Insulators Used in Electrical Lines and Their Types

Kathmandu: Every Friday, Urja Khabar publishes an informative column called 'Urja Gyan'. This week, we provide insights into insulators used in power transmission lines. In the previous week, we covered electrical cables, their types, and uses.

1. What is an Insulator?

Answer: An insulator is a material that opposes the flow of electric current. It prevents the accidental flow of electricity, ensures safety, and allows the smooth functioning of electrical systems. Insulators have very high resistance that helps in hindering the flow of electrical particles. The main work of an insulator is to prevent leakage of current.

2. What are the Types of Insulators?

Answer: Following are some of the most common types of insulators:

• Shackle Insulator: Used in 400/230-volt lines.
• Pin Insulator: Available usually in three types: 400 volts, 11 kV and 33 kV. 400-volt type is comparatively small whereas 11 kV and 33 kV types are larger. Pin insulators are used only on suspension poles. For example, a 66 kV pin insulator can be used in a 66 kV line as Balaju-Lainchaur 66 kV line mainly to provide ground clearance.
• Disc Insulator: As the name suggests, it is disc-shaped, usually 25 cm in diameter. Normally, one disc is used for 11 kV lines, and three discs are used for 33 kV lines. They are used on tension poles but can also be used on suspension poles. It is advisable to use two discs for 11 kV lines for continuous supply in case one fails. Although insulators can also be fabricated from other materials, such as polymers, the most used material is porcelain.
• Glass Disc Insulator: It has almost the same shape as a porcelain disc insulator. Unlike porcelain, glass breaks in case of damage and so it is identifiable from a far distance. 132 kV Marsyangdi-Kathmandu has glass disc insulators. Similarly, 66 kV passing through Balaju (Ring Road) uses glass disc insulators.
• Polymer Insulator: Known as third-generation insulators, they are lightweight and durable. 

However, a number of porcelain and glass insulators are still in use, for example, the Dhalkebar-Muzaffarpur (India) 400 kV line or the Khimti-Dhalkebar 220 kV line.

Polymer insulators are also being used in distribution lines. Unlike disc insulators, if one polymer insulator is damaged then the whole assembly needs to be replaced.

Multiple-Choice Questions and Answers

1. What is the main purpose of an insulator?

(a) To carry wires
(b) Prevent voltage flashovers
(c) Prevent current leakage
(d) All of above

Answer: (d) All of above

2. At which voltage line shackles insulator are normally used?

(a) 11 kV
(b) 33 kV
(c) 66 kV
(d) 400/230 volts

Answer: (d) 400/230 volts

3. Which one is the lightest among the materials used for making insulators?

(a) Silicone rubber
(b) Glass
(c) Porcelain
(d) Both (b) and (c)

Answer: (a) Silicone rubber

4. How many disc insulators are installed in a 220 kV line?

(a) 5–6
(b) 10–12
(c) 22–23
(d) 16–17

Answer: (d) 16–17

5. Which of the following are insulating materials?

(a) Aluminum
(b) Paper
(c) XLPE
(d) Both (a) and (b)

Answer: (d) Both (a) and (b)

6. Which equipment is used to check if an insulator is cracked or damaged?

(a) Multimeter
(b) Ammeter
(c) Megger/Insulation Tester
(d) Voltmeter

Answer: (c) Megger/Insulation Tester

7. Which materials are used to make insulators?

(a) Carbon
(b) Porcelain
(c) Glass, Polymer
(d) Both (b) and (c)

Answer: (d) Both (b) and (c)