Why this topic is important for ECET
In Transmission & Distribution (T&D), sag ensures safe clearance of conductors from the ground, while insulators prevent leakage and maintain mechanical support. ECET 2026 often tests direct formulas of sag, effect of tension, insulator string efficiency, and types of insulators. Mastering these gives you an easy edge in Power Systems questions (usually 3–5 marks in ECET).
📘 Concept Notes
1. Sag in Overhead Conductors
- Sag is the vertical distance between the conductor at the lowest point and the line joining the two supports.
- Proper sag is necessary because:
- Too little sag → Excessive tension → Conductor breakage.
- Too much sag → Unsafe clearance from ground.
Factors affecting sag:
- Span length
- Conductor weight
- Tension in conductor
- Wind/ice loading
- Temperature
Example:
For a span of 200m, conductor weight = 0.9 kg/m, tension = 1800 kg, sag can be calculated using formula below.
2. Insulators
Insulators support and separate conductors from physical structures without allowing leakage current.
Types of insulators used in T&D:
- Pin type – up to 33kV
- Suspension type – above 33kV, consists of porcelain discs
- Strain type – used at dead ends/pull points
- Shackle type – used in low-voltage distribution
Important concept – String Efficiency:
- In suspension insulators, potential distribution is not uniform.
- String efficiency measures how effectively the string is utilized.
Where:
= String efficiency
= Equivalent voltage across string
= Number of discs
= Average voltage across each disc
⚙️ Formulas
- Sag between level supports:
Sag with unequal supports:
String efficiency:
Tension in conductor:
Weight of conductor per unit length:
🔟 10 MCQs
Q1. Sag depends on which of the following factors?
a) Span length only
b) Conductor weight only
c) Tension only
d) Span, conductor weight, tension
Q2. A conductor has span = 250m, weight = 0.8 kg/m, tension = 2000 kg. Find sag.
a) 2.5 m
b) 3.1 m
c) 3.9 m
d) 4.5 m
Q3. If tension increases, sag will:
a) Increase
b) Decrease
c) Remain constant
d) Zero
Q4. Which type of insulator is used for 11kV distribution?
a) Pin type
b) Suspension type
c) Strain type
d) Shackle type
Q5. Suspension insulators are generally used above:
a) 6.6 kV
b) 11 kV
c) 33 kV
d) 66 kV
Q6. The string efficiency of a 4-disc insulator string is 80%. If total voltage = 100 kV, average voltage per disc is:
a) 20 kV
b) 25 kV
c) 30 kV
d) 40 kV
Q7. A 3-disc string has total voltage of 33 kV, but top disc has 15 kV. String efficiency is approximately:
a) 60%
b) 70%
c) 75%
d) 85%
Q8. For the same span and weight, if sag is reduced by half, tension becomes:
a) Same
b) Double
c) Half
d) Four times
Q9. Which insulator type is suitable at dead ends of transmission lines?
a) Pin type
b) Suspension type
c) Strain type
d) Shackle type
Q10. The major disadvantage of pin-type insulators is:
a) Expensive
b) Cannot be used beyond 33 kV
c) Heavy weight
d) Non-mechanical strength
✅ Answer Key
| Q.No | Answer |
|---|---|
| 1 | d |
| 2 | c |
| 3 | b |
| 4 | a |
| 5 | c |
| 6 | b |
| 7 | c |
| 8 | b |
| 9 | c |
| 10 | b |
🧠 Explanations
- Q1: Sag depends on all three (span, weight, tension). → (d)
- Q2:
→ (c) - Q3: Higher tension reduces sag. → (b)
- Q4: Pin type used up to 33kV; common for 11kV. → (a)
- Q5: Suspension used for voltages above 33kV. → (c)
- Q6:
→ (b) - Q7: Efficiency = (33 / (3 × 15)) × 100 ≈ 73% ≈ (c).
- Q8:
; halving sag doubles tension. → (b) - Q9: Strain type used at dead ends. → (c)
- Q10: Limitation = can’t be used beyond 33kV. → (b)
🎯 Motivation / Why Practice Matters
In ECET 2026, you’ll face direct sag formula problems and conceptual insulator questions. These are quick to solve if you know the formula–concept link. Students who practice these regularly can answer within 30 seconds in the exam, saving time for tough numerical problems. Speed + accuracy here gives you an advantage over others.
📲 CTA
👉 Join our ECET 2026 WhatsApp group for notes, tests & discussion:
🔗 Join Now
