Circuit Breakers - How they Work, What's Inside

A look inside a typical circuit breaker or MCB.
These particular ones are made by Crabtree, but most others will be similar inside.
These were removed from an installation after being damaged by a water leak, however there is no real evidence inside of why some of these stopped working - they all have continuity through them now.
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[@HandyKindaGuyUK]

Im on a course and your name echos. You explain things in such simple English. One of the guys because a patron for this reason. Thankyou mate, please don't stop making these.

[@Goodchappy]

I wish every school and college tutor I had was like you!

[@terretulsiak]

I don't know where you live or what you studied but in Florida they rarely even mention the practical application of anything until well into the profession- if at all. And many electricians don't remember if they ever knew the applied science of it. Most don't apprentice, but are tied to a single licensed electrician. Engineers sit in front of computers for the most part.

[@harveysmith100]

Thanks JW. Following a previous video I have kept an old one on my bench to open up to have a look inside. This video will make it much clearer for me to follow the internals.

[@madandu]

Thanks, that's what I was looking for. Excellent and crisp explanation of inner-workings.

[@matthewjacobs1578]

Extremely detailed video, keep up the great work please!

[@dejffjed]

BRILLIANT!!! Great lesson on the item here! Thank you!

[@jbr9813]

An excellent explanation. Just the right level, I think, for both the layman and the more experienced.

[@magna59]

Thank you for putting this up . Nice piece of work .

[@robviolin1]

Thanks John, that helps a lot. Keep posting.

[@BobMellor1954]

What an ingenious device, well explained, thank you. Kind regards Bob

[@clidiere]

Very good job explaining what many failed to explain ! Thanks!

[@supersesqui]

Excellent presentation.

[@volatileacid2281]

Great command of the English language and clear explanations - perfect!

[@rickyweird01]

Thanks John, perfectly explained... needed to know about the different types of MCB as having a garage re-wire.

[@lorenzhoernel3249]

Great explanation! Started kind of rocky but became very interesting half ways through. Very In-depth.

[@boristheslav2860]

Everything I needed to know about these at one place? Oh well, thank you!

[@patrickchiasson1009]

very professional presentation. Thanks from Canada

[@malachihayes6059]

Exactly what i was looking for. Thank you.

[@aerodicus]

Excellent presentation, very clear explanation of components!

[@michaelronan4928]

Excellent clear explanation, business as usual for JW !

[@shawnshawn911]

wow, thanks this is very informative especially the explanation of the various types of circuit breakers.

[@rakeshjasoliya736]

Very nice and detailed Explanation..Thanks Mr. John Ward..

[@marcusthomas3003]

Much appreciated video superbly detailed, many thanks.

[@seriesg7171]

Great illustration!

[@georgiecoo1577]

First class, clear and concise.

[@michaelmbogo8342]

Great presentation! Explained very well!

[@zchal66]

3:38 to get to the "How they work, what's inside" part of the video. Great explanation - Thank you!

[@james.black981]

I'm more entertained by the incorrect sub titles in this video, then the disection of the MCB'S. Great work!

[@app0the]

Probably would have been unsafe but it would be really awesome to see it trip while being open like that, probably on a high FPS camera for a slo-mo. Awesome material whatsoever! Thanks~

[@thexxangel]

Thank you so much , this was very helpful :)

[@leiphasw]

Nice explanatory video. Thanks!

[@chanchaljindal5716]

Great presentation.. Thanks John

[@paulmcconnell2086]

Great video very clear and fully understood,thank you.

[@nasaw2]

Thanks Sir is very clear explanation.I m an qualified electrician BS 7671I enjoy watching your videos.Keep up your good work.

[@ansarawan]

Thank you for such a great lecture. It was really helpful

[@marcocalzavara3206]

superb explanation, thank you

[@bobbyashrimp]

Very informative. Thanks for sharing.

[@davefranz8766]

Im a licensed electrician in the us .I think the UK and countries that use miniature circuit breakers as primary branch circuit protection need to reconsider especially at 240 volts to ground which IS NOT SAFE FOR A HOME. In the USA you can only use molded case breakers for protecting branch circuits even at 120 volts. Miniature breakers can ONLY BE USED AS SUPPLEMENTARY PROTECTION.

[@saurabh199530]

great explanation john thanks :)

[@noumman]

Fantastic video

[@pariah1234]

Very informative, thank you

[@ridha_theonlyone]

Nice explanation, gotta learn more

[@WaltWW]

Well explained sir!

[@111shamim]

nice explanation.

[@sandeepKumar-jz7sc]

Excellent explanation sir 👌👌 .. Thanks

[@juandagali]

Tks, very nice video

[@stonent]

I can't help but wonder if John Ward has ever been a news anchor for the BBC.

[@MonaichFother]

No never, he has however freelanced as a presenter for ITV's Magaluf Weekender.

[@enlightenednews5265]

Great video.

[@anthonyvolkman2338]

Very interesting how the arc breaking cooling mechanism is basically a safety spark gap such as when coilers use them as a safety gap for our Tesla Coils and their high voltage power supplies. It prevents over shoots due to resonant current and voltage rise on the LC network.

[@user_i.d]

Thanks for the video, totally love the way you explain it, nice and simple. Could you explain discrimination of fuses please.... And maybe you could do a video explaining the Regs book, maybe chapter by chapter because it still confuses my when i pick it up and its been some years now

[@user-lv2rv9rt8c]

Thank you for the information

[@strider5964]

I'm not even from the UK but these videos are so fascinating to watch. On a random note I notice circuit breakers throughout Europe all seem to use the same DIN rail connectors. Wish we had that kind of standardization here in the US because breakers can only fit into consumer units made by that manufacturer where as MCBs can fit into any box regardless of manufacturer. (Correct me if I'm wrong)

[@jwflame]

They fit onto the same rail, but connections to the busbar and spacing of the terminals can be different between manufacturers. Consumer units are type tested assemblies, and that approval only applies if all of the devices in it are from the same manufacturer, so although it's often possible to physically fit devices from other manufacturers in there, it shouldn't be done.

[@RichardNash100]

really excellent very useful video

[@surman1816]

great stuff

[@peterstiles1]

JW could you put a link to your next video when you mention one at the end like you did here, please? Having trouble finding it on your website. Thanks.

[@jeraldfooo]

Thank you Thank you Thank you

[@sbusweb]

Dear John, The layout of the ARC-suppression, might suggest that the MCB should only be installed vertically (so the arc might naturally travel 'up' into that gap, instead of going the other way where there is no suppression-materials). Is this actually the case, or can MCBs (generally, subject to manufacturer instructions) be installed in any orientation? With many thanks,

[@thawatchaichaichana1832]

Thank you very much 😀

[@sbusweb]

I request a similar teardown of an RCBO, with a discussion of the differences/reasons for some having an flying-lead earth (Functional Earth??) connection to them and others not. Also very interested in the compromises involved with the smaller sized RCBO unit over a large RCD. Similarly, single-pole-isolation (bad on TT supplies??) not isolating N-E fault from upstream could be worthy of mention =). With thanks,

[@bjtaudio]

C-curve is the most common these days, as B curve may trip even with lighting circuits as high initial surges are likely with many lightning products having SMPS such as led drivers.

[@johnharrison8810]

Dear John Could do a video on Portable appliance testing etc etc Thanks Great videos.

[@Graham_Langley]

Just pulled an almost identical Wylex NSB10 apart and the arc quench plates are copper-plated steel.

[@hiendo8973]

Thanks!

[@tskipwadoo3447]

Thanks, I wondered what went on in a breaker tripping.

[@jamesbarrymcnulty2087]

T, Skipwadoo ww

[@Neo-po2xw]

5:10 Tripping coil, so only when a large number of current flows through it in the event of short circuit then it will energie the pin enough to break the circuit.

[@terretulsiak]

I would like any opinions about what age children will be able to grasp and engage with this very fascinating and well presented information. I feel that we bypass the window of opportunity in many cases.

[@starky497]

Have you (is it possible to?) made a video showing the arc created when a fault current exceeds the breaking capacity of a MCB?

[@kanchankashelani2245]

hello sir ..can u plz give me information about d latest Circuit breakers used in ehvac transmission lines

[@seandoofer5720]

A recent kind of breaker is type K, specifically designed for motors.

[@akramal-hayek6285]

We installed 6A, type C, circuit breakers in head frame on the top of a light pole, protecting 600W, HPS , IP65 luminaire . During commissioning and in a very hot day with outside temperature 45- 50 Deg. C. the MCB trips. switching off the lamp's circuit. Obviously the thermal element inside the breaker reacts to heat and trips the device. what do you suggest to do in this case.

[@i4ittwasdis]

Akram Al-Hayek A HRC fuse my best guess.

[@piee683]

Hi john can you answer me why does my circuit breaker trip sometimes when I plug me my 110 v yellow transformer thx

[@jwflame]

Transformers appear as a short circuit before power is applied, so there is a significant surge when switching on. Once power is applied, the magnetic field of the transformer limits the current to almost nothing until something is connected to the output side. Whether it trips or not depends on where the AC mains cycle is - if it's at the peak value it will probably trip, but if at a low or zero value it probably will not.

[@nial8877]

wow its much more complex than i thought it would be inside...

[@OmarAlvi]

Thanks... very informative. in my house MCBs are connected other way round. input is given where output should be. i just noticed this after this video. will they work ?

[@jwflame]

Omar Alvi Yes, doesn't matter which way they are connected.

[@OmarAlvi]

Thanks you for replying this quick. I like your channel and watching all of your videos. Very informative and educational. Its a great work. Pls keep it up.

[@arrowstheorem1881]

Super!

[@kevinhopton7664]

Hi John - thanks for your very clear explanation. I've recently bought a 2nd hand 240v angle grinder from a certain online aution site.... I've tried plugging it into my garage 13a sockets (protected by a 'B' 16a MCB) and the MCB trips every time, almost instantly, spinning the cutting disc momentarily. However, when I plug it into the house ringmain sockets, protected by a 'B' 32a MCB, it will run continuously. Can you say please if my garage electrics are simply not up to the job & I just need to use the appropriate house circuit or could there be a fault with the appliance? I understand that these tools do draw more current during startup that when at operating speed.

[@jwflame]

Unlikely to be faulty, motors do have a significant surge when first switched on, often 5x-10x the running current, so can easily trip lower rated circuit breakers.

[@kevinhopton7664]

Thanks John.

[@sbusweb]

Is there any truth to comments that MCBs can take rather more current without tripping, if the current is very gradually 'ramped up' rather than added in lumps? Definitely wondering how that makes bimetallic strip behave, if its' allowed to 'warm-through' over time.... maybe this is a misnomer!

[@chrissnowdon8235]

change fridge bulb

[@dolong8668]

Dear Mr John, I have a question, Pls help me: What happen If I change position connect source and position connect to load? CB can protect if I change 02 position above?

[@jwflame]

They will work either way, it does not matter which is source and load.

[@dimitriderop8822]

@JW Can circuit breakers depreciate when they trip (too) often?

[@jwflame]

Yes, they don't last for ever. Repeated trips will cause damage inside and they will eventually fail.

[@dimitriderop8822]

@@jwflame Every product has a lifetime, that is clear. However, it looks like the performances are also effected (i.e. getting weaker: not able to withstand the same load and/or inrush peak current as before).

[@Kageitenshi]

I got a 240V 10A C type breaker as a resettable fault protector for a heater control system. It has a PID controller for temperature adjustments, an input VA meter, a 12V 0-999 second timer between the PID and the mains solid state relay and finally a SCR dimmer to adjust the wattage from pretty much 0 all the way up to what the breaker will allow. Now I'm wondering if an B type would've been better in case of an accidental short circuit in the exposed kanthal coils and whether or not the breaker will trip _before_ the 15A main circuit breaker does, assuming there's no other load? Maybe I'll just add a 13A quick fuse, that ought to do.

[@jwflame]

Kageitenshi For short circuit faults there will be little or no difference, both a B and C type will trip very quickly - typically less than 0.1 seconds. There won't be any discrimination between a 10A and 15A circuit breaker either, so either or both could trip.

[@Kageitenshi]

***** Thank you for clarifying. Though I may be a bit paranoid about overloading the breakers, resulting in a walk of shame, I'd prefer to be on the safe side: Until I realized that my newer water kettle was rated at 2.6 kW, instead of the 1.8 kW it replaced, I routinely ran it alongside an 800W microwave and a lamp. It wasn't until I tripped the kitchen appliances breaker that I realized I was drawing very close to its rated 15A from the ~230V mains. Came as a surprise every time until I learned not to do that, rather slowly, thanks to how the microwave operates on "medium" and "low" settings for thawing, randomizing the intervals. Well, despite not having enough resistance at the business end of the kiln's heater to limit the current below 10A, but closer to 15A, the SCR dimmer and VA meter should be sufficient controls. Safety first.

[@christastic100]

it would be interesting to compare a 6ka to a 10ka mcb

[@TranceNotes]

I would have thought that a good old-fasioned simple wired fuse would be more failsafe than a complicated contraption like that. The Circuit Breakers looks like it would be prone to manufacturing errors and such.

[@sajidullah]

Hey thanks. Is it possible to fix / recondition some of my obsolete Zinsco breakers ?

[@jwflame]

+Sajid Rafique No, if circuit breakers have failed, they must be replaced. Zinsco were never sold in the UK, but a quick search reveals they are the type which welds closed and cause fires, so probably better to replace the entire panel.

[@sajidullah]

+John Ward Thanks John, for informing me of the fire hazard ! Really appreciate that !

[@wjimenezu]

Where did you get that T-Shirt? It's pretty cool!

[@tomgreen2836]

Is that a One-punch man t-shirt?

[@PerMejdal]

Where are 6A circuits use in the UK?

[@jwflame]

Per Mejdal Rasmussen Lighting circuits usually, most homes have 6A lighting circuits with one for each floor, some older or smaller properties have a single 6A circuit for all of the lighting.

[@PerMejdal]

***** Interesting. Then you can use 1,0mm² instead of 1,5mm² thick wires.

[@twig3288]

John, Have you made a video about wiring RCBOs yet?

[@ranjan1436]

Mr Ward , how the MCB discriminates a short ckt current and overload current when both these current enters MCB then it goes through the solenoid and enters bimetallic rod. The Solenoid should trip the MCB in both cases.

[@jwflame]

The bimetallic strip is for moderate overloads and reacts slowly, trips from 1.13x to 1.4x the rated current and above. It will typically take many seconds or 10s of seconds to trip. The magnetic solenoid is for short circuits and for a Type B will trip near-instantly from 3x to 5x the rated current and above.

[@ranjan1436]

@@jwflame Thanks, Mr Ward.

[@andrewzhoe]

Cheers

[@jonbaldwin]

The screw which attaches the circuit to the top... is it an exposed live part?

[@julielowe7462]

No, this would normally be behind the Shroud, and you should only remove this once the circuit is Safely Isolated, and then only if you’re Qualified! - Julie

[@bhupendrasharma8255]

how to safe home equopment form high voltage

[@arunkumar-dn4nt]

sir John Ward I am seeing your videos i am gaining knowledge from your videos but please make subtitles for your videos then we can understand very well

[@bhupendrasharma8255]

there some time high voltage come in home which destroy the all equipment so is any device which avoid such problm ?? pls reply

[@jwflame]

bhupendra sharma yes, such devices exist. Either surge protectors for spikes / intermittent increases, or an overvoltage release for situations where the supply voltage increases above normal. Hager MZ209 is one example.

[@DogsBAwesome]

We used to have a Big Freezer that used to trip off at random times usually in the middle of the night, I assume the Circuit Breaker was the wrong type

[@jwflame]

brian whittle Possibly, larger motors often require the use of C type breakers as the starting current can cause B type devices to trip. Alternatively it could be that the motor had some problem such as sticking bearings, which would cause it to draw more current.

[@mlkirshad1862]

nyc information

[@elektronikmaleinfach16]

i do not understand why type b is avable! i use type d in my home because from my pc plus sterio amp b type trips often if i swith on boath diveces together! or b types trip from a 300VA toridial lamp transformer! i dont see a risk to destroy some wirering or sockets by using b types! what is the idea to use b types in homes???

[@rrpatra87]

i want to know more about electrical fields

[@rickyputt5305]

When is home time?

[@adithyakaravadi8170]

How does the coil mechanism even work? Aren't we dealing with alternating current here, so the rod or pole should not even move, obviously I'm missing something here..

[@shabinanaaz7581]

Does mcb protects the system from overvoltage

[@jwflame]

No, only overcurrent. Overvoltage and undervoltage trips are available as separate items.

[@kcarroll0913]

6 amps? I've never seen that before. In the US our lowest breaker is 15 for any residential and 20 for commercial. How many breakers are in a 2500 Sq ft home over there?

[@jwflame]

6 amps but that's at 240 volts, so the equivalent circuit power of a 12 amp 120V breaker. Newer properties generally have more individual circuits, older ones have a smaller number of higher power circuits that cover larger areas. 2500 sq ft would be a very large and expensive property in the UK, most are far smaller than that.

[@kcarroll0913]

@@jwflame lol my 2 bedroom apartment is almost 2500 Sq ft

[@sankarthayal2713]

sir its very useful for me but i need a difference between MCB and RCCB

[@jwflame]

sankar thayal MCB trips on overload or short circuit, RCCB trips due to current imbalance between line & neutral.