Circuit breakers for mains panel

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current, typically resulting from an overload or short circuit.

Its basic function is to interrupt current flow after a fault is detected. In small mains and low voltage circuit breakers, fault current is usually done within the device itself. Typically, the heating or magnetic effects of electric current are employed. Stotz’s invention (patented by Brown, Boveri & Cie in 1924) was the forerunner of the modern thermal-magnetic breaker commonly used in household load centers to this day. Thermal magnetic circuit breakers, which are the type found in most distribution boards, incorporate both techniques with the electromagnet responding instantaneously to large surges in current (short circuits) and the bimetallic strip responding to less extreme but longer-term over-current conditions.

Once a fault is detected, the circuit breaker contacts must open to interrupt the circuit. When a high current or voltage is interrupted, an arc is generated, and this situation must be handled properly. Low-voltage miniature circuit breakers (MCB) use air alone to extinguish the arc. These circuit breakers contain so-called arc chutes, a stack of mutually insulated parallel metal plates which divide and cool the arc. Circuit breakers are usually able to terminate all current very quickly: typically the arc is extinguished between 30 ms and 150 ms after the mechanism has been tripped. Circuit breakers are rated both by the normal current that they are expected to carry, and the maximum short-circuit current that they can safely interrupt.

The DIN rail-mounted thermal-magnetic miniature circuit breaker is the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe.


This type of circuit breakers are labeled with the rated current in amperes, preceded by a letter, B, C, or D, which indicates the instantaneous tripping current — that is, the minimum value of current that causes the circuit breaker to trip without intentional time delay. When supplying a branch circuit with more than one live conductor, each live conductor must be protected by a breaker pole in “common trip” breaker.

Typical domestic panel circuit breakers are rated to interrupt 10 kA (10000 A) short-circuit current.  International Standards, IEC 60898-1 and European Standard EN 60898-1, define the rated current In of a circuit breaker for low voltage distribution applications as the maximum current that the breaker is designed to carry continuously (at an ambient air temperature of 30 °C). The commonly available preferred values for the rated current are 6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A, 100 A,and 125 A.

One thing that needs to be sometimes taken into consideration also the power loss and heating generated on the circuit breaker. When fully loaded, the circuit breaker can generate several watts of power loss. For example Schneider document on calculating heat generated by molded case circuit breaker mentions that a circuit breaker manufactured by them generates/loses  7.9 Watts per pole. This causes energy loss, heating on the panel and some voltage loss.

The voltage loss is not typically issue on normal mains voltages, but when circuit breakers are used in low voltage applications, this needs to be taken into account (lower the current rating, typically more internal resistance and voltage loss at full load).


MCB Working principle

How it Works MCB

Thermal Magnetic Circuit Breaker Operation

Fuse vs Circuit Breaker │ Difference between Fuse and Circuit Breaker │

Circuit Breakers – How they Work, What’s Inside

How to Replace a Circuit Breaker By: Everything Home TV

Difference between MCB and MCCB

ABB Miniature Circuit Breaker clearing downstream fault

RCBO circuit breaker teardown and extreme overload test

Transparent Siemens Circuit Breaker

Transparent ABB Circuit Breaker


Links to more material:



  1. Tomi Engdahl says:

    Energy Regulations
    IEC 60898-1 and IEC 60947-2: a tale of two standards

    In response to the question “What’s the difference between standards IEC 60898-1 and IEC 60947-2?”, I’m tending to answer with another question: What do they have in common? They are both standards that specify requirements for low-voltage circuit breakers. Is there more in common? Let’s look into.

    IEC 60947-2, which I introduced at length in the first of these blog posts, governs CBs for industrial applications. They protect electrical power distribution of up to 1000 volts a.c. and 1500 volts d.c. with whole spectrum of rated current from 0,5 to 6300A. Utilities and manufacturing facilities use them: air circuit breakers (ACBs), molded case circuit breakers (MCCBs) and miniature circuit breakers (MCBs).
    As for IEC 60898-1, it relates to the a.c. low-voltage circuit breakers – MCBs – we find in our homes, schools, shops, and offices electrical distribution switchboards. The standard states that the top rated current is 125A, while the lowest is 6A and maximum value of rated short-circuit capacity (Icn) is 25kA.

    Many more differences and IEC 60898-1 and IEC 60947-2?

    Yes. The rated voltage currently required in industrial-use CBs is 440, 690 volts or higher. Compare those numbers to the 400V upper limit between phases for residential MCBs.

    Users confuse – particularly “prescribers”, e.g. those people who draw up specifications in invitations to tender. Procurement managers in utilities, too, can get things wrong. As to how they mix up the standards, I can’t say. But happen it does.

    Can IEC 60898-1 be used instead of IEC 60947-2?

    No. Consequences can be dire if residential CBs are used instead of industrial ones. An MCB designed for indoor, pollution-free conditions would be woefully inadequate for harsh, outdoor applications that require pollution degree 3.

    Typically IEC 60898-1 certified CBs meet minimally required performance to proof proper protection of household installations: Pollution degree 2, impulse voltage 4kV, isolation voltage is the same as nominal voltage 400V.

    Where should be used IEC 60898-1 certified CBs?

    These CBs are intended for use of indoor, pollution and humidity-free conditions: household or similar installations

    The most suitable solution for use are MCBs certified with both standards as their performance meets requirements of use for residential installations and high enough for use in industry and infrastructure applications.

    I can say is that mix-ups of standards happen.

  2. Tomi Engdahl says:

    Comparison of tripping characteristics for miniature circuit-breakers

    The requirements for “Protection for
    safety – Protection against overcurrent”
    are specified in IEC 60364-4-43.

    Miniature circuit-breaker are used for:
    −overload protection and
    −short circuit protection
    in electrical circuits as well as
    -protection against electric shock by automatic disconnection

    The combination of tripping curves of the electro-magnetic
    release and the thermal bi-metal release result in an overall
    tripping curve for overload protection.

  3. Tomi Engdahl says:

    Severe Overload – MCB and no MCB

    Test set up with an overhead line to a 20 Amp MCB protected piece of 1.5 mm² TPS cable.
    Later the MCB was bypassed to see what happened.
    The 1.5mm² TPS overheated rapidly and burnt out.
    A bolted short was made on the OH line.
    The overhead line 2.5mm² overheated rapidly and the pvc insulation melted as well as the line sagged.

  4. Tomi Engdahl says:

    Fuse – MCB or no Fuse [ and see what happens ]

    A test set up and visualize what happens when a rewireable fuse, MCB (miniature circuit breaker) or no fuse is fitted in an electrical circuit.
    Wire of 2.5mm² is used which is normally rated for 16 or 20 Amp power circuits in 240 Volts systems, depending how long the runs are.
    First test after the intro is a rewireable 15 Amp fuse which pops.
    Second test after the intro is with a 16 and a 32 Amp MCB which trip almost immediately.
    Then the protective device is bypassed and a 300 Ampere current is flowing making the 2.5 mm² wire acting as a fuse itself.

  5. Tomi Engdahl says:

    How the Regs are changing the Fuse Box we fit in your house.

    How Wylex are approaching the change to the Regs for domestic fuse boxes in UK

    single phase meter wiring diagram | energy meter | energy meter connection by earthbondhon

  6. Tomi Engdahl says:

    Types of MCB / Circuit Breaker, BCDKZ

    Different types of circuit breaker, tripping current and considerations when using them.

  7. Tomi Engdahl says:

    Consumer Unit Upgrade

    This video is only a rough display of what’s involved when replacing or upgrading a consumer unit. It doesn’t go into too much depth or show the final labelling, fire sealing with compound or testing.

  8. Tomi Engdahl says:

    Changing a total mess of a fuseboard – An Electricians Day in London

  9. Tomi Engdahl says:

    The SY2-D surge protector (SPD)

    Surge protection made easy for 18th Edition! Also available in (sober) article form at:

  10. Tomi Engdahl says:

    Torque Settings on Circuit Breaker (MCB) and How to Set a Torque Screwdriver up and use it

    Finding the correct torque setting on a Crabtree Starbreaker MCB and using a Wiha electricians torque screwdriver to correctly connect conductors at the required Nm torque setting. Video includes a full demonstration of setting up the torque screwdriver correctly and using it.


    How often do you calibrate your torque screwdrivers?
    How do you know it’s set right.
    What if the mcb is old.
    Torque screwdrivers are fine, if you’re working and on new equipment even then it doesn’t seem tight enough sometimes.

  11. Tomi Engdahl says:

    Circuit Breaker Panel Inspection & Common Problems

    Are you experiencing electrical problems? Would you like to learn how you can inspect your electrical panel to identify common problems as well as how you can correct them? If so, then this video is for you. I also explain how GFCI’s/Surge Arresters work, and show you useful test equipment.

    DISCLAIMER: If you’re not comfortable working with 120V/240VAC electrical wiring, or unaware of the dangers of working with 120V/240V wiring, then DO NOT attempt to work on your unit! Safety first! AC Mains Power has the potential to KILL or injure you.

  12. Tomi Engdahl says:

    electrical wiring-overloaded feeders

    Service call about main fuses blown.The main feeder had been taped with everything a person could find to keep the phase from shorting out

  13. Tomi Engdahl says:

    Electrical Panel Nightmare! (or not)

    In this video HHinspect owner Rick Belliveau shows you the GUTS of an electrical panel box and explains the secrets of healthy wiring job. You might want to stand back…. a little more…

    Sub Electric Panel Box Wired Incorrectly, Home Inspection in Austin, Texas

    Inspection of a residential sub electric panel box during a home inspection in Austin, Texas

  14. Tomi Engdahl says:

    how to repair replace broken circuit breaker – multiple Electric outlet not working – fuse box panel

    How To Change A Breaker Simple

  15. Tomi Engdahl says:

    Air Circuit Breaker (ACB) – Construction, Operation, Types and Uses

    A circuit breaker is used for switching mechanism and protection of the system. Other associated devises and components are also used for this purpose associated with circuit breakers like fuses, relays, switches etc. Circuit breakers are widely used in industries as well as power system for controlling and protection of different parts of the circuit like switch gears, Transformers, Motors, Generators/Alternator etc., which leads the system stable and reliable.

  16. Tomi Engdahl says:

    A look inside a British home electrical panel.

    Note that in this video the panel is new and has no external circuits connected yet. When wired in and active there is a lot of exposed live metalwork that poses a shock risk. Changing a consumer unit is not a simple DIY task due to the presence of a high current supply that poses a shock and burn hazard if touched or bridged.

    In the UK we have a really simple electrical system. Just a three phase system with 240V between each phase and neutral and 415V between any two phases. (230V/400V under European tolerance standards.)

    A typical home will get a single phase and neutral with the three phases spread amongst homes in a street, while a factory or commercial premises will usually get all three phases.

  17. Tomi Engdahl says:

    Typical Australian Domestic Switch board

    Just a view of the inside & backing board of a domestic Switch Board here in Sydney Australia.

    Inspection of the Switchboard is NOT part of a Pre Purchase Building Inspection as per our Australian Standard.

  18. Tomi Engdahl says:

    How to Replace / Change a Circuit Breaker in your Electrical Panel

    How to Change a Breaker

  19. Tomi Engdahl says:

    After some pondering, I concluded that for that short of a run from the service panel to the duplex receptacle, a 20A breaker shouldn’t have caused this. In disbelief, I pulled the datasheet for the 20A type THOL circuit breaker that was substituted. I noticed that at − 20°F ambient temperature, the let through current of the breaker is approximately 130%, meaning that the two heaters would have never tripped the breaker with its ambient temp referenced to the cold outside wall.

  20. Tomi Engdahl says:

    Inside a fake un-trippable circuit breaker.

    This is uninspiring. It’s a circuit breaker that looks and feels just like the real thing, but has no fault detecting ability at all. Why would somebody even make something like this?

    So an appropriate schematic would be two wires converging into a symbol of a person on fire? That one’s probably near the back of the ISO standards

  21. Tomi Engdahl says:

    Big Clive Tears Down an “Un-Trippable” Circuit Breaker, Finds a Dangerous Fake
    Externally identical to the real thing, this fake circuit breaker offers absolutely no protection whatsoever — despite a satisfying clunk.

  22. Tomi Engdahl says:

    Why won’t an MCB reset? What’s the difference between TYPE B and TYPE C MCBs?

    There are numerous reasons why it may not be possible to reset a miniature circuit breaker (MCB). Some are obvious others are related to the design of the installation and the electrical supply.

  23. Tomi Engdahl says:

    Your circuit breaker panel protects your home from problems caused by external power surges, circuit overload, and short circuits. A breaker cuts off the power to a circuit by tripping when it detects a circuit overload. If your circuit breaker panel fails, the electrical wires can get so hot that they start a fire.

    Unfortunately, this breaker failure happens every year in homes equipped with Federal Pacific Electric (FPE) Stab-Lok panels. Despite this information and the number of electricians and home inspectors who warn against this equipment, FPE Stab-Lok panels were never officially recalled.

    Millions of homes were built with FPE Stab-Lok panels between 1950 and 1990. If your home was constructed during this time frame, then it may contain one of these panels.

    Is It Safe to Own an FPE Stab-Lok Panel?

    The trouble with an FPE Stab-Lok panel is that it can function perfectly fine — until suddenly it doesn’t. It only takes one short circuit or overcurrent.

    CPSC recommends getting this type of panel inspected by a qualified electrician “to look for any signs of overheating or malfunction among the circuit breakers.”

  24. Tomi Engdahl says:

    Types of MCB / Circuit Breaker, BCDKZ

    Different types of circuit breaker, tripping current and considerations when using them.

    What is the difference between a type A and type AC RCBO or RCD?

  25. Tomi Engdahl says:

    Inside a fake un-trippable circuit breaker.

    This is uninspiring. It’s a circuit breaker that looks and feels just like the real thing, but has no fault detecting ability at all. Why would somebody even make something like this?

    These fake breakers weigh 53g (2oz) per module. Typical UK breakers weigh 100g (4oz) per module. But weight is not a guarantee of functionality.

    It’s made worse by the fact that there is no standard test to see if a circuit breaker is tripping at its rated current. There are specialist testers that use a low voltage transformer to test breakers, but they usually have to be removed from equipment for the test. A type C 32A breaker would also need to be tested at a minimum of 160A.

  26. Tomi Engdahl says:

    Smarter distribution boards with Acti9 Powertag – from Schneider Electric

    If you can measure it you can manage it. The Schneider Electric Powertag is a great solution to record, monitor and control power within commercial and industrial electrical installations.

    The Powertag modules fix directly to outgoing circuits and link wirelessly to enable circuit-level remote power monitoring.

    The system can be further expanded with input and output modules.

    Works with Acti 9 distribution boards.

  27. Tomi Engdahl says:

    A look inside a British home electrical panel.

    It is a pleasure to look at a DIN rail based British panel.

  28. Tomi Engdahl says:

    Inside a totally smoked circuit breaker. What went wrong?

    Initially I thought this breaker may have suffered heat damage from a loose connection, but it’s clearly been much more serious than that.

  29. Tomi Engdahl says:

    MCB & Panel Testing Made Easier

    Breaker Testing using HEX Stabs & RAPTOR. Why remove & replace if you are only Testing? Inject Directly Into the Breaker Lugs.

  30. Tomi Engdahl says:

    Learn How to Conduct Circuit Breaker Testing Using the ISA CBA1000

  31. Tomi Engdahl says:

    3 Station Calibration & Verification Test Bench for MCB

  32. Tomi Engdahl says:

    What’s the difference between an RCD and an RCBO? – Circuit protection.

  33. Tomi Engdahl says:

    Overload Protection vs Short Circuit Protection? |Overcurrent Explained

  34. Tomi Engdahl says:

    MCBs, how do they work?

    Let’s see working of an MCB in detail.

  35. Tomi Engdahl says:

    The table below gives the watts loss per pole at rated current.
    MTN Electrical Characteristics.
    MCB Rated current (A) 0.5 1 2 3 4 6 10 13 16 20 25 32 40 50 63
    Watts loss per pole 1.2 1.3 1.5 2.0 1.8 1.4 1.9 2.1 2.5 2.8 3.2 3.8 4.0 4.5 5.1

  36. Tomi Engdahl says:

    #TBT: Even the power grid needs circuit breakers. Giant giant circuit breakers.


    Tomorrow’s megavolt transmission lines need breakers that can withstand titanic forces

    The Guardian: KEMA Laboratories tests a circuit breaker under extreme conditions to ensure it won’t fail when it really matters.

    A BLAZINGLY HOT DAY IN CENTRAL CHINA, when all the air conditioners in every megacity are running at full blast. Through the remote mountains of Shanxi province, the major transmission lines that carry ultrahigh-voltage electricity to the cities are operating at close to maximum capacity. Heated by the sunshine and the flowing current, the transmission lines sag dangerously close to the treetops. Suddenly the current jumps from line to tree branch, finding the path of least resistance and pouring through the tree into the ground. There’s a bright flash as the current ionizes the air.

    During this short circuit, the abruptly unleashed current reaches 10 to 20 times its normal level within a blink of an eye. Now the power grid’s protection system must act fast. Within milliseconds, protection relays must recognize the fault and command the circuit breakers at both ends of the line to switch off the current, isolating the faulted line. The stakes are high: A sustained short-circuit current can trigger a chain reaction of failures throughout the grid and cause widespread blackouts, severely damaging expensive equipment in the process.

    just separating the contacts doesn’t stop the current. Instead, the current creates an electrical arc inside the breaker. That small space, which has a volume of just a few liters, now contains a roiling plasma that may reach temperatures of many thousands of degrees Celsius. The breaker can’t contain that plasma for long; if it’s not cleared away quickly, there will be a terrible explosion.

    Now the alternating nature of the AC current comes into play: Each time it changes direction (every 10 milliseconds in China’s 50-hertz system), the current temporarily becomes zero, and the energy supply to the arc plasma momentarily halts. It’s at one of these “current zero” moments that the fault current must be interrupted. At that crucial moment, a cooling system inside the circuit breaker injects a high-pressure jet of gas into the gap, blasting away any residue of the hot arc plasma.

    Immediately after the arc disappears and the fault is cleared, the power system ramps up again. In this recovery process, the voltage across the gap steeply rises to over 1 million volts before settling to its normal operational level.

    So in the microseconds before and after current zero, the contacts need to change over from channeling approximately 50 kiloamperes of current through the arc plasma to withstanding 1 megavolt of voltage. This rapid change puts enormous strain on the breakers’ components.

    Yet the circuit breakers must perform flawlessly, because the transmission line needs to go back into operation. They must work even though they may have been inactive for long stretches of time and through all kinds of weather.

    Tomorrow’s power grid will likely rely on large-scale renewable energy facilities such as hydropower plants, solar parks, and offshore wind farms, located far from power-hungry cities. To transport that energy across long distances, system operators are planning and constructing massive transmission lines. These lines must be high voltage, so they’ll lose only a small fraction of energy through resistance in the lines. Building these cutting-edge high-voltage systems is quite expensive. But many power companies are deciding that the ability to move huge amounts of energy across vast distances justifies the costs.

    Choosing to construct a high-voltage transmission system is the first step. The next step is to decide: DC or AC? High-voltage DC transmission systems are an increasingly attractive option, as DC overhead transmission lines require less space and lose less power than do AC lines. But AC technology is more mature, and the world’s most powerful transmission systems are still designed for AC. The latest AC supergrids use ultrahigh voltage (UHV) of at least 1,000 kilovolts, a staggering level not yet realized in DC.

    In UHV transmission systems, the most crucial piece of technology is the circuit breaker. The breaker is the system’s guardian: It must be eternally vigilant and prepared to act instantly. And it must function in all environmental conditions and despite great systemwide stress. At the KEMA test facility, in Arnhem, Netherlands, we put these breakers under extraordinary strain to provide an independent assessment of their performance. There’s a clear need for this service: About a quarter of the circuit breakers brought to our labs fail to pass their tests.

    Why not rely on simulations to study the stresses at work? Unfortunately, computer models aren’t yet up to the task of simulating microsecond-scale interactions between electrical circuits and extremely hot and chemically complex plasmas. A study carried out by CIGRÉ, the International Council on Large Electric Systems, evaluated the simulation tools used by seven major manufacturers. First, the good news: These different tools did model the electrical fields at critical locations inside a circuit breaker with great accuracy and agreement. But when the tools modeled a breaker’s failure—the point at which it succumbed to electric stress—they produced values quite different from each other and from the true tested value.

  37. Tomi Engdahl says:

    Circuit breaker supplying direction ?

    Hello Guys ,
    thank you for this helpful Forum

    my question is related to Circuit breakers position ,and its supplying direction

    i mean it is marked as always input or supply side to CB from upper terminals and it connecting to load side from lower terminals
    according to NEC ,and according to CB performance
    CAN we supply the circuit breaker from the lower terminals ?

    2- what is the preferable supplying the main CB or the enclosure from upper and other branch breakers be lower it ,i mean cable entrance the enclosure from the top or the bottom of the enclosure ?

    Unless a breaker is marked line and load, it can be fed from either terminals. Most typical is line at the top and load at the bottom.

    Another example example is a main lug only panel that is back fed from a breaker. Power is fed from into the lug terminals and fed to the bus from the stab connection.

    Whether cables and conductors enter an enclosure/cabinet on top or bottom is a matter of choice, design and ease of install.

    No matter what orientation a breaker is installed, it is preferred the line be connected to the end towards which the handle points when in the closed (ON) position. I say preferred because it is not a hard rule. You will find some bottom-feed panels with MCB at the bottom, the line terminals are at the bottom of the MCB and the handle must be flipped up to close the breaker. Some manufacturers avoid this discrepancy with the preferred status by mounting the MCB sideways.

    Providing that breaker is not marked LINE and LOAD, which it does not appear to be, the NEC does not prohibit any of those cases.

    Is that third breaker upside down?

    If so, it cannot be installed like that.

    240.81 Indicating. Circuit breakers shall clearly indicate
    whether they are in the open “off” or closed “on” position.
    Where circuit breaker handles are operated vertically
    rather than rotationally or horizontally, the “up” position of
    the handle shall be the “on” position.

    Yes, but some breakers, such as AFCI or GFCI or some electronic trip units which contain circuitry powered from the circuit they protect, or DC breakers with a preferred current direction, will be marked with Line and Load terminals for a good reason.
    And under the NEC, even if there is no theoretical justification for it, connecting the source to a marked Load side is a code violation.

  38. Tomi Engdahl says:

    Is there a specific input and output for an AC circuit breaker (ABB), or does it matter where to put them (from the upper or lower side)?

    Great answers here to your question.

    Totally agree going up is ON where the live side is and down is OFF to isolate the load.

    ABB make AC circuit breakers from 110Volt 5Amps to 1 200 000Volts 6300Amps. Wow that is one hell of a range.

    You know what in every instance up is on and down is off.

    It doesn’t matter which way around you use it but the convention is in at the top and out at the bottom. It’s good to know which side is live when the breaker is tripped!

    MCBs are used in more locations than just distribution boards…

  39. Tomi Engdahl says:

    An mcb will trip if the current exceeds it’s limit whichever way around you connect it. However they have an exhaust type mechanism on one end of them that will expel heat and whatnot should they have to deal with a massive short circuit. This is the reason that when connected into a board they need to be connected the right way round.

    A simple thermal-overload type circuit breaker is bi-directional. That is, the LINE and LOAD terminals can be arbitrarily assigned Up, Down, Left or Right. It merely depends on where the LOAD is, relative the circuit breaker.

    A simple thermal-overload type circuit breaker is bi-directional. That is, the LINE and LOAD terminals can be arbitrarily assigned Up, Down, Left or Right. It merely depends on where the LOAD is, relative the circuit breaker.


    1) If a main circuit breaker, MCB, is located above the distribution breakers, then the MCB load terminals are on the bottom and connected to the distribution bus bars.

    2) If a main circuit breaker, MCB, is located below the distribution breakers, then the MCB load terminals are on the top and connected to the bus bars.

    The above applies only to low Voltage miniature circuit breakers which are unit mounted or mounted in a distribution panel-board. The typical Medium Voltage, or rack mounted, circuit breaker has the LOAD terminals at the bottom of the breaker.

    Here is a catalogue page, showing busbars at the bottom, and therefore the load terminals at the top

    I lectured for many years on this stuff.

    I always said it was logical that you push the switch into the direction of the load.

    Therefore, because you push the switch up to close the circuit,

    the load is at the top.

    Incidentally switches go up to be ON because it was said

    that it was more likely that something would push a switch accidentally down,

    and therefore shut it off.

    What is the difference between a single pole and a double pole MCB?
    Single Pole MCB is used to break ‘single phase’ and Double pole is used to break ‘phase and neutral’.

    In other words, single pole controls 1 live wire and it trips the respective line when the fault current exceeds the pickup setting and double pole can control 2 live wires/ one live & one neutral.

    Should I use a single or double pole circuit breaker?
    Assuming this is a single phase system, you use a single pole breaker for a 120 volt circuit and a double pole breaker for a 240 volt circuit. You are only attaching the hot leads to the breakers not the neutral wires. If you do not know this, it is probably advisable to hire an electrician.

    Can single phase load connected to 3 pole MCB run normally? What will be the consequences?
    You can connect to any of the 3 poles and can act as a single phase breaker

    Or if you are goin to use it permanently you can do as shown in d attached picture so that you can use 3 poles of breaker even we are using single phase

    Can we use 2 double pole MCBs instead of one 4 pole MCB?
    I would say “ NO”

    Simply because a 2 pole MCB & a 4pole MCB are designed for different purposes.

    While a 2 pole MCB is designed for switching action of 1-phase circuit consisting of phase-neutral, the 4 pole MCB is designed for 3-phase systems with higher voltage levels ( approximately 1.7 times the single phase voltage).

    What is the operating current of MCB?
    MCB=Miniature Circuit Breaker.

    Operating current;-

    Actually there is no operating current (negligible). It is just a very low resistance conductor once the load consumes more power than rated power of CB it heats which results in tripping the breaker mechanically.

    So we could say there is no operating current in normal working and during overloaded condition a small amount of power will be consumed to heat the conductor which operates the tripping mechanism mechanically.

    If your question is the current during tripping action, then the answer is it depends on the load if the load is very high it ta

  40. Tomi Engdahl says:


    What happens when you use an impact driver to install a consumer unit? We know it’s a terrible way to carry out an electrical installation but we went ahead anyway.

    Manufacturers of circuit breakers and consumer units receive lots of warranty returns caused by electricians using impact drivers. So we decided to see exactly what happens to the terminals inside a consumer unit, Lewden RCBO and an expensive Wylex AFDD.

    With BS7671 amendment 2 just around the corner, the use of AFDD’s in consumer units is expected to increase. Is it worth the risk of damage using an impact driver on an expensive device?

  41. Tomi Engdahl says:

    Can you use a 20 Amp double pole switch to control a spur from a ring final circuit? BS7671

    In kitchens, it’s often desirable to be able to control a hidden 13A socket outlet remotely. For example when the socket is behind a washing machine or dishwasher.

    With reference to BS7671 can you use a 20Amp switch to control a socket connected to the spur off a ring final circuit?


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