Electrical Cables Interview Questions & Answers

What is the main reason for providing metallic sheath in underground cables?

The metallic sheath is provided around the insulation to protect it against the ingress of moisture, gas and other damaging liquids (acid or alkalies) from the soil and atmosphere.

What are the main requirements of the insulation materials used for underground cables?

Some the properties that insulation material require to possess are high insulation resistivity, high dielectric strength, good mechanical properties, non-hygroscopic, non inflammable, immune to attacks by acids and alkalies.

What are the advantages and disadvantages of oil filled cables?

Some of the advantages and disadvantages of oil filled cables are listed below:

Advantages:

• Smaller overall size and smaller weight for given voltage and kVA rating due to reduction in the thickness of the dielectric required
• No ionization, oxidation and formation of voids
• More perfect impregnation
• Smaller thermal resistance due to decrease in the dielectric thickness, so higher current rating
• More maximum permissible stresses
• Fault identification is easy

Disadvantages:

• Greater cost
• Complicated laying of cables and maintenance

Advantages of XLPE cables compared to PVC cables?

•  XLPE cable can withstand higher temperature (90⁰C) compared to PVC (70⁰C)
• XLPE cables can have higher over-load capacity
• XLPE cables are lighter in weight and smaller bend radius hence lower installation cost
• XLPE cable has high short circuit rating
• It has lower dielectric and constant power factor which can result in saving of power

What are different insulation materials used in cables for high voltage and medium voltage?

Medium Voltage:

• Polyethylene (PE)
• Cross linked polyethylene (XLPE)
• Tree retardant cross linked polyethylene (TR-XLPE)
• Ethylene-Propylene Elastomers (EPR)
• PILC

High Voltage:

• Cross linked polyethylene (XLPE)
• Paper/Oil
• Paper/Polypropylene
• SF6 gas