MCB (Miniature circuit breaker) Tripping curves - Page 1
Forum section: electronics / beginners

author: 2X
author_rank: Frequent Contributor
timestamp: March 08, 2021, 03:38:37 pm
content: Hello,I would your opinion about reading a trip curve from MCB. At the picture... at 4times the rated current the breaker maybe will trip between 0,01-2seconds from the electromagnet and if the electromagnet don't work it will trip between 2-6sec from the bi-metalic strip?https://img.c3controls.com/image/upload/resources/c3controls-Understanding-Trip-Curves.pdf

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 08, 2021, 06:16:17 pm
content: It’s not an opinion, it’s fact. That’s the way that particular breaker works. There are different trip curves and that is one of many. Note that the center picture with green and red areas is a different curve than the one you describe.

author: mikerj
author_rank: Super Contributor
timestamp: March 08, 2021, 09:28:31 pm
content: Quote from: 2X on March 08, 2021, 03:38:37 pmIt's not a case of the electromagnet "not working", the magnetic trip and the thermal trip are for different overload conditions. The thermal overload is for sustained low level overloads, the magnetic trip is for gross overloads.

author: 2X
author_rank: Frequent Contributor
timestamp: March 08, 2021, 09:55:00 pm
content: Yes but the magnetic and the thermal trip are overlay where I have the green line, so between them where it acts the magnet and where the thermal?

author: 2X
author_rank: Frequent Contributor
timestamp: March 08, 2021, 10:50:12 pm
content: Quote from: mikerj on March 08, 2021, 09:28:31 pmI read that the thermal trip is for inrush currents and the magnetic trip is for short circuit currents.

author: mikerj
author_rank: Super Contributor
timestamp: March 10, 2021, 07:59:16 pm
content: Not quite, the thermal element is effectively an integrator with the temperature increasing at a rate roughly proportional to the square of the current; a relatively small overload may take many minutes to trip the MCB. For large overloads the breaker must trip very quickly, and the magnetic circuit is responsible for this.For covering loads with different inrush currents the magnetic actuation is available with different trip limits, identified by the MCB "Type" e.g. Type B, Type C, Type D etc.

author: 2X
author_rank: Frequent Contributor
timestamp: March 10, 2021, 08:33:43 pm
content: Many thanks for the reply to all.So the magnetic is responsible to trip at all of the red area and for example at the 4times the rated current (green line) the magnet will trip between 0.01 sec to 6sec (first image)?At the below site of the Schneider Electric it shows that the magnet is responsible at pink area only (third image).https://www.se.com/th/en/faqs/FA346069/At second image says something like the Schneider Electric (second image). Are quite confusing I think.https://img.c3controls.com/image/upload/resources/c3controls-Understanding-Trip-Curves.pdf

author: tautech
author_rank: Super Contributor
timestamp: March 10, 2021, 09:18:06 pm
content: Quote from: 2X on March 08, 2021, 10:50:12 pmWrong.Thermal trip is for sustained overload and the breaker type determines how long that sustained period may be.I got occasional nuisance tripping of a 20A 3ph breaker supplying a welder but only at turn ON which I deduced would be near the peak of the mains sinewave and the problem was totally cured with the installation of a D curve (motor start) breaker.

author: 2X
author_rank: Frequent Contributor
timestamp: March 10, 2021, 10:52:30 pm
content: Quote from: tautech on March 10, 2021, 09:18:06 pmIt would be wrong if we say that the thermal trip is responsible for extend the period of light overload(inrush)currents and the magnet extend a little bit more "heavy" currents and for the short circuits to act immidiatelly (always the extension of time will vary along the MCB type like B, D etc).Inrush Current:https://youtu.be/GPoWH5xH8q8?t=1354

author: tautech
author_rank: Super Contributor
timestamp: March 10, 2021, 11:05:25 pm
content: Quote from: 2X on March 10, 2021, 10:52:30 pmLight overload ?Resolution:For C60 Multi-9 devices, ac rated, the following is the meaning of the trip curves:B curve: 3.2 - 4.8 times amp rating for UL489 and UL1077 (3 - 5 times amp rating for IEC)C curve: 7 - 10 times amp ratingD curve: 10 - 14 times amp ratingK curve: 10 - 14 times amp ratingZ curve: 2.4 to 3.6 times amp ratinghttps://www.se.com/nz/en/faqs/FA228794/

author: Alti
author_rank: Frequent Contributor
timestamp: March 20, 2021, 09:30:17 pm
content: Quote from: tautech on March 10, 2021, 09:18:06 pmKeep in mind the MCB is for protection, not for convenience. Swapping a circuit breaker to D requires half of circuit loop impedance of C and such swap is usually very challenging.

author: 2X
author_rank: Frequent Contributor
timestamp: March 20, 2021, 09:43:36 pm
content: Quote from: Alti on March 20, 2021, 09:30:17 pmThanks for the reply Alti.... At the below video it say's about the line's the impedance and how important it is.... I assume that there will be and something like a "special" instrument that will measure the impedance of the line?https://youtu.be/niZ01Dr_8CE?t=730

author: tautech
author_rank: Super Contributor
timestamp: March 20, 2021, 09:52:17 pm
content: Quote from: Alti on March 20, 2021, 09:30:17 pmIf the circuit cabling is adequate for continuous current rating there is no problem.The welder in question is a 400A unit so we can't and don't need to run it at full steam and fitting the D curve breaker only solved the occasional nuisance tripping at power ON.

author: Alti
author_rank: Frequent Contributor
timestamp: March 20, 2021, 09:58:29 pm
content: Quote from: 2X on March 20, 2021, 09:43:36 pmWell yes, sparkys use a dedicated equipment (it is actually an integrated: load + multimeter) to measure various parameters of the loop.But you can also measure loop impedance with any load and any multimeter. You won't get to precise 1% results but it is enough to know if you are within acceptable range or not.Since this is Beginner's section, I can recommend a kill-a-watt (this is a multimeter) and a kettle (this is the load). Disclaimer: be careful with boiling water, do not lick kill-a-watt.

author: themadhippy
author_rank: Super Contributor
timestamp: March 20, 2021, 10:13:47 pm
content: im intrigued, you can measure earth loop impedance with a power meter and a kettle ?,please tell me more, i could have saved a few quid instead of buying my loop testerQuoteNot always, long cable runs,high Ze, loose connections all add up and may put your Zs over the minimum,even though the cable is rated for the full load current and the volt drop is within spec

author: Alti
author_rank: Frequent Contributor
timestamp: March 21, 2021, 12:21:26 am
content: Quote from: themadhippy on March 20, 2021, 10:13:47 pmYou are right, thanks for pointing out.@OP: For the purpose of understanding how a loop impedance is measured, L-N loop impedance might be measured safely with mentioned instrumets (disclaimer). For other loops, including earth loop impedance, this is not a subject for beginner's forum

author: elekorsi
author_rank: Regular Contributor
timestamp: March 21, 2021, 07:37:53 am
content: Quote from: tautech on March 20, 2021, 09:52:17 pmCurrent carrying capacity of the cable is only one factor at designing the circuit. Many times it happens that because of relatively high loop resistance the fault current is not large enough to reliably trip the breaker in the magnetic regime at introducing short circuit... This is very dangerous and people don't understand this. I saw many times that people replace their B breaker with C or even D, but when you measure the fault current ot is way to low even to change just from B to C. This is mainly a problem in domestic installations.In industrial there is usually more of a problem with to high fault currents and people chosing the equipment that is not rated for such high fault currents... MCB are usually rated for up to 10kA, anything more than that and you need a bigger breaker or classic fuses...

author: tautech
author_rank: Super Contributor
timestamp: March 21, 2021, 07:54:46 am
content: Quote from: elekorsi on March 21, 2021, 07:37:53 amUnderstood and maybe you missed ^^Not possible to fully power a 400A welder on a 20A circuit and that's all we have at this installation. 20A serves our needs perfectly well apart for the power ON occasional nuisance tripping that we don't have when using the welder to the capabilities we require that are well short of its maximum capability.

author: perieanuo
author_rank: Frequent Contributor
timestamp: March 22, 2021, 10:44:12 am
content: make it simple,>>big sudden short>>current rise quickly, electromagnetic protection cames into play>>cut-off>>loooong over-current (below electromagnetic threshhold but higher than the thermal one)>>thermal trip acts>>cut-offthoes 2 areas are distinct, like 2 separate graphs

author: 2X
author_rank: Frequent Contributor
timestamp: March 22, 2021, 09:11:58 pm
content: Quote from: perieanuo on March 22, 2021, 10:44:12 amThanks a lot for the reply... please could you tell me if I read with the right way the two graphs about the tripping times at the pictures?

author: tautech
author_rank: Super Contributor
timestamp: March 22, 2021, 10:11:03 pm
content: While MCB tripping curves are clearly stated in documentation one needs be aware after numerous trips these things degrade to where they will no longer hold their rated current.My sparky mate replaces good numbers of them in for example power point circuits with numerous splitter boxes on them where circuits can often draw near their rating and then any inrush results in nuisance trips and as they start adding up the MCB starts degrading.It's a balancing act.....use an ordinary MCB with a rating that matches the wiring or use a C or D curve that will handle some overload yet not nuisance trip and so give good life.

author: 2X
author_rank: Frequent Contributor
timestamp: March 22, 2021, 10:36:07 pm
content: Quote from: tautech on March 22, 2021, 10:11:03 pmThats so true.. I have personal experience of that thing... and also it is not right someone to turn-off a device/lights from the MCB because in every turn-off with your hand it degrades slightly every time resulting in that he say the "tautech". If we want to turn-off a load from the electric box we have to place an electric switch in series with the MCB and turn-off the load from the switch and not the MCB. As you can see a switch has a bigger number of swiching cycles.