MCB / MCCB / ELCB Quick Reference (Part-2)


Selection of MCB

MCB curveType of LoadResidentialCommercial
B curveResistive LoadsIncandescent lightsIncandescent lights
Geyser Boilers
 Heater Heaters
Fan blower heatersOil radiator heaters
Slight Inductive LoadsFlorescent LightsFlorescent lights
Small motors (FHP)High pressure mercury vapor lamps
C curveSlight Inductive LoadsFans & small pumpsSodium vapor lamps
Window / Split ACs
Lights with ballasts
 General household equipment
D curveInductive LoadsWater lifting pumpsFlorescent lights
UPSID & FD fans
Small control transformers
Medium size motors
Refrigerators for commercial use


Type of MCB

MCB CurveType of LoadResponseTrippingApplicationUses
B curveResistive loadsMCBs react quickly to overloads3 To 5 times F.L current (0.04 To 13 Sec)Domestic & Commercial applicationsSuitable for incandescent lighting, socket-outlet, bulbs, heaters etc. Protection of DG sets (since DG sets have low short-circuit capacity)
C curveSlightly inductive loadsMCBs react more slowly,5 To 10 times F.L current  (0.04 To 5 Sec)Commercial and Industrial applications


Highly Inductive loads such as motors, air conditioners, fluorescent lighting lights, fans & household electrical appliances.
D curveInductive loadsMCBs are slower10 To 20 times F.L current  (0.04 To 3 Sec)Commercial and Industrial applications


Very high inrush Inductive currents, Small transformers, welding machines. UPS, small motor & pumps, x-ray machines etc. Note, however, that MCBs with Type K characteristics may provide better protection in some applications of this type.
K curveInductive loadsMCBs are slower8 To 10 times F.L current  (0.04 To 3 Sec)Placing them between the traditional Type C and Type D breakers. In most cases, they allow improved cable protection to be provided in circuits that include motors, capacitors and transformers, where it would previously have been necessary to use Type D devices. This enhanced protection is achieved without increasing the risk of nuisance tripping.


Selection of RCCB

Type of RCCBSensitiveApplication
Type ACSensitive to AC Currents OnlySuitable for most domestic and commercial applications.
Type ASensitive to AC Currents + Pulsating DC Currents (Produced by Rectifier, Thyristors)Used where there are a lot of “electronic” loads, such as computer equipment or lighting systems with electronic ballasts.
Type BSensitive to AC Currents + Pulsating DC Currents+ Pure DC CurrentsUse in photovoltaic (PV) solar energy installations because the PV panels produce a DC Output and some types of fault can result in the leakage of DC Currents to Earth.
Type B+Similar to Type B, but respond to ac leakage currents over a wider frequency rangeType B and Type B+ devices can be used wherever a Type AC or Type A device is specified, as they provide the same functionality as these types and more.



TYPEAC Current 50HzAC Current 50Hz To 1KHZPulsating Current with DC ComponentMulti Frequency Current Generated By 1Phase InverterMulti Frequency Current Generated By 3Phase Inverter


Sensitivity of RCCB

30 mApersonal protection domestic installation / direct contact
100 mAlimited personal protection / indirect contact
300 mAbuilding / fire protection


MCB Enclosure Size

MCB Rating (A)Min. Enclosure Size
300A to 400A506mm381mm153mm
600A to 800A520mm420mm200mm
1000A to 1200A704mm554mm173mm
1600A to 3000A1016mm608mm615mm


Switch Gear Protection

Switch GearProtectionIsolationControl
Over LoadShort Circuit


Type of Faults

Types of FaultReasonConsequences Protective Device to be used
OverloadWhen Equipment tries to run beyond its rated capacity, or there is a fault in the equipment E.g. When you keep a heater on without any water in it.It can lead to reduction in life of equipment, Failure of insulation and hence damaging the equipment.MCB / RCBO
Short CircuitInsulation Failure, Shorting of the Phase to Phase or Phase and Neutral Wires.High Inrush Current, causing permanent damage to equipment and may lead to a Fire.MCB
Earth FaultShort circuit between Phase and Earth Conductor.Can result in Fire due to sparking.RCBO / RCCB
Earth LeakageHuman Body Touching Live Wires. Insulation failureRCBO / RCCB
Over VoltageOpening of Neutral Connection increase in Phase To Phase Voltage of 440V, Surge through Lighting or transients, Over voltage from Utility.Damage to sensitive Electronic Equipment.OV protection Device
Under VoltageDrop in supply voltage, starting of heavy loadsDamage of Equipment, Flickering of Lights.UV relays


 MCB Type (BS EN 60898-2)

Trip Typeinstantaneous
Trip (< 0.1 s)
Load TypeTypical Load
B3 to 5 In (AC)ResistiveHeaters, showers, cookers, socket outlets.
4 to 7 In (DC)
C5 to 10 In (AC)InductiveMotors, general lighting circuits, power supplies.
7 to 15 In (DC)
D10 to 20 InHigh InductiveTransformers, motors, discharge lighting circuits, computers


Relays for Transformer 

Capacity of TransformerRelays on HV SideRelays on LV SideCommon Relays
Generator Transformer3 Nos Non-Directional O/L Relay– –Differential Relay or
1 no Non-Directional E/L RelayOverall differential Relay
and/or standby E/F + REF RelayOver flux Relay
Buchholz Relay
OLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
220 /6.6KV Station Transformer3 Nos Non-Directional O/L Relay3 Nos Non-Directional O/L RelayDifferential Relay
1 no Non-Directional E/L RelayOver flux Relay
and/or standby E/F + REF RelayBuchholz Relay
OLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
132/33/11KV up to 8 MVA3 Nos O/L Relay2 Nos O/L RelaysBuchholz Relay
1 no E/L Relay1 no E/L RelayOLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
132/33/11KV up to 8 MVA to 31.5 MVA3 Nos O/L Relay3 Nos O/L RelayDifferential Relay
1 no Directional E/L Relay1 no E/L RelayBuchholz Relay
OLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
132/33KV, 31.5 MVA & above3 Nos O/L Relay3 Nos O/L RelayDifferential Relay
1 no Directional E/L Relay1 no E/L RelayOver flux Relay
Buchholz Relay
OLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
220/33 KV, 31.5MVA & 50MVA , 220/132KV, 100 MVA3 No O/L Relay3 Nos O/L RelayDifferential Relay
1 no Directional E/L Relay1 no Directional E/L RelayOver flux Relay
Buchholz Relay
OLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
400/220KV 315MVA3 Nos Directional O/L Relay3 Nos Directional O/L RelayDifferential Relay
1 no Directional E/L relay.1 no Directional E/L relay.Over flux Relay
Restricted E/F relayRestricted E/F relayBuchholz Relay
3 Nos Directional O/L Relay for actionOLTC Buchholz Relay
PRV Relay
OT Trip Relay
WT Trip Relay
Over Load (Alarm) Relay


Relays for Transmission & Distribution Lines Protection

Lines to be protectedRelays to be used
400 KV Transmission LineMain-I: Non switched or Numerical Distance Scheme
Main-II: Non switched or Numerical Distance Scheme
220 KV Transmission LineMain-I : Non switched distance scheme (Fed from Bus PTs)
Main-II: Switched distance scheme (Fed from line CVTs)
With a changeover facility from bus PT to line CVT and vice-versa.
132 KV Transmission LineMain Protection : Switched distance scheme (fed from bus PT).
Backup Protection: 3 Nos. directional IDMT O/L Relays and
1 No. Directional IDMT E/L relay.
33 KV lines Transmission LineNon-directional IDMT 3 O/L and 1 E/L relays.
11 KV lines Transmission LineNon-directional IDMT 2 O/L and 1 E/L relays.


Selection Chart for 3Ph Induction Motor

Motor Rating,415V,3PhFull Load Current (A)CONTACTOR (A)OVER LOAD RELAY (A)BACK UP FUSE (A)Cable Size
DOL StarterSTAR-DELTA Starter
0.750.521.6161.0 To 1.641.51.5
10.752161.6 To 2.561.51.5
21.53.5163.0 To 4.5101.51.5
32.25164.5 To 7.0101.51.5
53.77.5166.5 To 10161.51.5
7.55.511161610 To 154.5 To 7.0162.
107.514161613 To 206.5 To 10202.
12.59.318251613 To 2010 To 152542.542.5
151121251615 To 2213 To 20256464
201528321824 To 3013 To 2032106106
2518.535402525 To 3015 To 225016101610
3022.540502532 To 5024 To 305016161616
352647703232 To 5025 To 306325162516
503766704057 To 7032 To 508035253525
604580955070 To 10532 To 5010050355035
755510012570100 To 15040 To 5710070507050
9067.512014070100 To 15057 To 7016095709570
1007513514095100 To 15070 To 10516095709570
1259016512570 To 10516015095
150110200125100 To 150200185150


MCB Selection Chart For Motor Protection

KwHP1Phase 230V DOL
3Phase 400V DOL
3 Phase 400V Star Delta
Full Load CurrentMCB SelectionFull Load CurrentMCB SelectionFull Load CurrentMCB SelectionMCB Selection — — —
0.250.343.2101.22 — — —
0.370.53.5101.22 — — —
0.550.744.8161.83 — — —
0.751.016.22023 — — —
1.11.478.7252.66 — — —
1.52.0111.8323.510 — — —
2.22.9517.5504.410 — — —
1013.4 — —2150215032
1520.11 —27100276340
18.524.8 — —3212532 —50
2229.49 — —381253863
3040.21 — —5112551 —63



CodeType of Relay
1Master Element
2Time-delay Starting or Closing Relay
3Checking or Interlocking Relay
4Master Contactor
5Stopping Device
6Starting Circuit Breaker
7Rate of Change Relay
8Control Power Disconnecting Device
9Reversing Device
10Unit Sequence Switch
11Multifunction Device
12Over speed protection
13Synchronous-Speed Device
14Under speed Device
15Speed or Frequency Matching Device
16Data Communications Device
17Shunting or Discharge Switch
18Accelerating or Decelerating Device
19Starting-to-Running Transition Contactor
20Electrically-Operated Valve
21Distance protection Relay
21GGround Distance
21PPhase Distance
22Equalizer circuit breaker
23Temperature control device
24Volts per hertz relay
25Synchronizing or synchronism-check device
26Apparatus thermal device
27Under voltage relay
27PPhase Under voltage
27SDC under voltage relay
27TNThird Harmonic Neutral Under voltage
27TN/59N100% Stator Earth Fault
27XAuxiliary Under voltage
27 AUXUnder voltage Auxiliary Input
27/27XBus/Line Under voltage
27/50Accidental Generator Energization
28Flame Detector
29Isolating Contactor
30Annunciator Relay
31Separate Excitation Device
32Directional Power Relay
32LLow Forward Power
32NWatt metric Zero-Sequence Directional
32PDirectional Power
32RReverse Power
33Position Switch
34Master Sequence Device
35Brush-Operating or Slip-ring Short Circuiting Device
36Polarity or Polarizing Voltage Device
37Undercurrent or Under power Relay
37PUnder power
38Bearing Protective Device / Bearing Rtd
39Mechanical Condition Monitor
40Field Relay / Loss of Excitation
41Field Circuit Breaker
42Running Circuit Breaker
43Manual Transfer or Selector Device
44Unit Sequence Starting Relay
45Atmospheric Condition Monitor
46Reverse-Phase or Phase Balance Current Relay or Stator Current Unbalance
47Phase-Sequence or Phase Balance Voltage Relay
48Incomplete Sequence Relay / Blocked Rotor
49Machine or Transformer Thermal Relay / Thermal Overload
49RTDRTD Biased Thermal Overload
50Instantaneous Overcurrent Relay
50BFBreaker Failure
50DDCurrent Disturbance Detector
50EFEnd Fault Protection
50GGround Instantaneous Overcurrent
50IGIsolated Ground Instantaneous Overcurrent
50LRAcceleration Time
50NNeutral Instantaneous Overcurrent
50NBFNeutral Instantaneous Breaker Failure
50PPhase Instantaneous Overcurrent
50SGSensitive Ground Instantaneous Overcurrent
50SPSplit Phase Instantaneous Current
50QNegative Sequence Instantaneous Overcurrent
50/27Accidental Energization
50/51Instantaneous / Time-delay Overcurrent relay
50Ns/51NsSensitive earth-fault protection
50/74Ct Trouble
50/87Instantaneous Differential
51Phase Inverse Time Overcurrent IDMT (Time delay phase overcurrent )
51GGround Inverse Time Overcurrent
51LRAC inverse time overcurrent (locked rotor) protection relay
51NNeutral Inverse Time Overcurrent
51PPhase Time Overcurrent
51RLocked / Stalled Rotor
51VVoltage Restrained Time Overcurrent
51QNegative Sequence Time Overcurrent
52AC circuit breaker
52aAC circuit breaker position (contact open when circuit breaker open)
52bAC circuit breaker position (contact closed when circuit breaker open)
53Exciter or Dc Generator Relay
54Turning Gear Engaging Device
55Power Factor Relay
56Field Application Relay
57Short-Circuiting or Grounding Device
58Rectification Failure Relay
59Overvoltage Relay
59BBank Phase Overvoltage
59PPhase Overvoltage
59NNeutral Overvoltage
59NUNeutral Voltage Unbalance
59PPhase Overvoltage
59XAuxiliary Overvoltage
59QNegative Sequence Overvoltage
60Voltage or current balance relay
60Voltage or Current Balance Relay
60NNeutral Current Unbalance
60PPhase Current Unbalance
61Density Switch or Sensor
62Time-Delay Stopping or Opening Relay
63Pressure Switch Detector
64Ground Protective Relay
64FField Ground Protection
64RRotor earth fault
64REFRestricted earth fault differential
64SStator earth fault
64SSub-harmonic Stator Ground Protection
64TN100% Stator Ground
66Notching or Jogging Device/Maximum Starting Rate/Starts Per Hour/Time Between Starts
67AC Directional Overcurrent Relay
67GGround Directional Overcurrent
67NNeutral Directional Overcurrent
67NsEarth fault directional
67PPhase Directional Overcurrent
67SGSensitive Ground Directional Overcurrent
67QNegative Sequence Directional Overcurrent
68Blocking Relay / Power Swing Blocking
69Permissive Control Device
71Liquid Switch
72DC Circuit Breaker
73Load-Resistor Contactor
74Alarm Relay
75Position Changing Mechanism
76DC Overcurrent Relay
77Telemetering Device
78Phase Angle Measuring or Out-of-Step Protective Relay
78VLoss of Mains
79AC Reclosing Relay / Auto Reclose
80Liquid or Gas Flow Relay
81Frequency Relay
81OOver Frequency
81RRate-of-Change Frequency
81UUnder Frequency
82DC Reclosing Relay
83Automatic Selective Control or Transfer Relay
84Operating Mechanism
85Pilot Communications, Carrier or Pilot-Wire Relay
86Lock-Out Relay, Master Trip Relay
87Differential Protective Relay
87BBus Differential
87GGenerator Differential
87GTGenerator/Transformer Differential
87LSegregated Line Current Differential
87LGGround Line Current Differential
87MMotor Differential
87OOverall Differential
87PCPhase Comparison
87RGFRestricted Ground Fault
87SStator Differential
87SPercent Differential
87TTransformer Differential
87VVoltage Differential
88Auxiliary Motor or Motor Generator
89Line Switch
90Regulating Device
91Voltage Directional Relay
92Voltage And Power Directional Relay
93Field-Changing Contactor
94Tripping or Trip-Free Relay
Abbreviation Code
AFDArc Flash Detector
CLKClock or Timing Source
CLPCold Load Pickup
DDRDynamic Disturbance Recorder
DFRDigital Fault Recorder
DMEDisturbance Monitor Equipment
ENVEnvironmental data
HIZHigh Impedance Fault Detector
HMIHuman Machine Interface
LGCScheme Logic
METSubstation Metering
PDCPhasor Data Concentrator
PMUPhasor Measurement Unit
PQMPower Quality Monitor
RIORemote Input/output Device
RTDResistance Temperature Detector
RTURemote Terminal Unit/Data Concentrator
SERSequence of Events Recorder
TCMTrip Circuit Monitor
LRSS Local/Remote selector switch
VTFF Vt Fuse Fail

Suffixes Description

AAlarm, Auxiliary Power
ACAlternating Current
BBus, Battery, or Blower
BFBreaker Failure
BLBlock (Valve)
BTBus Tie
CCapacitor, Condenser, Compensator, Carrier Current, Case or Compressor
CHCheck (Valve)
DDischarge (Valve)
DCDirect Current
DCBDirectional Comparison Blocking
DCUBDirectional Comparison Unblocking
DDDisturbance Detector
DUTTDirect Under reaching Transfer Trip
FFeeder, Field, Filament, Filter, or Fan
GGround or Generator
GCGround Check
HHeater or Housing
LLine or Logic
MMotor or Metering
MOCMechanism Operated Contact
NNeutral or Network
PPhase or Pump
PCPhase Comparison
POTT Pott: Permissive Overreaching Transfer Trip
PUTTPutt: Permissive Under reaching Transfer Trip
RReactor, Rectifier, or Room
SSynchronizing, Secondary, Strainer, Sump, or Suction (Valve)
SOTFSwitch On To Fault
TTransformer or Thyratron
TDTime Delay
TDCTime-Delay Closing Contact
TDDOTime Delayed Relay Coil Drop-Out
TDOTime-Delay Opening Contact
TDPUTime Delayed Relay Coil Pickup
THDTotal Harmonic Distortion
THTransformer (High-Voltage Side)
TLTransformer (Low-Voltage Side)
TTTransformer (Tertiary-Voltage Side)
UUnder or Unit


Harmonic Effects

HarmonicR PhaseY PhaseB PhasePhase Rotation SequenceHarmonic Effect
3th3×0°=3×120°=360°=0°3×240°=720°=0°No Rotation          (In Phase)Adds Voltages or Currents in Neutral Wire causing Heating
5th5×0°=5×120°=600°=(600-720)=(-120°)5×240°=1200°=(1200-2400)=(-240°)Rotate Against Fundamental (-) (B-Y-R)Motor Torque Problems
7th7×0°=7×120°=840°=(840-720)=(+120°)7×240°=1680°=(1680-1440)=(+240°)Rotate with Fundamental (+) (R-Y-B)Excessive Heating Effect