There is a very specific way to overcome this issue, that involves downrating the voltage rating of the mcb.
It requires a unconventional way of wiring a multi- pole mcb. Not really something that the average sparky is up for.
Unless the arc extinguishing concept is understood it is not advised. Please avoid.
Check if the Mcbs are magnetic, 2mins of your life.
I think I should explain some arc quenching concepts.
AC current has natural zero current points due to the whole sine wave thingy.
DC does not, consequently a DC arc will self sustain.
What does that mean?
In a nutshell, an arc ionises the air and makes it a conductor. A very good conductor, just like a piece of wire.
To blow the arc out, well there are a host of mechanisms including literally blasting it with high pressure air to blow it out.
But, that is big breaker stuff, in the domestic mcb the idea is to stretch the arc and to cool it down until it breaks.
To that end there are various mcb constructions that rely on the heat of the arc itself to stretch upwards into multiple thin plates that stretch it further whilst cooling it.
That process relies on the principle of heat rises, and therefor so does the arc.
Another way of forcing the arc to stretch is to use a permanent magnet. Current direction represents a specific magnetic polarity
As we know like poles repel, and this is the mechanism used to drive the arc into the thin plates.
However, if the current direction is otherwise, the arc is not repelled, but attracted. And therefore will not be broken resulting in the catastrophic failure of said mcb.
The +~ signs are not a reliable indication of whether the mcb is bi-directional or not. Whether the mcb is magnetic or not is. You should be able to feel the magnetic effect externally without disassembling the mcb.
I havenāt been fully comprehensive about everything, but hopefully enough so you can make decisions in youāre own application.
Presume the MCB needs to have a current flowing.
If I understand you correctly, I need to put a knife next to the MCB:
If I feel a magnetic field, that is bad.
If not, that is fine.
Yeah, the +/- signs on my NoArks are āconfusingā (only the first time).
It depends on if the current comes in from the bottom or the top.
You connect the wires accordingly to the +/-.
The output on the other hand, purely based on the +/- signs, can lead to some confusion, as it now seems āreversedā, but it is not, the actual current flowing is still +/-.
Quite clear once you āget itā.
Nope, it is the presence of a permanent magnet you are looking for.
Correct.
These signs are critical in directional mcbs, the thing is I have seen them on non-directional mcbs as well. On a non-directional mcb they matter not-a-jot to functionality. A non-directional mcb will do its job regardless.
This thread scares me! I thought installing solar panels wasnāt too difficult. Well, I suppose it is if you donāt worry about doing it right. āRightā seems to be not so clear cut.
āRightā is very clear-cut, just not very well-known.
Is there a diagram available of the āRightā way, with brands/model numbers etc.?
Yes, from reputable manufacturers, there will be. The problem is reputable manufacturers are the first to attract counterfeits.
That is why I have detailed a way to discover for yourself.
Checked. No magnetic field felt. So that is āsortedā.
BUT, this No-Ark model I bought way back when Combiner Boxes were the latest coolest thing ever, mine the coolest one of them all (for the time), Iām waiting on confirmation that it canāt do the job. No one ever mentioned the model at the time. So no harm, no foul.
I donāt like what @justinschoeman said. Not one bit!!! 
ā¦ Iād be stupid if I ignored it.
Buck stops with me ā¦ Caveat Emptor.
Do what Anton did ā¦
I couldnāt find the non-directional breakers when I did this combiner box:
When I did it I was aware of the issue with reverse currents, but given that thereās fuses as well I didnāt dwell on it. Now I am wondering whether I should replace those breakers. They are NOT cheap.
1 Like
Why are they even there? No legal requirement, and no practical requirement for them that I can see.
The polarised breaker on the output is fine, as you can not get reverse currents there.
I believe I was aware at the time of the reverse current issue, and adding fuses was an attempt to mitigate the risk. For what itās worth the fuses are 15A and the breakers 16A. (I put in fuses in all subsequent installs as well, for some redundancy and also because theyāre cheap.)
If thereās a massive reverse current then a fuse should blow and prevent a fire, but Iām now worried about what would happen if thereās a mild reverse overcurrent that causes the breaker to (attempt to) trip without blowing a fuse. I didnāt contemplate this scenario back then.
Right, I have the wrong NoArk breakers.
Thanks @justinschoeman for pointing it out.
Now to sort it, cause I like the option to switch the strings off under load, to test them once in a while.
Easy to sortā¦ Add a fuse to each stringā¦ I have fuses as well as NoArkās (The wrong ones but I only use them to disconnect).
Nope - you have fuses.
Add another DIN rail to the top of the box for a fuse for each positive?
What I am fearing is that the breaker opens before the fuse blows, and then because the current is flowing in the wrong direction it is unable to extinguish the arcā¦ and starts a fire.
What is the Imp of the string? I think a 15a fuse would be fineā¦
Yeah, was thinking the same based on Antonās design.
Thinking of putting the fuses between the panels and the breaker, and I can still switch off under load to test.
15a fuses yes, for me. Breakers are 16a per string.