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Tuesday, 18 February 2014

What are ELCB, RCCB, RCBO and RCD's?

Residual Current Circuit Breaker
RCD stands for Residual Current Device and this term can cover a multitude of devices with other acronyms such as ELCB, RCCB, RCBO and RCD. I will try to explain in this post what they are, where they might be used, what they detect and how they detect it. For a more blatant and pithy comparison of any combination of two of the above see my post "Difference FAQ...."


  1. ELCB  (Earth Leakage Circuit Breaker)

  2. RCCB  (Residual Current Circuit Breaker)

  3. RCBO  (Residual Current Breaker with Overload)

  4. So where does the current go, is it leaking to earth?

  5. So how does an RCCB or RCBO, detect that rogue current is leaking out? 

  6. RCD  (Residual Current Device)

  7. What are passive/active or latching/non-latching RCD's?

Earth Leakage Circuit Breaker (ELCB), Residual Current Circuit Breakers (RCCB) or Residual Current Breaker with Overload (RCBO), are frequently found in the mains switchboards or consumer units.  They protect the consumer supplied sockets and lighting from a specific fault condition such as a leakage of current to earth.  These units will trip when the device detects small amounts of stray current.  The most common trigger value is 30mA as it is also stipulated in many wiring regulations for domestic applications. Demonstrating how sensitive they can be, it is possible for these devices to trip before the cable insulation has been fully compromised by a physical short between conductors.

In fact it is a legal requirement in countries, such as the UK, for all mechanically unprotected (no armour) internal wiring circuits to be protected by a residual current detecting device as their first line of protection.

ELCB  (Earth Leakage Circuit Breaker)

An ELCB comes in two types: First the rare and now phased out voltage operated (VOELCB), which required a third connection to an associated earth terminal. The operational requirements for this to trip made it unsuitable in many situations. The earthed connection was used to detect any voltage difference between the earth and neutral indicating that there was a current flow into the earth being monitored.
  • If the fault current from the protected circuit was to a different earth or the monitored earth was connected by supplemental bonding, to another low resistance equipotential bonding point, then the VOELCB may not trip correctly.

The second version is the current detecting version which is now more commonly known as an RCCB.

RCCB  (Residual Current Circuit Breaker)

Residual Current Circuit Breaker
Residual Current Circuit Breaker
The RCCB detects a current difference between the live and neutral conductors.  This current is known as a residual current. Correctly functioning circuits will always have a balanced live and neutral current and a residual current only occurs when there is a fault or potential shock hazard. It is an RCD class of product and the Circuit Breaker (CB) part of its name indicates that it sits within the consumer unit with other circuit breakers.
  • The current imbalance is usually detected by directing the current path for both the live and neutral conductors to wrap around a toroidal transformer contained within the RCCB body.  The generated magnetic flux is directly proportional to the current drawn through either the live or neutral conductors. The windings are constructed so the live and neutral currents induce magnetic fields in opposite directions. The balanced magnetic fields cancel each other out, making a net zero magnetic flux within the toroid. Even a small difference in current will result in a net magnetic flux greater than zero. When the net flux is proportional to the leakage current trigger point, the unit will trip out.

The RCCB cannot, and should not, be used on its own as it only provides detection for residual current (previously known as earth leakage) detection.  They often have a current rating but this only indicates the level of current the RCCB can handle. This should not be confused with the current rating of an MCB which gives the trigger current it will trip out at.

When looking at RCCB labels, an RCCB may be described as:  2P 230v 80A 30mA

  • 2P == 2 Pole - meaning it switches both the live and neutral.
  • 230v == The rms voltage rating of the device.
  • 80A == The rms current handling of the device.  - Meaning the maximum rated current it is safe to use with it. (Not the trip current)
  • 30mA == The maximum differential current between the live and neutral circuit before it will trip (leakage/residual current)

Note that RCCBs work not only at a specific voltage but also at a specific frequency of the supply. For example an RCCB rated for 230v @ 50Hz would not be suitable for 120v @ 60Hz installation and neither visa versa.

  • It is a common mistake to assume an RCCB provides overload protection. It's really a supplement to the overload protection provided for each individual circuit. Due to the expense of an RCCB they tend to be used to protect banks of circuits where each individual circuit would have its own overload protection.  Usually this would be in the form of a fuse or resettable fuse such as the relatively modern Miniature Circuit Breaker (MCB).

  • The down side is that when the RCCB trips, all subsequent circuits are powered off.

  • In the past, to protect a single circuit would require the purchase of one RCCB and one MCB fitted in a separate consumer unit or the rewiring/replacement of the original consumer unit to include an RCCB. With the development of RCD technology, if you need combined overload and residual current protection you now have the option to fit an RCBO (Residual Current Breaker with Overload). See below.

RCBO  (Residual Current Breaker with Overload)

The full name of an RCBO is a residual current circuit breaker with overload protection.

So what is an RCBO? It is a combination of a residual current device and a miniature circuit breaker in one package.

Voltex dual pole RCBO internals - Made for Australian market
Voltex DP RCBO for Australian market
When a current leakage fault occurs the internal residual current detecting element will trip the whole circuit.  Equally, if the circuit is overloaded the internal thermal/magnetic circuit breaker parts are able to trip the device in the same way. RCBO’s come in dual pole and single pole variations. The single pole was intended as a direct replacement for an  MCB offering an upgrade path for the additional residual current protection. Essentially by retrofitting to an old consumer unit.

  • Note with a single pole device you still need to feed through the Neutral conductors of the circuit but only the Live conductor is actually switched.
  • Although the RCBO can be considered an upgrade it is important to ensure that it will actually fit the consumer unit in question.  The best way is to ensure that it is the same series and manufacturer of the CU and MCBs it is to replace. In the UK, although all breakers must fit a standard "din" rail in the consumer unit, sadly the UK regulations do not specify that the sizing or location of the busbar connectors are the same.  Consequently there are literally dozens of types of MCB with questionable or no compatibility with other makes and models.

It's a common mistake to confuse RCCB with an RCBO (Residual Current Breaker with Overload). The full name of an RCBO is a residual current circuit breaker with overload protection.

  • For many years, I too assumed that the 80A marking on the RCCB within my consumer unit was the overload trip current. It is not, the 80A refers to the maximum rated (continuous) operating current of the device for safe operation. These are totally different things. If I somehow managed to draw 90A through the device it would not trip out while an 80A RCBO would trip.

When looking at RCBO labels, an RCBO may be described as:   SP 230v 32A 30mA Type B

  • SP == 1P == 1 Pole - meaning it switches the live only.
  • 230v == The rms voltage rating of the device.
  • 32A == The trigger rms current rating of the device.  - Meaning the overload trip current
  • 30mA == The maximum differential current between the live and neutral circuit before it will trip.
  • Type B == Meaning the overload current profile to cause a Trip.  This stops it being prone to trip during a surge such as switching on a large power amplifier, which could take way over 32A but for less than 20ms.
  • The values go from A to D where A is immediate and D the most reluctant.

So where does the current go, is it leaking to earth?

The detection circuitry merely detects a difference, it does not differentiate where the current is actually going (or coming) only the fact that it has gone astray.  Anyone familiar with an electricians screwdriver, with a neon indicator in the handle, will know that you don't actually need to be connected to anything for it to function.  The current is leaking/radiating into the air around you.  Similarly the current might go into another adjoining wire, in a live to live or neutral to neutral, of a completely different circuit (assuming they are not protected by the same RCCB or RCBO).

RCCB tend to protect banks of circuits in a domestic consumer unit while RCBO tend to protect individual circuits allowing for better granularity.

So how does an RCCB or RCBO, detect that rogue current is leaking out?

Detection is simply a balance check of the current between the live and neutral lines.  In normal conditions, the current between the live and neutral will be identical.  The RCCB usually uses a magnetic balance technique, through a transformer, to detect any difference between these currents.  The transformer is wound and connected so that the flux generated by the currents in the live and neutral conductors cancels out. This method allows for successful detection of leakage, or residual current, even when high currents are being drawn through the circuit being protected. This design inherently compensates for sudden changes in load that may occur normally. Any difference in current between the conductors will result in a net flux greater than zero. It is this resultant, net flux that electromechanically triggers the trip action of the circuit breaker.

The leakage current trigger point can come in several values but the most common is 30mA.

The RCCB portion of an RCBO is usually electronic, rather than electromechanical. This allows for a much smaller form factor, enabling the RCBO to fit into the tighter space requirements of an MCB, in a typical consumer unit.

  • An additional note is that it is only a difference in current that is detected. So, a fault condition can include a leakage into or out of the protected circuit. Anything that causes an electrical imbalance is considered a fault, not the direction of the leakage!

RCD  (Residual Current Device)

Technically, all the above are Residual Current Devices (RCD's), as it is an umbrella term for this type of protective technology. In day to day language, the term RCD generally refers to the power socket level implementation of RCD technology.  These devices can either be built into a power socket or alternatively, they plug between the power socket and the plug of the appliance to be protected.  This application of RCD differs from the RCBO in that it does not trip at a particular current overload and so is more akin to an RCCB.

The RCD would usually be rated at the maximum current of any countries socket. In the UK it would be 26A for a built in double socket  or 13A for an extension or plug in unit .  The current rating will usually be written on the manufacturer's product for the country in question.

  • In the UK the consumer appliance's cabling from the residential socket is protected by a fuse within the plug. Commonly there is no fuse in the RCD and it is rated at the same rating of the socket.

What are passive/active or latching/non-latching RCD's?

RCD's come in latching (passive) and non-latching (active) versions. The difference is that if there's a mains power failure, the non-latching version will trip out and remain so until manually reset by the user.  The latching RCD remains set if the supplied power fails, which allows the power to be immediately available to the appliance on resumption.

  • Non-Latching RCD’s are good for power tools or where a supervised starting procedure is necessary, such as most (but not all) computers. 

    • Note that the operating trigger is held by an electromagnet so a small amount of power is required to hold it in the on state.  This is why it is also known as an "Active" RCD.

  • Latching RCD’s are good where you have devices that are power critical, such as a freezer or security lighting.

    • Note that if during a power outage a leakage fault condition occurs, a latching RCD will not trip. When in a fault condition, only the return of power will enable it to trip as a latching RCD requires power to operate its trigger. This is also known as a "passive" RCD, as the operation mode does not require power only the triggering action.

    • Note that RCBO's and RCCB's tend to be passive (or Latching) RCD's

For further reading an references: Please read my resource page.


  1. Very helpful article about difference between rccb and rcbo !

  2. very helpful one.. Thank you

  3. Hi, my father's house has a three phase(4 wire) supply but no earthing is there in the house.
    Will installing a RCCB provide any protection to people and appliances?

    What I really want to know is will an RCCB ( without earthing) detect the disconnection of neutral wire outside the house or from the electricity meter which is also fitted outside the house and trip?

    1. Hi SS, This is too much of an open-ended question to give a really satisfactory answer. I am not sure which country you are from so please note that your wiring regulations take precedents and this answer is just my opinion and does not constitute advice. You must enlist the services of a qualified professional if your regulations stipulate it, but generally it is a good idea to use their services, regardless.

      "will an RCCB ( without earthing) detect the disconnection of neutral wire outside the house or from the electricity meter which is also fitted outside the house and trip?"

      Answer is: No, an RCCB cannot detect a floating neutral which has occurred on the distribution side of the RCCB rather than the consumer or appliance side. RCCBs are designed to detect residual current faults, which are not floating neutral faults. If you had a floating neutral on the appliance side, and it managed to get earthed, the RCCB would trip but only due to the leakage. It would not trip because of the fact the neutral was floating or high resistance.

      Your scenario of your father's house having a 3 phase, 4 wire system with no earth asks many questions to the age, set-up, configuration of your father's house. I would suggest that you ask either the metering company/distributor/supplier which earthing arrangement it is listed as possessing. Is is an IT or TT installation? Certainly in the UK I would expect it to be at least a TT installation for a domestic dwelling. If so then I believe the international standard IEC 60364 makes the RCD protection a requirement.

      Assuming that there was no protective earth conductor it would be difficult to guarantee that a rogue voltage or current would not be brought into the building by an extraneous path seen or unseen. Therefore you would still need to electrically bond pipes and accessible building metalwork. If so, fitting the RCCB will definitely offer protection to people and appliances.

      Just a note that the best way to detect a floating neutral is to check for wandering phase voltages. In a perfectly loaded 3 phase system, the net voltage of the neutral is zero volts. When asymmetrically loaded, the phase voltages will wander to make the net current in the neutral zero. For a correctly attached neutral the current will be conducted back to source, preserving the general symmetry of the phase voltages.

  4. Hi,

    I am quite electrical illiterate and the above info is much helpful but still i have doubt.

    At present My house wiring (single phase line) have good earthing and simple two fuse. There no any other switches such as MCB, ELCB etc etc. we never habe any problem with appliances or current leakage but since i came across this article, i would like to add safety.

    So what you suggest to install? RCBO only or something else? *note it is small family house with 4 Fan, 6 Tubelights, 1 42' Led, 1 Fridge and one washing machine.

    I am limited with budget but at same time i don't want to compromise safety.

    Thank you for your help and keep up informing people like me.

    1. Hi Jignesh,

      I am glad you have gained some insight from the information contained in this website and it has encouraged you evaluate the safety of your installation at home. Consideration to your safety, your family's, friends and property should always be paramount. Sadly, I cannot provide advice remotely and in reality it would be dangerous for me to do so. There are too many scenarios which could mean that advice is inappropriate for your circumstances.

      You have not said what voltage your country uses but I cannot express enough the potential danger in mains electricity and if you do not know, are not sure, or just generally puzzled; you must use a qualified electrician. In certain countries it is mandatory that any electrical work is carried out by a qualified professional. I am a strong believer that, "knowledge is power." But this maxim must be countered by the equally relevant maxim of, "A little knowledge is a dangerous thing." It is easy to be seduced into a false sense of security by attaining some knowledge but without a full understanding of the wider implications. The danger of serious injury and death by electrocution or fire because of bad construction or poor installation is a very real and present danger.

      What I do suggest is that you consider several points. This is not an exhaustive list.

      1 Are the RCBO RCCB MCB readily available to you?

      2 If so do they have a manufacturers guarantees as well as standard accreditation relevant to your country?

      3 Purchasing fake or substandard goods is a recipe for disaster, so avoid this route even if it is cheaper.

      4 Goods with the correct standard and accreditations are not necessarily expensive, especially if they are the preferred choice by local professionals. Often the high turnover causes the purchase cost to reduce.

      5 You may find that fitting RCCB or RCBO may require substantial rewiring especially if your wiring is old and misspecified. So find out what the job will entail by talking through the job with the electrician.

      6 If you only need to rewire the consumer unit or breaker panel then consider options where you can purchase a system/package relevant to your country. Usually they are the box, switches and trips already populated. If you go down that route, make sure that you can get hold of replacements readily and cheaply. Sometimes these consumer unit packages are cheap to buy initially but then it is near impossible to find a replacement if something fails. Such an event could result in a large expense of having to buy and wire in another consumer unit and trips.

      7 At this moment you are making modest demands on your electricity supply so it is worth considering if you should take this as an opportunity to think ahead. For example wire in such a way that future expansion would be easy to implement.

      8 Installation of an RCD in the form of an RCBO or RCCB can show up faults which you never knew existed so be prepared. What may initially appear to be a simple rewire ends up turning into a major fault diagnostic exercise chasing a "neutral earth" fault. Having a good qualified professional should allow you to quickly isolate such a fault if it exists on your installation.

      RCBO are good as you have a 30ma leakage window per circuit for each RCBO fitted.
      RCCB (old electromechanical) are good as their price has plummeted and they are tried and trusted technology plus they tend to be dual pole.

      The RCBO can be more expensive but if you only have two circuits such as one for lighting and one for appliances then if the appliance trips you don't loose the lighting circuit.

      As RCCBs tend to protect banks of circuits, one trip results in all subsequent circuits being powered down.

      As far as the best choice for your installation enlist the advice of a local qualified professional they really are worth the money.

    2. Hi.
      I have 2P RCBO without N (ge DPC100)
      How do i wire these modules without N wire?
      How can it even work without N ???
      I bought these in hope that I can save space in my fusebox...

    3. If you do not have a supplier designated neutral for your supply and it is single phase, it would suggest that you have an isolated supply such as an IT mains. Neutral is normally the phase line which is referenced to earth at some point from the distribution transformer to your dwelling. Ignoring the AC conundrum, if you consider one end of the supply conductor as the feed and the other conductor as the return, then power comes from the feed and returns through the return conductors. A RCBO would need to be connected to the feed and return, which in most cases would be the live and neutral conductors. It is your distribution supplier who would designate which is the live and neutral as it is their responsibility to connect the mains distribution cables as they see fit. As long as the RCBO, or any residual current device, is connected so that it is in circuit then it will detect a residual current.

      Note, with a purely isolated supply there would need to be at least two faults. A scenario such as one fault at the distribution end and one on the consumer side circuitry, for it to detect a residual current fault. Similarly if there is a fault between an RCBO protected circuit and another fault on a different circuit but not protected by the same RCBO and supplied from the same isolated supply then the RCBO will trip. Having only a single fault would not usually provide a sufficiently strong current path for the RCBO to trigger.

      Another possibility is that you're combining the 115v phases of a two phase system to make a 230v supply. Again the RCBO would be fitted in the fusebox to each phase line but not connected to the neutral, as the neutral would not be part of the supply circuit to the appliance. The neutral is still technically in circuit but provided by the distribution supplier and not connected at the RCBO. Assuming the neutral is connected to earth by the distribution supplier, then if either phase has a consumer side residual earth fault, the RCBO would trip correctly.

      So as described above, in such a specific scenarios the RCBO may not need to be connected to a neutral conductor for it to provide protection.

      I hope that you had advice in your choice? An RCBO usually has a neutral connection and the dual pole device is designed to switch both the feed and return in the event of a fault. Usually the feed is the live and the return is the neutral in a single phase set up.

      Please note that the form-factor of the RCBO device isn't necessarily compatible in all fuseboxes so it might not even fit properly if they are not the same make I found a Dutch language installation guide for the dpc100 which shows that it is supposed to be connected to feed and return. Please note that the supply (distribution side) and source (consumer side) need to be wired to the correct terminals for the device to operate as designed. If you cannot read Dutch I would suggest you request the English installation guide direct from General Electric as their internet presents may not be optimised for general use. If you're unsure about anything, even just slightly, seek professional advice as electricity is far too dangerous to just "suck it and see."