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Saturday, 15 March 2014

What is an MCB, AFCI, AFDD and GFCI?

In a previous post I wrote about types of RCD such as an ELCB, RCCB, and the RCBO.  In this post I want to discus other types of breaker you might find in a consumer unit or breaker box. The most common that you might find in Europe is the MCB or Miniature Circuit Breaker. Although the MCB is used as a replacement for fused wire, it is designed to have a number of specific properties.  I would like to discuss the makeup of this unit, which is very similar to the Circuit Interrupters as used in the North America. I would also like to discuss the workings of and operation of an Arc Fault Circuit Interrupter (AFCI) AKA Arc Fault Detection Device (AFDD).


What is an MCB (Miniature Circuit Breaker) and how does it work?

What is a (combination) AFCI (Arc Fault Current Interrupter) or AFDD (Arc Fault Detection Device) and how does it work?

What is a GFCI (Ground Fault Current Interrupter)?

What is an MCB and how does it work?

Very common in European wiring installations is the Miniature circuit breaker (MCB) that has an equivalent in the USA called a circuit breaker, which can be found in their breaker panel.  The MCB is, in its simplest terms, a protective switch, which has three modes of operation.

Anatomy of the Miniture Circuit Breaker
Anatomy of an MCB

  1. Manual.

    • The first is self-explanatory and allows the disconnection of the circuit, it is fitted to, from the mains and is operated from the lever on the front.  The most common type is a single pole installation and only allows the disconnection of the live conductor, not the neutral. For both the live and neutral conductors to be switched together would require a dual pole MCB, which would also need to be fitted in a compatible consumer unit. Where there is a fault condition on the circuit, any attempted manual operation to reengage the power can be overridden by the internal workings of the MCB.  This stops a forced attempt to manually engage and hold the switch in the on position.

  2. Short circuit protection.

    • The short circuit protection is usually operated by a coil inside the MCB. The coil forms part of the current path, ensuring all the current passes through it. The magnetic field generated by the current is concentrated by the windings and is able to operates an armature, in a very similar fashion to a relay.  If a short circuit condition occurs, then the resultant current induces a sufficiently large magnetic field to move the armature and thus trip the unit into the off, or disconnected, state.

  3. Thermal over-current protection.

    • The overload protection of the MCB is afforded by running the current path of the circuit along a bimetallic strip. The current flow through the bimetallic strip causes it to heat up and the hotter it gets the more it will bend.  If the current is too high then the bimetallic strip bends sufficiently to trip the unit.  The reason why thermal detection is employed, as the method of overcurrent detection,  is so the MCB has an innate tolerance to minor surges. These minor surges can occur when switching on appliances and are usually manifested as a large current but of a very short duration. Commonly, surges can be cause by transformers, incandescent lights, fluorescent lights, heating elements, etc.

    • These surges are usually short lived but can draw many Amps over the rated current of the circuit.  The additional energy is so small that it does not present a risk to the circuit or wiring, and the delayed reaction of the thermal detection is sufficient to stop any unwanted tripping. The time and level can be set by the manufacturer to suit the customer's requirements. The delay and profile of that delay is described by a "type" rating followed by a letter, see the table below for some of the types available. Sometimes the, "type," is called the, "curve."

      Trip current ratings for MCBs
      Type B trips between 3 and 5 times full load current.
      Type C trips between 5 and 10 times full load current.
      Type D trips between 10 and 20 times full load current.

Current path through MCB when switched when it is on
Current path through MCB

In all cases, the MCB interrupts the power flow, which could be drawing quite some current and so is likely to internally generate an arc. The fact that European mains voltage is at 230v coupled with a potentially larger current flow, during a fault generated trip, means the more likely an arc will be sustained as the arc energy ionises the atmospheric gas contained internally. Ionised gas has a low resistance along its length and can conduct electricity readily. The energy of the high current maintains the high temperature of the ionised gas and consequently the low impedance connection.

Within the MCB there a structure called an arc chute comprised of arc runners and an arc divider. It's designed as a path for the arc to follow, which draws the hot energised gas away from the switching contacts and into an arc suppressor or arc divider. This action helps to preserve the switch contacts within the MCB. The arc movement within the MCB is very like that of a Jacobs ladder in motion in that, as the hot ionised gas rises the conductors, or arc runners, diverge, so increasing the distance the arc needs to jump to maintain conduction. This extends the length of the arc and increases the resistance path of the current through the arc's ionised gas. At the top of travel the hot ionised gas rises into a cooling matrix, or arc divider that further stretches and cools the ionised gas back to high resistance gas and effectively snuffs the arc. This mechanism ensures that the arc is cooled and extinguished quickly as a matter of safety. According to the original patents the fins of the arc divider are partially magnetic to assist in drawing the arc into the divider matrix extending its length and turning the arc into a difficult zigzag pattern.

I have included a short video from a company called ABB, which at the time 35 seconds into the video, has some high speed footage of an arc flash being quenched in an arc chute and divider. Please ignore the unhelpful marketing information but just concentrate on the amazing footage. The footage is taken over a 3ms (millisecond) period. Note the zero crossing points of a 50Hz and 60Hz mains are at 10ms and 8.3ms intervals respectively.

Arc path through MCB when switched off
Arc path when MCB switches off

  • I must say that I was not aware of the arc snuffing abilities of a typical MCB, but it does mean that there is a, "right way up," for the best and correct operation of the device.  If upside down or fitted on its side the less dense hot gasses of the arc cannot be drawn into the cooler or arc divider. (see manufacturer's specifications)

I have found a video, which I think best explains the internal workings of an MCB nicely.

What is a (combination) AFCI (or AFDD) and how does it work?

Siemens Arc Fault Circuit Breaker
Siemens AF Circuit Breaker
The Arc Fault Circuit Interrupter (AFCI), or the European name of Arc Fault Detection Device (AFDD), is a device commonly used in North America and some parts of Europe, which is designed to detect the arcs within the circuit it protects. In the USA, it is a requirement for all living areas such as the bedrooms and day rooms like the hallway, parlour, etc.

There are actually two types of AFCI used in North America.  The first is the "AFCI" and the second is the "Combination AFCI."

Other than the word "combination" what is the difference you may ask?  The answer is the types of arc faults they detect.

  • The plain old "AFCI" is the older of the two and only detects high current arcs or what is loosely termed a "parallel" arc. The detection may use discreet circuity and simple logic.

  • The combination AFCI detects both high and low current arcs or what is loosely knows as a "parallel" and "serial" arcs respectively. The detection is performed actively via an internal microcontroller and its ancillary circuitry.

    • The use of the prefix "combination" appears to be fairly fluid in the information I've collected, which leads to alot of confusion as to what is actually being used on which part of a domestic installation in North America.

    • This confusion has lead to a sizable number of people with the belief that AFCIs don't actually work when they have used various "ad hoc" test methods.

The term "combination" is to distinguish between the level of arc detection and not to do with other possible added features, which can be bundled with the breaker such as CB/MCB or GFCI/RCCB. Such a breaker might be called a Dual Function Circuit Interrupter (DFCI) or "System Combination AFCI"

  1. Parallel Arc.

    • The parallel Arc is caused by a resistive short between the live and neutral conductors.  It's effectively an intermittent short between the conductors. Potentially, this fault can draw enough current to pull the circuit breaker out but not necessarily before it has caused sufficient heating at the arcing point to start a fire.  Common causes could be physical damage such as a trapped cable or flex where the insulation has been mechanically compromised by a conductive object or liquid.  Also where the conductors may have been crushed together by an external force such as a door or building movement.

  2. Serial Arc.

    • The second type is the Serial Arc, which is harder to detect, being more subtle than the parallel arc. That's due to the fact that the current may be limited by the appliance load, which is being supplied by the faulty cable.  Common causes would be where screw terminals or connectors have worked lose or mechanical movement has broken a conductor internally within its insulation.  The serial arc still generates heat but much more slowly as the load limits the energy drawn through the circuit. The appliance is effectively acting as a ballast limiting the ferocity of the arc.

    • The Serial Arc may happen on either the live or neutral conductors independently of the condition of the other conductor.
    • Only a "combination AFCI" is capable of detecting this type of arc.

The job of the arc detection is made much harder by the fact that arcing is much more common than you might realise.  Whenever you switch something on or off via a mechanical switch there is usually an arc.  Common items such as lights, vacuum cleaners, toasters, power tools, etc. Arcs will be created during both the switch on and switch off, frequently due to switch bounce (unless your appliance has electronic zero volt switching or solid state switches). The Arc is brief and short lived and only lasts milliseconds but it's still an arc.

  • Anyone familiar with programming micro-controller keypads and switches will be familiar with the concept of key bounce.  Essentially, in the world of electronics 10 milliseconds or 100th of a second is a very long time.  It is possible to detect many switch bounces during this time where the switch is connected, disconnected, reconnected, etc for 20 or 50 milliseconds.  Thus if a transition is detected it is ignored briefly and rechecked after the preset switch bounce delay, ensuring the bounce has definitely stopped and the transition can be confirmed. Similarly when an arc is detected it is briefly ignored then rechecked to ensure that it is not a typical switching generated arc. You might think you're fast but a microprocessor is much much faster.

Arcs typically have very big current transients, which are comprised of extremely high frequency components.  This is especially true of the parallel arc, which is easier to detect.  The serial arc is more difficult to detect, being a much lower current due to the ballast effect of the load it's supplying.  Even so it will still have high frequency transients, which will be random in nature that can be detected by the combination AFCI/AFDD electronics.

Another hurdle is to differentiate the arc from machinery such as the motor of power tools, which also produce arcs as a natural function of their operation.  Under normal electrical standards the manufacturer of these tools must suppress internally generated electrical noise to meet with the countries regulations.  This means the manufacturer of electrically noisy appliances must effectively remove the confusing harmonics and mains born noise caused by internal switching and arcing.  This makes the job of the AFCI easier as it does not need to specifically differentiate between an internally generated arc, produced as designed, from that of a arc generated by a fault. Sadly not all manufacturers are rigorous in holding to these rules and poorly designed non-compliant equipment can cause nuisance trips, which is entirely the fault of the appliance and not the AFCI.  For the user who is plunged into darkness, they may not see it that way!

If you are interested in the types of waveforms which may be observed normally and those of an arc fault, then please look at my further reading and references at the end of this post.

Interestingly AFCIs/AFDDs have moved the protective power electronics into the modern age by incorporating programmable devices, such as a microcontroller, within them.  They tend to be a Hybrid of an MCB with additional waveform detecting electronics.

  • The tried and trusted electromechanical MCB technology gives some, "fail safe," certainty that you will not be left with nothing in the event of a processor crash. 

You might be thinking that if they can deduce the waveform of an arc then, probably, they could also do the job of an RCBO.  The answer is, "yes they can," but it doesn't stop there.  They can do so much more, such as overvoltage detection, remote operation and monitoring, and data logging. Potentially analysis such as determining power factor.

Sadly it is not all a bed of roses and there are serious issues with nuisance trips.  AFCI/AFDD are a young technology (late 1990s) and there is still a great deal of development to do before they can really be considered a mature technology such as the RCCBs.  I hope to touch on this again in my next post.

We do not use AFCI in the UK but they do use them in other European countries (at least in Germany!).  Please see the video below which is the Siemens' version of the AFCI which they call an AFD.

What is a GFCI?

In the North America there are items such as Ground Fault Circuit Interrupters (GFCI) and this is really identical to an RCD but rather than the RCD's 30mA the North American GFCI uses a 5mA threshold. The term, "Ground Fault" in its name is misleading as it is detecting a difference in current between the hot and neutral conductors.  It is assumed that if the current is not returning within the circuit it must be going to ground.  The GFCI doesn't care where the rogue current is going it just measures that there is a rogue current element.  When this current hits 5mA the Interrupter trips.

The essential makeup and working of a GFCI are the same as an RCCB and you can see the link here where I also describe how it works.  The GFCI uses electromechanical design to detect a residual leakage fault and is early 70s technology, in the main part. Modern GFCIs and RCCB will use discrete electronics, which gives it a robust reliable operation. AFCIs on the other hand are late 90s technology and incorporate a microprocessor in the form of a microcontroller.

This allows the AFCI to be versatile. It is possible that the GFCI may be replaced by the AFCI, which already can incorporate GFCI functionality.  AFCI are currently only limited by their programming criteria and a number of mains noise based issues. 

Here is a video I found which beautifully sums up the history of mains circuit protection.

For further reading and references: please see my resource page

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