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).


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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


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