GROUPING FACTOR

GROUPING FACTOR

Generally cables laid in air ground, when number of cables laid for a plant then it is laid in a trench which will be Grouped or laid one after one which is called Grouping of Cables. Grouping factor means since the cables are arranged in a row, their current carrying capacity will be reduced which is given below.

Derating factor

       

 4 cable                  Touching                                              0.75

 6 cable                  Touching                                              0.67

 9 cable                  Touching                                              0.54

12 cable                Touching                                               0.51

30 cm spacing between cables

4 cable                  Touching                                              0.79

6 cable                  Touching                                              0.71

9 cable                  Touching                                              0.61

12 cable                Touching                                              0.57

45 cm spacing between cables

4 cable                  Touching                                              0.81

6 cable                  Touching                                              0.74

9 cable                  Touching                                              0.63

12 cable                Touching                                              0.6

Ground Temperature

According to the temperature in ground Cable selection should be as per table given below.

Derating value

                        15 c                        -                              1.14

                        30c                         -                              1.0

                        45c                         -                              0.8

Specific resistance: S

Resistance of Cables = X S L/A

S = Is constant for different types of cables.

L = Length of Conductor or cable

A = Area of cross section of cables

Voltage drop calculation (Cables):

Generally voltage drop calculation is done for selecting the cable size for particular motor and the distance involved. As distance increases voltage drop occurs due to increase in resistance of cables for this reason we should find the voltage drop.

Operating Voltage                     440

Length of Cable                        500mtrs

Motor Hp                                 100

Full load Current                      120

2% Voltage of drop                  8.8V

Resistance of cable                  8.8 V =0.073

                                                                          120A

Area of cross section = S I/R=0.0172X500/0.073 = 117sq.mm.                        Say 120 sq.mm

Cable gland : Cable glands are used to Mechanical damage, to give earth continuity and fo entering in junction box, MCC etc, for PILC Cable lead covered are used because in Refinery, hydrocarbon gases will damage the PVC insulation, for this purpose lead cover is used. For this purpose plumbing type glands are used.

In explosion proof areas double compression gland are used to avoid these gasses entering in the electrical apparatus, drawing / colour codes / ferruling will be done for easy identification with use of lugs. Lugs are to two types. One is crimping type and the other one is soldering type.

Fault Location: If a cable becomes faulty, then identifying the fault is done with cable fault locator kit. By injecting high frequency impulse with audible set the fault can be identified with the CRO by selecting the cursor on the fault so that we can identify the distance.

Another type is by giving power source and using a Wheatstone Bridge.

MURRAY LOOPS TEST: For selecting the cables, manufacturers give the voltage drop/A/Mtr. It should be selected according to the size and the reference and theory point of it.