards (like IEC, IEEE, BS, NFPA 70/NEC) but adapts them to the local context and climate.

• Saudi Electricity Company (SEC) Standards: As the primary utility provider, SEC has its own set of technical specifications and approval requirements for cables and other electrical equipment connected to its network.

• International Electrotechnical Commission (IEC) Standards: Many cables manufactured and used in Saudi Arabia comply with IEC standards, particularly IEC 60502-1 for power cables with extruded insulation for rated voltages from 1 kV to 30 kV.

• Saudi Standards, Metrology and Quality Organization (SASO): SASO issues national standards that products must meet to be sold in the Kingdom.

When selecting LV cables in Saudi Arabia, factors such as voltage rating, current carrying capacity (ampacity), conductor material, insulation type, environmental conditions (e.g., temperature, moisture, chemical exposure), mechanical protection requirements (armored/unarmored), and fire performance (e.g., LSHF) must be carefully considered to ensure compliance with local codes and safe, reliable operation.