April 23, 2021

CABLES

1. General:

Today’s industries, such as Petro-chemicals, fertilizers, power plants, etc., require a reliable source of power supply. Any discontinuity in the power source of even smallest duration shall result into considerable disturbances in the product quality or production loss or a long shutdown due to shocks exerted to the process equipment.

To maintain a high reliability of source power, electrical system needs attention at various stages like project design, purchase of equipment, erection, installation & commissioning and subsequently predictive & preventive maintenance

In electrical system cable network play a vital role for transmitting electrical power from generating station to consumer ends at various voltage levels. It works as main nerves system of the electrical configuration. Hence if cables are properly selected, installed, tested & maintained, they will be the most faithful & trouble free means to supply uninterrupted power to the various system & plants

2. Terms regarding Cables:

2.1. Cable:

A single stranded conductor with insulation and protective covering or two or more such conductors laid up together.

2.2. Core:

A single conductor with its insulation but not including protective covering. A cable may be

– Single core or single conductor type, or

– Double core or two conductor type, or

– Multicore or multi conductor type.

2.3. Cable insulation shielding:

Metallic tape or braid or wire over the insulation of conductor, also known as screening.

2.4. Screened cable or shielded cable:

A cable in which each conductor is separately enclosed in a conducting film in order to ensure radial electrical field surrounding the conductor. These conductors have electrical connection with each other and with the metallic sheath of the cable which is usually earthed. Shielded cable may be solid, oil filled or gas filled. “Shielded cable can have lead sheath or plastic sheath in addition to the shield”. Sheath should be of non-magnetic material.

2.5. Armour:

Metallic wrapping (usually tape or wires) over the insulation and sheath for the purpose of mechanical protection. In single core cable, armour should be of non-magnetic material

In three core cable, common armour is provided for all the three cores and the material of armour may be galvanized steel.

2.6. Belt:

Insulating wrapping over the insulation of all the three cores.

2.7. Belted Cable:

A three or more cored cable in which part of the insulation is on each conductor individually and remainder is in the for if overall belt.

2.8. Filler:

An insulating material such as treated hemp or rubber or a thermo plastic material etc. which is used for filling up the space inside the sheath / shield.

2.9. Jacket / Sheath / Covering:

It is provided over the insulation for mechanical protection to avoid moisture entry.

Metallic sheath: Lead / Al

Non-metallic sheath: Rubber / PVC.

2.10. Insulation:

Part of the core which provides insulation over individual conductor or over a ground of insulated conductors.

3. Types of Cables :

Broadly the cables are categorised and dealt in two main groups.

— Power cable

— Control cable

4. Power Cables:

Power cables are specified as follows:

4.1.Voltage Levels :

4.1.1. Low tension cables:

Usually used up to voltage below 1100 Volts i.e. 1.1 kV grade. PVC, Paper insulated & XLPE cables are mostly used. Recently FRLS Fire Retardant Low Smoke cables and FS – Fire Survival cables are used in hazardous plants.

4.1.2. HT Cables:

Up to 11kV level – These are usually of PILC, PVC & XLPE type cables. The three core belted type PILC cable has the core wormed together, each circular / sector shaped cores being insulated. The three cores are being further insulated with impregnated paper & finally a lead sheath covers the whole to prevent the moisture from entering the cable. The cable may then be provided with armoured steel wire with overall PVC / Jute served.

4.1.2.1. HT Cables up to 22/33 kV:

The “H” type cable has each cores suitably insulated and insulation is surrounded by a metallic conducting layer which is maintained at earth potential. The arrangement is equivalent to a 3 core single cables.

4.1.2.2.Extra High Tension (EHT) Cable: –

These types of cables are used beyond 66 kV either by using thin oil under pressure / using gas pressure / XLPE.

4.2. Conductor Material :

  • Aluminium 
  • Copper

4.3. Insulation Material

  • Paper Cables
  • Rubber Cables
  • varnished Cambric Cables
  • Asbestos Cables
  • PVC Cables
  •  XLPE Cables
  • PE Cables
  • EPR Cables
  • FS Cables
  • Glass Cables
  • Silicon Cables
  • Mineral Insulated Cables

4.4. No. of Cores:

  • Single Core
  • Three Cores
  • Multi Core

4.5. Mechanical Construction

A) Armoured Cables

  • Galvanised Steel Wire
  • Galvanised or plain steel tape. 
  • Galvanised or plain steel strip. 
  • Aluminium strip.

B) Unarmoured Cables

5. Selection of the Cables :

Before selecting the proper size and type of the cable for the required application, following factors are to be considered.

1. Load i.e. continuous current rating

2. System Voltage.

3. Insulation

4. Short Circuit Rating of conductor / sheath / screen / armour.

5. Short time (over load) rating

6. Installation and environmental condition

7. Economics 8. Safety

9. Protective covering

Load :-

Precise information regarding actual load conditions, occasional over loads & future expanding capacity should be considered from current carrying capacity details of the cable charts. Voltage drop are also carried out for longer length cable to determine the size of the cable to be used.

System Voltage :-

Selection of cable of appropriate voltage class depends upon the type of system i.e. a.c. or d.c. supply, single phase or three phase supply and system earthing arrangement, whether neutral is earthed or unearthed.

Insulation: –

Type of insulation at various levels plays an important role. Various types of insulations are Rubber, PVC, XLPE, Polyethylene (PE), and Ethylene Propylene Rubber (EPR). Since last decades the use of XLPE cables from PILC cables has increased tremendously due to higher current capacity temperature withstand capacity, ease in cable jointing and end terminations with low cost of cables.

Economics: –

Replacement of existing cable is thought only when it is not fit for introduction of any joints and when cable operates on its full rated capacity. Being high cost of the cable proper planning is carried out for replacement schedule.

Cable failure generates two types of failure cost.

1. Cost of restoration of service.

2. Cost of production loss / revenue during failure and resumption of power. Considering above cost when cable failures become preserve, a decision is made to replace the cable.

Short Circuit Rating: –

The approximate size of the conductor of a cable solely depends upon the load current it has to carry. During a short circuit there is a sudden rise inrush current until the protection operates. The increase in temperature caused by the short circuit current is a main factor in deciding the size of conductor. Short circuit rating of cable is expressed as

Where 

 Ish – S.C. rating in kA.

A – Conductor Area in Sq mm.

K – Constant depending upon conductor insulation material

t – Duration of S.C. in second

Installation: – Cables are installed:

Directly under ground In readymade underground R.C.C. Cable trench / duct. Overhead racks in air.

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