In this video, Roland raises the awareness about using DC-MCBs as switching devices within the (high- voltage) PV-DC circuit.
This is a safety issue which must be tackled very carefully. t

 

 

To be fair, he also discusses the issues arising in low voltage (48Vdc) applications... 

I've had two '125A' polarised breakers fail (almost catching fire) when never subjected to more than 80A and never operated (switched) under load.

Both failed the same way as Roland W's one in his powerwall - insufficient contact area leading to excessive heat, leading to... well, this:

After the first one, I assumed I'd just somehow messed up the termination.  Swapped it out, taking extra care the second time.  It lasted a week.

When I dissected them both, it became pretty clear that the tiny surface area of the contact was just too small to do the job, and the CSA of the single braid looked totally insufficient.  These were branded 'TOMZN', but I suspect they might have been fake (even though they came from a reputable UK seller).  Even the arc-deflecting 'magnets' appeared to just be pieces of steel (unless the heat killed them too, that is).

I tackled the issue by installing mega-fuses and vowing to never use polarised breakers again if I can possibly avoid it. 🙂

50 minutes ago, HedgeSlammer said:

To be fair, he also discusses the issues arising in low voltage (48Vdc) applications... 

I've had two '125A' polarised breakers fail (almost catching fire) when never subjected to more than 80A and never operated (switched) under load.

Both failed the same way as Roland W's one in his powerwall - insufficient contact area leading to excessive heat, leading to... well, this:

After the first one, I assumed I'd just somehow messed up the termination.  Swapped it out, taking extra care the second time.  It lasted a week.

When I dissected them both, it became pretty clear that the tiny surface area of the contact was just too small to do the job, and the CSA of the single braid looked totally insufficient.  These were branded 'TOMZN', but I suspect they might have been fake (even though they came from a reputable UK seller).  Even the arc-deflecting 'magnets' appeared to just be pieces of steel (unless the heat killed them too, that is).

I tackled the issue by installing mega-fuses and vowing to never use polarised breakers again if I can possibly avoid it. 🙂

Thanks for a great picture but as you say this seems to be heat from the contact area and not the arc quenching of polarised breakers. I have always had my doubts about polarised where there is current flowing in 2 directions. 

Roland explained the pitfalls very well in his video.