Working Principle of ELCB and RCB

Working Principle of ELCB and RCB:

  • An Earth Leakage Circuit Breaker (ELCB) is a device used to directly detect currents leaking to earth from an installation and cut the power and mainly used in TT earthing systems.
  • There are two types of ELCBs,
  1. Voltage Earth Leakage Circuit Breaker (voltage-ELCB)
  2. Current Earth Leakage Current Earth Leakage Circuit Breaker (Current-ELCB).
  • Voltage-ELCBs were first introduced about sixty years ago and Current-ELCB was first introduced about forty years ago. For many years, the voltage operated ELCB and the differential current operated ELCB were both referred to as ELCBs because it was a simpler name to remember. But the use of a common name for two different devices gave rise to considerable confusion in the electrical industry. If the wrong type was used on an installation, the level of protection given could be substantially less than that intended. To ignore this confusion, IEC decided to apply the term Residual Current Device (RCD) to differential current operated ELCBs. Residual current refers to any current over and above the load current

 Voltage Base ELCB.

  •  Voltage-ELCB is a voltage operated circuit breaker. The device will function when the Current passes through the ELCB. Voltage-ELCB contains relay Coil which it being connected to the metallic load body at one end and it is connected to ground wire at the other end.
  • If the voltage of the Equipment body is rise (by touching Phase to metal Part or Failure of Insulation of Equipment) which could cause the difference between earth and load body voltage, the danger of electric shock will occur. This voltage difference will produce an electric current from the load metallic body passes the relay loop and to earth. When voltage on the equipment metallic body rose to the danger level which exceed to 50Volt, the flowing current through relay loop could move the relay contact by disconnecting the supply current to avoid from any danger electric shock.
  • The ELCB detects fault currents from live to the earth (ground) wire within the installation it protects. If sufficient voltage appears across the ELCB’s sense coil, it will switch off the power, and remain off until manually reset. A voltage-sensing ELCB does not sense fault currents from live to any other earthed body.


  • These ELCBs monitored the voltage on the earth wire, and disconnected the supply if the earth wire voltage was over 50 volts.
  • These devices are no longer used due to its drawbacks like if the fault is between live and a circuit earth, they will disconnect the supply. However, if the fault is between live and some other earth (such as a person or a metal water pipe), they will NOT disconnect, as the voltage on the circuit earth will not change. Even if the fault is between live and a circuit earth, parallel earth paths created via gas or water pipes can result in the ELCB being bypassed. Most of the fault current will flow via the gas or water pipes, since a single earth stake will inevitably have a much higher impedance than hundreds of meters of metal service pipes buried in the ground.

  • The way to identify an ELCB is by looking for green or green and yellow earth wires entering the device. They rely on voltage returning to the trip via the earth wire during a fault and afford only limited protection to the installation and no personal protection at all. You should use plug in 30mA RCD’s for any appliances and extension leads that may be used outside as a minimum.


  • ELCBs have one advantage over RCDs: they are less sensitive to fault conditions, and therefore have fewer nuisance trips.
  • While voltage and current on the earth line is usually fault current from a live wire, this is not always the case, thus there are situations in which an ELCB can nuisance trip.
  • When an installation has two connections to earth, a nearby high current lightning strike will cause a voltage gradient in the soil, presenting the ELCB sense coil with enough voltage to cause it to trip.
  • If the installation’s earth rod is placed close to the earth rod of a neighboring building, a high earth leakage current in the other building can raise the local ground potential and cause a voltage difference across the two earths, again tripping the ELCB.
  • If there is an accumulated or burden of currents caused by items with lowered insulation resistance due to older equipment, or with heating elements, or rain conditions can cause the insulation resistance to lower due to moisture tracking. If there is a some mA which is equal to ELCB rating than ELCB may give nuisance Tripping.
  • If either of the earth wires become disconnected from the ELCB, it will no longer trip or the installation will often no longer be properly earthed.
  • Some ELCBs do not respond to rectified fault current. This issue is common for ELCBs and RCDs, but ELCBs are on average much older than RCB so an old ELCB is more likely to have some uncommon fault current waveform that it will not respond to.
  • Voltage-operated ELCB are the requirement for a second connection, and the possibility that any additional connection to earth on the protected system can disable the detector.
  • Nuisance tripping especially during thunderstorms.


  • They do not detect faults that don’t pass current through the CPC to the earth rod.
  • They do not allow a single building system to be easily split into multiple sections with independent fault protection, because earthing systems are usually use common earth Rod.
  • They may be tripped by external voltages from something connected to the earthing system such as metal pipes, a TN-S earth or a TN-C-S combined neutral and earth.
  • As electrically leaky appliances such as some water heaters, washing machines and cookers may cause the ELCB to trip.
  • ELCBs introduce additional resistance and an additional point of failure into the earthing system.

 Can we assume whether Our Electrical System is protected against Earth Protection or not by only Pressing ELCB Test Switch?

  • Checking the health of the ELCB is simple and you can do it easily by pressing TEST Push Button Switch of ELCB. The test push-button will test whether the ELCB unit is working properly or not. Can we assume that If ELCB is Trip after Pressing TEST Switch of ELCB than your system is protected against earth protection? Then you are wrong.
  • The test facility provided on the home ELCB will only confirm the health of the ELCB unit, but that test does not confirm that the ELCB will trip when an electric shock hazard does occur. It is a really sad fact that all the while this misunderstanding has left many homes totally unprotected from the risk of electric shocks.
  • This brings us or alarming us to think over second basic requirement for earth protection. The second requirement for the proper operation of a home shock protection system is electrical grounding.
  • We can assume that the ELCB is the brain for the shock protection, and the grounding as the backbone. Therefore, without a functional grounding (Proper Earthing of Electrical System) there is totally no protection against electrical shocks in your house even if  You have installed ELCB and its TEST switch show proper result. Looking after the ELCB alone is not enough. The electrical Earthing system must also be in good working order for the shock protection system to work. In addition to routine inspections that should be done by the qualified electrician, this grounding should preferably be inspected regularly at shorter intervals by the homeowner and need to pour Water in Earthing Pit at Regular interval of Time to minimize Earth Resistance.

 Current-operated ELCB (RCB):

  • Current-operated ELCBs are generally known as Residual-current devices (RCD). These also protect against earth leakage. Both circuit conductors (supply and return) are run through a sensing coil; any imbalance of the currents means the magnetic field does not perfectly cancel. The device detects the imbalance and trips the contact.
  • When the term ELCB is used it usually means a voltage-operated device. Similar devices that are current operated are called residual-current devices. However, some companies use the term ELCB to distinguish high sensitivity current operated 3 phase devices that trip in the milliamp range from traditional 3 phase ground fault devices that operate at much higher currents.

  • Typical RCB circuit:

  • The supply coil, the neutral coil and the search coil all wound on a common transformer core.
  • On a healthy circuit the same current passes through the phase coil, the load and return back through the neutral coil. Both the phase and the neutral coils are wound in such a way that they will produce an opposing magnetic flux. With the same current passing through both coils, their magnetic effect will cancel out under a healthy circuit condition.
  • In a situation when there is fault or a leakage to earth in the load circuit, or anywhere between the load circuit and the output connection of the RCB circuit, the current returning through the neutral coil has been reduced. Then the magnetic flux inside the transformer core is not balanced anymore. The total sum of the opposing magnetic flux is no longer zero. This net remaining flux is what we call a residual flux.
  • The periodically changing residual flux inside the transformer core crosses path with the winding of the search coil. This action produces an electromotive force (e.m.f.) across the search coil. An electromotive force is actually an alternating voltage. The induced voltage across the search coil produces a current inside the wiring of the trip circuit. It is this current that operates the trip coil of the circuit breaker. Since the trip current is driven by the residual magnetic flux (the resulting flux, the net effect between both fluxes) between the phase and the neutral coils, it is called the residual current devise.
  • With a circuit breaker incorporated as part of the circuit, the assembled system is called residual current circuit breaker (RCCB) or residual current devise (RCD). The incoming current has to pass through the circuit breaker first before going to the phase coil. The return neutral path passes through the second circuit breaker pole. During tripping when a fault is detected, both the phase and neutral connection is isolated.
    • RCD sensitivity is expressed as the rated residual operating current, noted IΔn. Preferred values have been defined by the IEC, thus making it possible to divide RCDs into three groups according to their IΔn value.
    • High sensitivity (HS): 6- 10- 30 mA (for direct-contact / life injury protection)
    • Standard IEC 60755 (General requirements for residual current operated protective devices) defines three types of RCD depending on the characteristics of the fault current.
    • Type AC: RCD for which tripping is ensured for residual sinusoidal alternating currents

Sensitivity of RCB:

  • Medium sensitivity (MS): 100- 300- 500- 1000 mA (for fire protection)
  • Low sensitivity (LS): 3- 10- 30 A (typically for protection of machine)

Type of RCB:

Type A: RCD for which tripping is ensured

  • for residual sinusoidal alternating currents
  • for residual pulsating direct currents
  • For residual pulsating direct currents superimposed by a smooth direct current of 0.006 A, with or without phase-angle control, independent of the polarity.

Type B: RCD for which tripping is ensured

  • as for type A
  • for residual sinusoidal currents up to 1000 Hz
  • for residual sinusoidal currents superposed by a pure direct current
  • for pulsating direct currents superposed by a pure direct current
  • for residual currents which may result from rectifying circuits
    • three pulse star connection or six pulse bridge connection
    • two pulse bridge connection line-to-line with or without phase-angle monitoring, independently of the polarity
    • There are two groups of devices:

Break time of RCB:

  1. G (general use) for instantaneous RCDs (i.e. without a time delay)
  • Minimum break time: immediate
  • Maximum break time: 200 ms for 1x IΔn, 150 ms for 2x IΔn, and 40 ms for 5x IΔn
  1. S (selective) or T (time delayed) for RCDs with a short time delay (typically used in circuits containing surge suppressors)
  • Minimum break time: 130 ms for 1x IΔn, 60 ms for 2x IΔn, and 50 ms for 5x IΔn
  • Maximum break time: 500 ms for 1x IΔn, 200 ms for 2x IΔn, and 150 ms for 5x IΔn

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 13 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

62 Responses to Working Principle of ELCB and RCB

  1. Dharminder says:

    Thank a lot Sir,

  2. GEE says:

    any body have note about relay calibration?? like over current relay, earth fault relay, earth leakage relay. i want to learn using secondary injection test. and i must know about its calculation….. please….

  3. GEE says:

    anybody can teach me about testing of TX Ratio using variable???

  4. mkl says:

    very very good but more drawings are needed please for the answeres and other places.

  5. keith says:

    My ELCB at home trips every day when the water heater runs for some time. What is the solution to fix this problem?

  6. Sankalp says:

    Why does ELCB in generator is connected to Y phase while overcurrent protection in R and B phases?

  7. sanjaykumar says:

    a lot of thank’s sir for a nice article

  8. manjunatha says:

    Do you have all this articles in one book ,

  9. Toink says:

    Its possible to installing ELCB 2 pole with 2 Phase (L1 and L2) ?

  10. santosh says:

    I want to know the working principle of ELCB,anybody can help me?

    • It is already explain in this Post!!

    • MOHANKUMAR. C says:

      Voltage-ELCBs were first introduced about sixty years ago and Current-ELCB was first introduced about forty years ago. For many years, the voltage operated ELCB and the differential current operated ELCB were both referred to as ELCBs because it was a simpler name to remember. But the use of a common name for two different devices gave rise to considerable confusion in the electrical industry. If the wrong type was used on an installation, the level of protection given could be substantially less than that intended. To ignore this confusion, IEC decided to apply the term Residual Current Device (RCD) to differential current operated ELCBs. Residual current refers to any current over and above the load current

  11. Muzi says:

    Really good information for understandind elcb and rcd

  12. Gejo Raphy says:

    Thanks Mr. Jignesh

  13. B.S.Khan says:

    ELCB meant for human safety or equipment safety please explain.

  14. zarul ashraf says:

    what the Similarity elcb and rcd.

  15. A.MANJUNATH says:


  16. A.MANJUNATH says:

    Every time there is a heavy rain, there is a power surge in our area and and which ever equipment is ON like fan, cfl tubes gets damaged. and this has happened several times.. Local electricians say it is due to neutral and phase wire fault at main the supply line.. In this case which ELCB will help. This is a regular problem in several houses in our area…

  17. t.k.nayyar says:

    I like this article.well explained

  18. dushyant kumar says:

    good explen about RCCB & ELCB

  19. jay says:

    Does a portable generator need an ELCB? Are there ELCBs for 400Hz power?

  20. Mohammad says:

    Useful info. Thanx a lot.

  21. russxy says:

    How does 4P RCD works? Red , Yellow & Blue have currents and may not be balanced. The return current is zero.

  22. RAFI AYYARIL says:

    OVR/UVR Controller will protect any loose connection issues on the DB?

    • John Williams says:

      My 3 phase RCB (100-milli amps) trips when the electrician shorts the phase and earth in a power socket, but it does not trip when the neuter and earth is shorted and the electrician says it is a defect of the wiring. Is it true?

  23. lalit says:

    AC area count by Tr capacity but how we can count it?

  24. vinod says:

    Very very useful info.
    I have legrand rccb 2 pole with 25A switch. which has 4 holes. How to do wiring ? please help me?. i have bought for home used (1 bhk)

    • 2 Nos for out going 2 Nos for incoming..for single phase supply…
      Please concern any electrician if you do not have basic idea of electricity.
      Don’t take any rick or experiment because it may be resulted in Fatal Accident…

  25. Tapas Barua says:

    Dear Mr. jignesh,

    Good morning. I read your detailed explanation of ELCB. Many many thanks for effective explanation. But i am confused about the operation of 4 pole ELCB. This ELCB operates on the residual current flowing through three phases i.e. vector sum of three currents of three phases.But in our electrical system some single phase loads are included. As a result the vector sum of currents is not equal to zero which leads to the tripping of 4 pole ELCB. Is it possible to install the 4 pole ELCB in that case? it is very helpful for us if you kindly give advice us.

    Thanks and Best regards

    Tapas Barua.

  26. Gautam Y. Pathak says:

    I have a three story residential building with 20 KW load equially distributed on all floors. I have a three phase connection (three phase and one neutral). I am connected each phase and a common neutral in each floor. Do I need THREE ELCB on each phase. Any one, please advise. Thanks.

  27. Manohar M says:

    For 2 kw heater which ELCB can I Use 30ma or 100ma please let me know.

  28. jagannath nayak says:

    if i change a circuit from raw power to emergency and connect only phase and neutral of raw power to phase and neutral of emergency do not connect earth of raw power to emergency then can ELCB trip please suggest me ASAP thank you

  29. Leonard says:

    This kind of discussions really build confidence and expansion of knowledge. keep it up

  30. Hameed. m.S. says:

    Mr. Jignesh,
    I want to get a solutions from you. There is one 4P ELCB on the main incomer line. Test button works of the same as tripping. Phases, R, Y, B testing by test lamps with Earth also tripping the ELCB. But when we are testing N and Earth of the outgoing side of ELCB not tripping. Why? what is the reason for not tripping when shorting with N and E?
    Please let me know.

  31. James Selvarai says:

    Very good info and advice keep up the good work.

  32. rajeev singh says:

    Thanks sir

  33. Pravin Sawant says:

    Can ELCB gives us protection from electric shock, means what happen, if we touch live wire which is not a part of elcb circuit. and what is proper position of elcb in home wiring (in SLD)

  34. HERTZ says:

    How the rccb work as the neutral is always zero potential. ?

  35. Gift Matchado says:

    nice one

  36. Dear Jignesh
    I am working as a Distribution engineer in one of the leading distribution company of Gujarat.
    I have a query regarding the operation of RCCB. We want to incorporate the feature of RCCB as per IS13947 (Part-2):1993 Annexure-B in our LT Distribution box for 25KVA to 100KVA transformers. Our main intention is to isolate the circuit instantly on occurrence of ground fault, such a snapping of LT line conductor. However, we anticipate that considering the sensitivity of RCCB, (which is maximum upto 30Amp), it will result in to nuisance tripping also if there is leakage current on consumer side. Moreover, it is also anticipated that in some one is using the load through earth (without using the system neutral to prevent the energy recording in meter) in such case also, RCCB will trip. Looking to the huge numbers of connections connected on 100KVA transformers, it would not be possible for field engineer to identify such dishonest consumer in shortest possible time and this will affect large numbers of honest consumers connected on that transformer.

    Will you provide some technical solution to this problem..? or suggest some other alternative..?

    Moreover, please also suggest the what should be the maximum Rated Short time withstand current for Distribution box for 25KVA, 63KVA and 100KVA transformer.

  37. Dhir Chandan says:

    dear jignesh g iam feel very glad to be connect with u or learn somthing from u ,thanks a lot again .

  38. raju says:

    ensure used light

  39. Mahesh says:

    We have 10 light fixtures (LED Fixtures) connected in one circuit and connected through 30 mA RCCB. The neutral to earth voltage is 3 volts and we have frequent tripping issues of the RCCB. Is it this high neutral to earth voltage causing the RCCB tripping? Please explain.

  40. Farid Ahmed says:

    dear sir ,
    i have a question how i can connect ELCB in a panel as in coming breaker where magnetic contactor of 220 volt is operating and tripping the ELCB.
    Abul Basher

  41. Rishik kareer says:

    Charging mobile with UPS power ll lead to RCCB triping? if yes please explain

  42. tarak says:

    dear sir ,can i use ELCB FOR 125A INDUSTRIAL LOAD ,PL SUGGEST

  43. Dilip Rane says:

    Can ELCB or RCCB protect against floating neutral?

  44. kumaresan says:

    Dear sir,
    we need minimum 5A breaker, when we go to ELCB/ RCCB its starting range is 25A. In this condition can i use 25A RCCB. if can doesn’t it damage our components (LED Display) when there is over current flow?

  45. senthil says:

    Very useful messages, thank you

  46. Kelvin Moodley says:

    Good explanation, thanks. Could I swop the, is it safe to swop the inputs with load.

  47. Arafat mukiibi says:

    Am a little bit different on the topic,but would want to know in case if i need your assistance, can i be helped

  48. Fazal says:

    What is the reason to used 30ma(RCD), why not use more than 30ma

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