MCB - Circuit Breaker - How does it work?

In this video we will look at the popular Miniature Circuit Breakers and find out how they work. Using very simple drawings we will discover the two modes of operation and look at how to interpret and use the time and current tables to which they must all adhere.
Understanding how they work and the two modes of operation is very useful in fault finding on circuits that have tripped. Knowing in what manner they tripped can often give clues as to where to start the fault finding process.
This video is No.116 on our RUclips channel where we have videos on a wide range of electrical topics.
To see a full list of all our LearnElectrics videos, click on the link below.
/ @learnelectrics4402
You can also view all our videos by typing LearnElectrics all one word into the RUclips search bar.
Always work safely, electricity can be dangerous. If you are unsure, stop and seek advice.
This video is provided for guidance only. Every electrical situation is different, please check and ensure that you are applying this learning information to your own individual circumstances correctly and safely. If in doubt, stop and seek competent advice.

[@mb-electricalservices]

These videos are the equivalent of finding money you forgot you had. Brilliant guys! 😎👍

[@learnelectrics4402]

Thanks Mike, great feedback and much appreciated. Dave.

[@peterrobinson850]

Hi Dave, From an old spark from the 60's. Yet another excellent well presented explanation you've got the gift just like Joe Robinson, I've seen others try to explain the same thing and made it overly complicated, top marks.
Peter.

[@learnelectrics4402]

That's a great comment Peter, thank you. I try and make them understandable.
Thanks for your support. Dave.

[@johnhalse2779]

Thanks Dave - you present very well. Cheers Halsey

[@learnelectrics4402]

Thank you kindly John, much appreciated.

[@orestismiaris4928]

This is gold thank you very much!

[@learnelectrics4402]

That's great feedback. Thank you Orestis, really appreciated. Lots more to come. Dave.

[@spcyman7209]

Very good explanation, Dave. Keep them coming.

[@learnelectrics4402]

Will do. Lots more to come. Dave.

[@hypermotard617]

Thanks for the great videos, please keep up the good work 👍

[@learnelectrics4402]

Thanks, will do Scott. Dave.

[@30mAkills]

Simple to understand

[@learnelectrics4402]

Glad it helped, thanks for the support. Dave.

[@ifyougetstucklookitup237]

Thank you.

[@learnelectrics4402]

You're welcome, glad you like them. Dave.

[@stuffoflardohfortheloveof]

Thank you, very interesting and I now understand both the mechanism and the table. My only comment would be that when you showed the reg table (and then your simplified, coloured version) you didn’t state what the line (with the knee) ACTUALLY signified…..ie if you simply said “this line is when/where the MCB trips” it would have immediately made it all clear. Not a criticism, just pointing it out. Thanks for posting 👍🙂

[@learnelectrics4402]

It does actually say in the video. The point at which the MCB operates almost instantaneously. The MCB can trip before the knee too if it is a thermal response. Have another look at the video, it is there. Hope this helps. Dave.

[@Trade920]

Great video Dave so simple to follow, i do have a question tho please.
How would the cable hold up with such long tripping times as explained at 10:30?
lets say the 20amp circuit scenario is a 2.5mm twin and earth cable and was carrying the mentioned overload current for 2000 seconds or 33 minutes.
That cable is designed to only withstand 26-27 amps normally thats if it is not buried in insulation, so i appreciate 30 amps isn’t a great deal more than what the cable can withstand, however,
What could be done more to protect the cable from overheating/melting or in more serious scenarios where there is more fault current flowing potentially causing a fire under these slow trip times?
Sorry if this is a daft question but i’m just trying to better myself
Keep up the great work love the channel

[@learnelectrics4402]

That's a good question and logically it doesn't add up but this is the standard to which the MCBs have been manufactured and we assume that they know best. I just use 30 amps as a classroom example to show that not every overload will cause an instant trip. Most often though it may be two 13A devices (kettles) and that is less than the cable max. Not a daft question at all my friend. keep them coming and thanks for watching. Dave.

[@Trade920]

Thanks fpr replying dave 👍

[@hussamalsalem2438]

Thank ssssssssss

[@learnelectrics4402]

Thank you Hussam, appreciate your comments.

[@zazugee]

this explains why TN schemes need very low impedance earthing, it's because those MCB need a very high current to trip faster

[@learnelectrics4402]

You've got it. Thanks for watching, hope it helped. Dave.

[@deborahskeete]

Please do a video on the whirlpool jetbath

[@learnelectrics4402]

That's a thought Thanks.

[@learnelectrics4402]

Thanks for watching.

[@maxwang2537]

I don't understand why slowly changing current will have no, or little, electromagnetic effect. I believe this is wrong. I suspect you are confused by the difference between this and the property of inductor, which resists rapid change of current. Please correct me if I'm wrong of course.

[@learnelectrics4402]

Thanks for your input.

[@maxwang2537]

@@learnelectrics4402 thanks for your video. It’s still very helpful.