Every Miniature Circuit Breaker (MCB) sucks, cheap chinese circuit breaker rocks!!! Really?

[Peter]

Hi guys,

I got confused or I dont understand something or I just found out very important thing.. I will be glad for your opinions.

I need some reliable circuit breaker for my solar van.

What I need is a device that when I say max 50 A in this part of the circuit and there will be 65 A for more than few seconds, it will trip very soon and it will disconnect energy source. Simple thing, right?

I was thinking about some Miniature Circuit Breaker (MCB), they are industry standardized and from some bigger vendor..

It should be great, right? I dont think so.

These MCBs have a standard saying when they trip.. There are curves which standardize this tripping behavior. Most common curves look like this:

In my example I want 50A and I expect it trips on 65A. Simple.

When I pick MCB 50A with most sensitive behaviour - B curve, what happens?

Curve B says that it will not trip sooner than after 1000 seconds, but it may never trip... Uff.

If I put 100A on this 50A MCB, B curve... Industry standard says that it should trip anytime between 10-100 seconds.

I just think/feel its not much pleasant for devices and wires having double amps for 1,5 minute.

Then I found this video where guy is testing cheap chinese circuit breakers.

https://youtu.be/aDmNto-qC7Y?t=501

These circuit breakers trip with relatively slight over current in approximately 5 seconds!

This is exactly what I am looking for!

Sooner in the video guy also tests short circuit situation and it trips almost instantly.

https://youtu.be/aDmNto-qC7Y?t=288

Now I am thinking about filling my solar van with huge 15kWh LiFePo4 battery with these chinese breakers.

The question is if this breaker can handle few thousand amps from big lithium battery, thats why for extra safety I would add one more T-class ANL fuse as close to the battery as possible.

What do you think?

Do you have some other solution?

Is this cheap chinese stuff best for my purpose?

Peter

[Dacian Todea]

Peter,

Those MCB's have two separate tripping devices inside one is thermal a bimetal part that heats up and if temperature exceeds a certain value they will flip and trip the breaker and that is the slow responding one in the graph so at 20% above the rating it may never trip or take a very long time to trip depending on ambient temperature around the breaker but you are sure it will not trip at the rating current. Then there is the electromagnetic protection that reacts super fast in case of a short circuit and for B type that is 3x the rating but tripping times at that current is super fast below 10ms.

At 100A the 50A rated MCB will trigger based on the bimetal thermal protection and depending how hot the bimetal it is already like maybe you have 0A before this 100A so everything is cold or you had maybe 50A before and then everything is already preheated thus the trip time will be around 10 seconds based on that graph for 100A and about 50 seconds at 75A

The black ones are just thermal breakers so the slow tripping as on the MCB and the fact that they trip.  In video it mentioned that he had 30A trough that and it tripped in about 5 seconds that mean that that breaker is not properly rated as it should never trip at 30A and so the correct rating for that should be around 15A (seems sort of standard for many low quality products just to declare a 2x capability :) ).

Then you get one of those 30A rated thermal only breakers and put 18 or 20A through them and they at random intervals maybe a half hour or an hour just trip. 

[brianjonesphoto]

This cheap Chinese breakers are crap. I use on from my shore power charging circuit. The power supply is a cc cv witha 40 amp max. the first 50amp breaker I used as well as the 60 amp one both tripped after 30 minutes of use. I had to go with an 80amp in order for it to reliably not trip. Luckily my wire run can handle higher current if I were to need the breaker to actually be a breaker vs an occasional disconnect as I use it mostly. I know it's not the best solution but it's working for now.

If you have 15kWH invested maybe some quality protection is cheap insurance.

[Peter]

Hello Dacian,

I know about electromagnetic part of MCB and thats clear to me.

I see your point that the chinese breakers will probably trip even below stated amp rating.

Actually I found the video proving exactly this: https://youtu.be/ksWy5Uld_cM?t=204

And brianjonesphoto says the same thing.

OK then.. bimetal definitely cannot do fast sensitive precise disconnect as I dreamed about it.

When not chinese breakers, I will use MCBs then.

At least short circuit protection is faster on MCBs thanks to electromagnetic mechanism (under 0.01 sec versus roughly 0.1-0.8 sec that bimetal has; these figures are from curves-picture above).

And it seems to me that I should prefer B-curve MCBs because short circuit protection comes sooner (roughly around 4x amp rating).

Dacian, in SBMS0 manual on page 3 you have C curve breakers, is there any thought behind it? Why not B?

But what could I put between battery and all loads?

I guess MCB rated 150 or 200 A does not exist.

So fuse it is,  maybe with simple battery isolator?

Thank you guys!

Dne středa 12. května 2021 v 1:43:48 UTC+2 uživatel brianjonesphoto napsal:

[Dacian Todea]

Peter,

Not sure you will worry about short circuit current with LiFePO4 as any LiFePO4 battery even a small one will put out significant short circuit current.

If you get the B curve MCB then that will react to around 3x the rating and that may not be what you want in some circumstances where you may have some capacitive loads that are not pre charged or there are short peak loads exceeding that 3x rating.  The C curve will react at around 5.5 to 6x the rating and so is less likely to trip due to some inrush current on one of your devices. Keep in mind that it will trip in very few ms.

I can give you an example with an inverter.  Say you have a 12V 500W inverter that will mean 42A average if you do not consider the inverter efficiency at full load so say with the efficiency it needs 50A and you use a 50A MCB to protect that.

A pure sine inverter will have that 50A average current but in reality the current will be a half sine curve starting from 0A up to 1.41x the average 100x per second (assuming 50Hz AC).

So your current will be 0A then in about 5ms it will increase to 50A x 1.41 = 70.5A then the next 5ms will slowly drop back to 0A averaging 50A

If the inverter is a modified sine inverter then current will be a rectangular shaped pulse with half the time at 0A and the other half at 2x the average so in this example 0A for 5ms then 100A for another 5ms

You are now already at 2x the breaker rating so not enough to trip the B-curve MCB but not that far and so if there is an inrush current as you connect maybe a non resistive load to the inverter and since most inverters can provide at least 2x surge if not 3x for some then you can see especially with a modified sine wave inverter how you will exceed the B-curve on the MCB and it will trip.

The MCB's are in two versions the slim ones up to 63A and the double width ones that are rated up to 125A

But then there are larger breakers just not DIN style and you can see some examples in my post here https://groups.google.com/g/electrodacus/c/CflxSzTsKaY/m/N4CVd6G_AQAJ