With respect to PVC insulated cables, briefly describe the effects of the following on the insulation

With respect to PVC insulated cables, briefly describe the effects of the following on the insulation :

            (a)        at temperature below 0ºC                                                                   (5m)

            (b)        at temperature above 20ºC                                                                  (5m)

Answer :-

(a)                Effects on PVC insulated cables at temperature below 0ºC :

1)      It’s become hard and brittle and finally crack at low temperature (-8ºC). Thus live conductors will be exposed and as such short circuit can take place.

(b)               Effects on PVC insulated cables at temperature above 20ºC :

1)      at normal temperature, PVC tends to flow and become disturbed under mechanical stress, thus occurrence of necking at cable glands, causing gland to loose its watertight properties and integrity.

2)      An increase in temperature, its insulation resistance falls rapidly off to one hundredth of value at 20ºC.

3)      Prolonged exposure to heat leads to chemical breakdown of PVC and hydrochloride acid is formed with severe corrosion of metal parts at a range of 115 to 120ºC.

4)      Plasterers used for PVC coating become volatile at 70ºC, thus tends to soften and flows.

5)      PVC is flame retardant.  When burnt, formation of hydrochloric acid gas tends to blanket the area, but these fumes are toxic.

6)      PVC melts at 150ºC.