1. Frequency of Fault Occurrence in Different Links of a Power System
Equipment Percentage
Overhead Lines 50%
Cables 10%
Transformer 10%
Switchgear 15%
  1. Fault in Overhead Lines
Sr. No. Type of Short Circuit Fault Percentage Occurrence
1. Single phase to ground (L-G) 70
2. Phase to Phase fault (L-L) 15
3. Two phases to ground (L-L-G) 10
4. Phase to phase and third phase to ground 2 or 3
5. All the three phases to ground 2 or 3
6. All  the three phases shorted 2 or 3
  1. The ratio of minimum fusing current and the current rating of fusing element is known as fusing factor.
  2. Fusing factor always greater than unity.

Types of Fuse

  1. Rewirable fuse (Kit Kat fuse), Standard ratings of fuse are 6, 16, 32, 63 and 100A.
  2. Cartridge Fuse
  3. HRC Fuse
  4. Switch Fuse (The ratings of switch of fuse units are in the range of 30, 60, 100, 200, 400, 600 and 800 Amperes).

Circuit Breaker

  1. A circuit breaker is a device, which can (i) make or break a circuit either manually or by the remote control under normal conditions. (ii) Break a circuit automatically under fault conditions. (iii) make a circuit either manually or by remote control under fault conditions.

Classifications of Circuit Breaker

Oil Circuit Breaker,

  • This type of circuit breaker employs some insulating oil (e.g. transformer oil) for arc extinction.
  1. Oil circuit breakers are divided into two types (1) bulk oil circuit breaker (2) Low oil circuit breaker
  2. Bulk oil circuit breaker has two types (a) plain break oil circuit breakers (b) Arc control oil circuit breakers
  3. Plain break oil circuit breakers are used for low capacity installation for voltage not exceeding 11KV.

Air Blast circuit Breaker,

  1. In which high pressure air-blast is used for extinguishing the arc.
  2. Air blast circuit breaker used upto voltages of 15KV and capacity of 2,500MVA.

Sulphur Hexafluoride (SF6) Circuit Breaker

  1. Sulphur Hexafluoride (SF6) gas is used for arc extinction.
  2. Sulphur Hexafluoride (SF6) gas has electronegativity property.
  3. Sulphur Hexafluoride (SF6) circuit breaker can be used from 3.3KV to 760KV.
  4. Sulphur Hexafluoride (SF6) circuit breakers require maintenance once in four to ten year.
  5. 145KV SF6 Circuit breaker need only about 30 Kg of SF6 gas for first filling and no replacement is required for about 5 Years.

Vacuum Circuit Breaker

  1. In this type of circuit breaker vacuum is used for arc extinction.