1. Switchgear & protection
Lectures per week= 3 HRS
Tutorials= 1 hr
Marks= 100 Test 25 marks
Prerequisites: types of faults,Fault analysis,

2. UNIT 1
• Protective relaying
• Relay is an electromechanical device which
senses the abnormal fault condition &sends a
trip signal to the circuit breaker tripping
mechanism, thereby energising the trip coil of
circuit breaker , due to which its moving
contacts separate out from fixed contacts &
the circuit breaker trips or opens.
• Thus a relay only sends a signal to the circuit
breaker after it senses the abnormal fault
condition . It only assists the C.B. in providing

3. Requirements of Protective relay
• Selectivity S
• Speed S
• Sensitivity S
• Stability S
• Reliability R
• Adequateness A
• Economy / cost E
• Dependability D SSSSREAD

4. Unit 1
Protective relays, fundamental requirements of
relays
Classification of relays such as electromechanical
relays, induction type of relays such as induction
disc relay& induction cup relay. Overcurrent &
earth fault relay
Directional relay, its working principle & types.
Types of overcurrent relay- Inverse time current
relay, definite time overcurrent relay, inverse
definite minimum time relay, (I IDMT Relay)
Types of distance relays – PLAIN IMPEDENECE
RELAY, DIRECTIONAL IMPEDENCE RELAY ,
effects of arc fault resistance on reach of relay.
Reactance relay , directional reactance relay,
Effect of arc fault resistance on the reach of relay

5. SOLID STATE RELAYS, its types ,
advantages &disadvantages
Numerical relays- its general block
diagram & functional block diagram
Advantages & disadvantages

6. SELECTIVITY
• A relay selects between normal & abnormal
condition, healthy part & faulty part of the circuit.
Discrimination is an act of distinguishing between
faulty part & healthy part of the circuit . A relay
should operate for a fault within its zone of
protection . In case a relay is not selective it will
respond for a fault outside its zone of protection,
causing disconnection of large e portion of the
system , due to which there is embarassment to
supplier & consumer

8. Rapid clearance of fault
• Relay should be quick acting. Relay operating
time should very small, but not too small to the
extent that relay will operate unnecessarily
causing unwanted interruption of supply system
• A definite time lag must be permitted in order
• To permit discrimination between main & back
up protection
• To prevent unnecessary operation of relay under
transient starting current & permissible load
fluctuations

9. Sensitivity
• The smallest value of actuating quantity at
which protective scheme starts operating in
relation to minimum value of fault current
• Stability : is the quality of protective system
by virtue of which a relay remains inoperative
& stable under certain specified conditions
such as through fault, transient & system
disturbances

10. Reliabilty means Trustworthiness
• A protective relay should not operate for fault occuring
beyond its protective zone. Reliability of operation of relay
is marked by reliability of all components of a relay
Adequateness: It is judged on the basis of following aspects:
1. Rating of protected system
2. Location of protected machine
3. Probability of occurrence of abnormal condition due to
internal or external causes
4. Cost of machine
5. Effect on continuity of supply due to the failure of
protected system

11. ECONOMY / COST
• The cost of protective scheme should be
justified . The cost of protective system
depends upon
• The probability of failure of individual
components of protection system
• Cost of protected equipment
• Importance of protected system & location

12. Unit 2
Different protection of alternator
Types of faults , Types of protective
schemes, Types of earth fault
protections, restricted earth fault
protection, 100% earth fault
protections , sensitive earth fault
protection , leakage to frame

13. Protection of transformer
Factors affecting the protective scheme used.
Possible fault conditions and the protective
scheme used. Differential scheme of
protections & its drawbacks.
Merz prize differential scheme of protection ,
types of earth fault protections.Gas actuated
devices for protection , such as sudden pressure
relief relay, Rate of rise of pressure relay,
Buchholz relay .

14. Protection of motor
• Types of faults in motor, single Phasing, its
causes & effect & Protective scheme used,
phase reversal,, causes, protective scheme
used causes of field failure, its effects on
working of motor, protective scheme used ,
rotor protection against rotor faults

15. Unit 3
Protection of transmission line
• Types of protections provided, factors affecting the
protective scheme used , types of graded schemes of
protections – time graded , current graded , time &
current graded scheme of protection
• Lightning phenomenon, types of lightning strokes ,
causes of overvoltage, concept of insulation
coordination, types of lightning arrestors, surge
absorber & surge arrestor, types of system earthing –
Isolated earthing, solid grounding, resistance
grounding, reactance/ arc suppression coil / peterson
coil grounding/ resonant earthing . concept of isolator
earthing switch.

16. Fuses & circuit breaker
Characteristic of fuse, classification of
fuse, difference between fuse &
circuit breaker, Theory on methods
used in arc extinction in circuit
breaker. Methods of arc formation ,
& arc maintenance. Rating of circuit

17. Reference books
Switch gear & protection -
S.S. Rao
Electrical Power – J.B.
Gupta