DC MCB Not Working?

Hi All,

So I have installed a DC MCB between my battery and my PIP, specifically the "TOMZN" 63A single pole. specs for ref:


Now it said somewhere on the aliexpress listing that the "load" side should be the bottom, so since the PIP is the device creating the load I have wired the battery positive side to the top of the MCB and then the bottom side to the PIP battery positive terminal.

Now I wanted to test this setup so I applied a 100A load to the battery for about 10 seconds to see if it would trip, it did not. I swapped the connections on the MCB around just in case I/they got it wrong and the load should come out the top but the results were the same.

Am I doing something wrong here? My understanding was if you applied a current higher than the rated current for the MCB that's what would cause it to trip...?

Sonic01 said:

My understanding was if you applied a current higher than the rated current for the MCB that's what would cause it to trip...?

Click to expand...

In principle yes, but there is quite a large leeway to prevent nuisance tripping. Below is a link to some extensive explanations, but to make it short, have a look at the graph below of a typical breaker.
Most breakers have 2 tripping mechanisms:

1. A bimetallic switch (light blue in the graph), that slowly heats up under overload conditions. It's designed to mimic the heat buildup in the electrical wiring, and trip long before the wiring can start a fire. At 1.59x rated current (100A / 63A) , this mechanism should trip no earlier than 30s, and no later than 300 seconds. Environmental conditions such as temperature or airflow may affect the timing.

2. An electromagnetic coil (solenoid, gray in the graph) that can trip extremely fast, but only under extreme overload condition (eg. short circuit). This requires at least 3x the rated current to activate, so does not come into play in your test case.

aaahh ok, thank v.much for this. It says "Trip type: thermal magnetic" so I guess its the heat type. I'll try it again but for longer.

Sonic01 said:

It says "Trip type: thermal magnetic" so I guess its the heat type. I'll try it again but for longer.

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That probably means it has both the "thermal" bimetallic heat switch, and the [electro-]"magnetic" switch. That's pretty much the standard with these kinds of breakers.

It should trip real fast for a short circuit. Putting everything in safety you could try that with a cheap source and a dummy load. Please do write here after you do the test, we must be sure you're still alive

italianuser said:

It should trip real fast for a short circuit. Putting everything in safety you could try that with a cheap source and a dummy load. Please do write here after you do the test, we must be sure you're still alive

Click to expand...

my cells are individually fused so I'll pass on the short circuit, I don't fancy spending the next few weeks checking and refusing 1500 18650's

Best I can do is run about 112A through it, its a 63A trip so I'll wait a couple of minutes for the thermal trip.

@DavidPoz did a youtube on a circuit breaker he liked - and discovered that it took 10 minutes of significant overload before it tripped. You can see this starting around 15:00 thru the end of the youtube below. The lesson: its wise to understand the trip specifications/characteristics of the circuit breaker chosen. My Midnite breakers trip in a more 'normal' fashion - e.g. like a US AC breaker in a panel / quickly but not all breakers are the same and this youtube was an eye-opener for me.
View: https://youtu.be/q-uDyzkKwVA

Hi all,

JFYI I tested this again based on what I learned in this thread, I ran roughly 130A through a single TOMZN 63A single pole and it took 3m30s for it to trip, so seems to be working ok.

I think when I get this all wired into my house in August I'll probably switch to JTEC though as these are UK produced so will meet UK regulations, only £10 each.