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Continuity of metallised paper ensures less voltage gradient, hence preventing puncture of insulation. 

Continuity of sheath / armour ensures that grounding is maintained, so no over voltage is induced, and easy to detect earthfault in cables. 

To take care of the both earthed / unearthed systems. 

In 6.6 kV system AC (unearthed) grade having stranded aluminium conductors cables are used. Their insulation’s are as follows.

1. XLPE – Crossed linked polyethylene insulation. 

2. FRLS PVC – Fire retardant low smoke insulation with PVC inner and outer insulation. In RB copper conductor stranded cables are used. 

3. FS – fire survival insulation. 

4. HR PVC – heat resistant insulation.

In 415 V system 1100 V grade copper or aluminium stranded cables are used. Their insulations are HR PVC and FS type. 

To avoid galvanic corrosion failure.  

For mechanical strength, protect against damage by impact of an object. 

Normally cables are rated for 40°C 

a. Absorbs moisture. 

b. Cable termination/sealing problems. 

a. Easy routing at heights

b. Easy maintenance

c. Large current (90 ºC) 

d. No sheath (no fatigue) 

e. No paper tape wrapping technique

f. High dielectric strength

g. Very little deformation even at high temp, 

h. More rated current, overload, short ckt capacity

i. Low tan delta and hence suits long routes

j. Very light

k. Good mechanical properties. 

1 core cables -- sheaths/shields/armours grounded at one end only, other end insulated to prevent the circulating current through sheaths. 

3 core cables -- grounded at both ends but not including core balance CT’s, since even small induced current causes 50N operation. 

Mineral insulated (MI) cables. 

The specified cable life 50 years.

a. Avoid sagging of cables 

b. Give mechanical support