This document discusses low voltage switchgear, including definitions and examples. It describes common types of low voltage switchgear like air circuit breakers, MCCBs, contactors, relays, and fuses. It provides information on their ratings and standards. The document focuses on contactors, explaining their operation, applications, selection parameters, and utilization categories. It also discusses motor protection concepts and thermal overload relays, including their operation and selection parameters.

1. LV SWITCHGEAR
Presented By:
Sumeet Singh AEE(E)-137454
Shivendra Kumar Mishra AEE(E)- 137190
Shivesh Chandra Gupta AEE(E)- 136395

2. WHAT IS SWITCHGEAR?
 A device or a combination of devices, primarily intended for the purpose of
making, breaking, carrying electric currents in circuits under normal
conditions as well as under abnormal (faulty) conditions.

3. EXAMPLES OF LV SWITCHGEAR
 Air circuit Breaker (ACBs)
 MCCB
 MPCB
 MCB
 ELCB
 RCCB
 Air Break Contactors
 Vaccum Contactors
 Thermal Overload Relays

4. EXAMPLES OF LV SWITCHGEAR
 Switch/ Disconnectors/ Switch-Disconnectors
 Changeover Switch
 HRC Fuse
 SFU
 FSU
 SDF Units
 CT
 VT/PT
 Selector Switches
 Push Buttons

5. Switchgear As per IS/IEC 60947 Part 1
 A general term covering switching devices & their combinations with
associated control, measuring, protective and regulating equipment, also
assemblies of such devices & equipment with associated interconnections,
accessories, enclosures & supporting structures, intended in principle for use
in connection with generation, transmission, distribution & conversion of
electric energy.
 Definition of Low Voltage:
 Worldwide:
IEC:60947: less than 1000V

6. Switchgear Distinguishing Features
Type Current
Rating
Fault
operating
capability
Frequency Of
operation
Life
expectancy
(No. of
operations)
ACB High High Infrequent Low
MCCB Medium Medium Infrequent Low
Fuse Medium/Low High Once Once
SDF Medium/Low High/Medium Frequent Medium
Contactor Low Low Very frequent High

7. Switchgear Switching Protection
Contactor Y N
Relay N Y
Starter Y Y
Switch/SD Y N
HRC Fuse N Y
SFU/FSU/SDF Y Y
ACB Y Y
MCCB Y Y
MPCB Y Y

8. Switchgear- Functions
 Switching- (ON/OFF, Close/Open, Connect/Disconnect, Start/stop)
i.e. MAKE, CARRY, BREAK under normal & abnormal conditions
 Protection- Abnormal conditions
O/C- O/L & S/C
Earth Fault

9. Product Standards
IS/IEC: 60947
 Part 1 – General rules of all products
 Part 2- CB
 Part 3- SDF
 Part 4- Contactors/Relays/starters
 Fuse- IEC 60269
 MCBs- IEC 60898

10. LV switchgear- Range
 Contactor- 9 to 550A (On/Off)
 Relay- 0.6 to 450A (O/L+ Single Phasing)
 Fuse- 2 to 800A (S/C-100kA)
 ACB- 400 to 6300A (On/Off+O/L+S/C-100kA)
 MCCB- 16 to 1600A (On/Off+O/L+S/C-50/65kA)
 MPCB- 0.16 to 63A (On/Off+O/L+S/C-50/65kA+ Single phasing)
 MCB- 0.5 to 100A (On/Off+O/L+S/C-10kA)
 RCCB/ELCB- 25 to 100A
 RCBO- 6 to 32 A
 EL+MCB- 6 to 63A

11. SLD

12. CONTACTORS (IS/IEC 60947 Part 4)
A Electromechanical switching device capable of
making, carrying and breaking currents under
normal condition including operating overload
conditions.

13. Contactor Cross-section

14. Closing of Contactor
 Coil gets supply
 A magnetic field is produced.
 A operating force is produced
 Operating Force > Restraining Force
 Moving Magnet moves
 Moving Contact assembly moves
 Main contacts close
 Parallel operation of NO/NC

15. Opening of Contactor
 Supply to coil is cut.
 Magnetic coil collapse
 Operating force becomes zero
 Restraining force > operating force
 Moving Magnet returns
 Moving Contact returns
 NO/NC regain normal state

16. Contactors- Application
 Switching of motors
 Switching of Lighting loads
 Switching of heaters
 Switching of capacitors
 Switching of DC Loads

17. Why Contactors?
 High Operational Life
 High Frequency of operation
 Remote Operation
 Automatic operations
 Offers No volt Protection

18. Contactors-Selection Parameters
 Rated operational voltage- (Ue)
 Rated Operational Current- (Ie)
 Rated Normal Condition & Overload condition Breaking Current
 Rated Normal Condition & Overload condition Making Current
 Rated conditional SC Current
 Utilization Category
 Electrical & Mechanical Life
 Frequency of operation
 Coil voltage

19. Contactors-Selection Parameters
 Number & combination of Aux contacts
 Dimensions
 Terminal Size
 Ease of Maintenance
 Availability of Spares

20. Contactors- Utilization Category
 To ensure correct operation of the contactor, first, its rating has to be
selected, keeping in mind the intended switching application.
The most common utilization category are:
 AC-1
 AC-2
 AC-3
 AC-4

21. AC1
For Non Inductive/Slightly Inductive Loads/ Resistance Furnaces
 Required making capacity-1.0Ie
 Required breaking capacity- 1.0Ie

22. AC2
For Slip Ring Induction Motors- Starting & Stopping
 Required making capacity-2.5Ie
 Required breaking capacity- 2.5Ie

23. AC3
 For Squirrel cage induction motors- Starting & stopping during running
 Required making capacity-6.0Ie
 Required breaking capacity- 1.0Ie

24. AC4
 For Squirrel cage induction motors- Starting, plugging & inching
 Required making capacity-6.0Ie
 Required breaking capacity- 6.0Ie

25. Contactors-Making & Breaking Capacities
Utilization
Category
During normal Operations Ultimate capacity
Im Ib Im Ib
AC1 1.0Ie 1.0Ie 1.5Ie 1.5Ie
AC2 2.5Ie 2.5Ie 4.0Ie 4.0Ie
AC3 6.0Ie 1.0Ie 10.0Ie 8.0Ie
AC4 6.0Ie 6.0Ie 12.0Ie 10.0Ie

26. Contactors- Utilization Category
 DC-1- Switching of Non Inductive or Slightly inductive Loads/ Resistance
furnaces
 DC-3: DC Shunt Motors- Starting, Plugging, Inching, dynamic Braking
 DC-5: DC Series Motors- Starting, Plugging, Inching, dynamic Braking
 DC-6: Switching of Incandescent Lamps

27. DC Contactors
Advantage of DC Coil System
 No Eddy Current & Hysteresis loss
 Less coil failure
 Greater coil Reliability
Disadvantage of DC Coil System
 Coil is Bulky
 Contactors are bigger in height.
 Costly

28. Starters for 3ph Induction Motors (IS/IEC
60947 Part 4)
 Starting is a process in which a motor’s rotor is brought from zero speed to
rated speed.

29. Direct On-Line Starting-Power circuit

30. DOL Starter- Power Schematic Diagram

31. DOL Starter- Control Schematic diagram

32. DOL Starter
Advantages of DOL Starter:
 Most economical Starter
 Simple to establish, operate & maintain.
 Simple control circuitry.
 Easy to understand & troubleshoot.
Disadvantages:
 High Starting current
 Thermal stress on motor
 Mechanical stress on load leads
 Voltage drop in system

33. Primary Resistance Method
 Resistors are connected in series with each phase, between the isolation
contactor & the motor.
 Starting current is 3 to 4 times Full load current.

34. Limitations:
 Difficult to change resistance.
 Dissipate a lot of heat.
 Limited no. of starts per hour.
 Hard to start high inertia loads.
 Starting characteristics may change.

35. Auto transformer Starting
 Reduce voltage during starting.
 Transformer has range of voltage taps.
 Line current is less than actual motor current.

36. Auto Transformer Starting Method

37.  Drawbacks:
 Frequent maintenance & inspection
 Voltage flickers & mechanical jerks
 Produces dangerous current spikes.

38. Star delta starting method
 Motor is initially connected in Star and then, after preset time motor is
connected in delta configuration.
 Whwn motor speed reaches greater than or equal to 80% of its rated speed
changeover action is initiated.

39. Power schematic Diagram of Star delta
starter

40. Control Schematic of Star Delta Starter

41. Fully Automatic Star Delta Starter

42. Soft Starter
• Voltage applied to motor is regulated with help of SCR in series with supply of
motor.
• By pulsing a thyristor, it switches from ‘off’ to ‘on’ until the current stops
flowing through it- which occurs every half cycle in AC Supply.

44. Motor Protection Concepts & Thermal
Overload Relay (As per IS / IEC 60947 Part 4)
 Causes of Motor Failure:
 Overload
 Heavy Starting
 Locked Rotor
 Supply Variation
 Unbalance in supply
 Number of starts
 Single Phasing
 Mechanical problems

45. Electrical Motors- Classes of Insulation
Class of Insulation Maximum Attainable Temp (in Celsius)
(As per IEC 60085)
O/Y 90
A 105
E 120
B 130
F 155
H 180

46. Electrical Motors- Classes of Insulation
 Motors are generally insulated to Class ‘B’
 Nowadays, motors are insulated to Class ‘F’
 Even if insulated to class ‘F’ temperature rise is
limited to that of Class ‘B’ only
 So, for all practical purposes, they can be considered
to be Class ‘B’ only

47.  What causes insulation failure?
 Temperature
 Best form of motor protection would be by sensing the temperature.

48. Motor Overload Protection-
Temperature Sensing
 Thermostat
 Examples- Automatic electric iron box, electric gyser,electric rise cooker.
 Accuracy - +/- 10 (in degree Celsius)
 Advantage : Low Cost
 Disadvantage : under utilization of motors
 Thermistor
 PTC Thermistor/ NTC Thermistor
 Accuracy : +/- 1(in degree Celsius)
 Advantage : Accuracy , Consistency
 Disadvantage : Cost

49. Current Sensing Type Motor Overload
Protection:
 Electro-mechanical type over current Relay
 Electronic Motor Protection Relay
 Bimetal or Thermal Overload Relay
 Electro-mechanical type over current Relay
 Advantage : Simple to implement / Simple Working Principle
 Disadvantage :
 Thermal memory relay ( Suitable only for protection during cold starts of
motor)
 Accuracy , Size and Cost

50. Operation of Bimetal O/L Relay
 Two metals with different temperature coefficient of expansion are fused
together
 This action results in bending of bimetal
 Advantages :
 Thermal Memory
 Compact
 Accuracy protection
 Long Life
 Ambient Compensation

51. Bimetallic Relay
 Single Slide Mechanism
 O/L Protection
 Double Slide Mechanism
 O/L Protection
 Single Phasing

52. Schematic- Single Slide

53. Schematic- Double Slide
Overload Protection

54. Schematic- Double Slide
Single Phasing

55. Relay Characteristics-O/L Protection

56. Relay Characteristics- Single Phasing

57. Selection Parameters Trip Class

58. Relay I-t Characteristics and Motor
Damage Curve Superimposed

59. THANK YOU