Causes of Failure of Underground Cables
Q. What are the causes of failure of underground cables ?
1. Mechanical Failure : The various Layers like sheath, conductor, Insulation a be injured. Due to injuries the cable may fail to work.
Causes may be due to
(A) Accidents
(B) Carelessness
(C) Excessive pull during installation
(D) Sharp bends
(E) Vibrant, fatigue
(F) Explanation / contraction alternately
(G) Injury during digging
(H) Excessive pressure
(I) Improper routing and holding in position
2. Di-electric Failure : Increases in dielectric loss cause failure of cable. Increase in dielectric loss may be due to
(A) Increase in service voltage
(B) Overloading which increases temperature.
3. Thermal Failure : Insulation is degraded due to :
(A) Increase in temperature
(B) Sudden increase in temperature
(C) Unequal temperature rises (Due to installation, construction surrounding).
(D) Overloading increases temperature.
(E) Insulation in hot spot gets ionized.
(F) Void formation
(G) Partial discharge
(H) Leakage current
All these are the causes of thermal degradation of insulation
4. Chemical Failure : Corrosion of sheet is cause of failure which is due to :
(A) Chemical contamination in soil
(B) Rusting of racks or pipes
(C) Dissimilar soil effects acidity / Alkali in conduits.
(D) Underground earth currents between earthed neutrals.
5. Failure Due to Ageing : If the underground cable is laying for years together, due to ageing if fails to work due to wearing of serving, insulation etc.