L. T. CONTROL CABLES
Cables shall only be operated at their full rating if the minimum current at which
circuit protection is designed to operate does not exceed 1.5 times (in case of
cables in air or in ducts) or 1.3 times (in case of cables laid direct in the ground)
the values tabulated in Tables I to III.
NORMAL CONDITIONS OF INSTALLATION
|
1
|
Maximum Conductor Temperature
|
70°C For GPPVC Insulation and
85°C For HRPVC Insulation
|
2
|
Thermal Resistivity Of Soil
|
150°C cm/w
|
3
|
Thermal Resistivity Of PVC
|
650°C cm/w
|
4
|
Ground Temperature
|
30° C
|
5
|
Ambient Air Temperature
|
40° C
|
6
|
Depth Of Laying
|
75 cms
|
SHORT CIRCUIT RATING IN K. Amp. (For One Second Duration)
|
Area of conductor sq. mm.
|
Maximum Conductor Temperature
|
G.P. PVC
|
H.R. PVC
|
1.50 sq. mm.
|
0.169
|
0.153
|
2.50 sq. mm.
|
0.283
|
0.255
|
4.00 sq. mm.
|
0.452
|
0.406
|
6.00 sq. mm.
|
0.679
|
0.612
|
FORMULA FOR CALCULATING SHORT CIRCUIT
|
|
I1 |
|
I1 = Short Circuit Current for one seconds.
|
It = |
--------- |
|
It = Short Circuit Current for t seconds.
|
|
√t |
|
t = Duration in seconds
|
Maximum Conductor temperature during short circuit is 160°C for both G.P. PVC &
H.R. PVC Cables.
|
NOTE :
For cables in air no reduction in current ratings is necessary provided that:
1. The horizontal clearance between circuit is not less than six times the overall diameter of an individual cable. Not less than the overall width of individual circuit, except the horizontal clearance in any case need not exceed 150 mm.
2. The vertical clearance between circuits is not less than 150 mm.
3. If the number of circuits exceeds 4, they are installed in a horizontal plane.