DC MCBs on panels (and batteries) - Page 1
Forum section: electronics / general

author: paulca
author_rank: Super Contributor
timestamp: April 14, 2023, 03:01:51 pm
content: I got sold into the idea and my primary reason was not their "fault breaking" capability, but rather their dual pole isolation (for each panel string) + the total.However. On that breaking ability. I tested a 16A 12-240VDC MCB on the bench. With the highest sustained DC load I could create of 180W at 20A it did nothing for at least a minute.Deciding to get heavy handed with it, I set the psu to the max I felt confortable setting it to for this test which was 24V and 20A. The PSU will sustain that, although I knew it wouldn't have to.Sticking a metal file across the output of the breaker, sparks and all and no breakage. Held in place the total power fell to 35W because of the PSU current limiting to about 1.x volts. Bouncing it on and off felt like a way to provoke it, but I stopped as that also sounded like a way to provoke the PSU or it's DC brick release smoke going from 0% to 100% back 10% and repeating. The sparks where fun. No breaker popage and a very hot metal file is the only results.The minimum voltage rating of 12V suggests to me that to pop this MCB you would need to sustain over 16+A and at least 16*12~=192W through current long enough to get the thermal trigger hot enough.So, for low voltage DC short circuit protection they are about as good as a chocolate tea pot.I don't have the equipment or the nerve to consider testing the 100A MCB, and I an not going to attempt a battery short test with it.They will from now on, be considered as "maintenance isolation solutions" and never be relied on for fault current breaking.Time to spend more money on shifting priorities to proper fuses or more specialist breakers. $$$.Note. 12V, 24V DC with 40-100V panels. Being considered here.Your MCBs may do a lot more for you fault breaking in larger and higher voltage and higher wattage systems.Is it worth trying to back calculate the low voltage compensated MCB size for a 10A panel rail say? Work out what lower rating of breaker WILL pop at 12A or so on that rail? It's not protecting any wire, so whats the point?Other than foriegn voltage faults or "back faults" from inverter side back to the DC side, is there much actual benefit on MCBs for fault breaking versus a dump dual pole switch? Arc breaking on the contactor maybe? Their ease of availabilty maybe.

author: themadhippy
author_rank: Super Contributor
timestamp: April 14, 2023, 05:14:16 pm
content: maybe your psu was limiting the current before the breaker had a chance to do its thing,and a 16A device is going to need a bit more than 20A before it thinks of tripping

author: paulca
author_rank: Super Contributor
timestamp: April 14, 2023, 05:32:39 pm
content: Quote from: themadhippy on April 14, 2023, 05:14:16 pmYea. Very likely the PSU responding faster than the short detect in the MCB.There are ways that I can test that. However, as you point out, to pop it, I will either need substansially more current sustained for at least a few hundred milliseconds, like 10 times rating.... or I sustain 20-30A for quite a while.On the battery and inverter they make a bit more sense as the actual available current is greater than the cabling or any of the equipment asides the battery would be upset by. So putting a 100A breaker on a 100A wired and fitted circuit I think, does at least make sense.Maybe backed up by an actual fast blow fuse rated closer to the battery (and or BMS) specs. Personally updated with today experiement I think I want to go Software->Device max 'pull/push' rating->MCB->BMS->FUSEs in that order. With a 105Ah pack capable of a datasheet 210A or higher discharge, but 100A cables, I should calculate the highest current between the two I would like to "see" the cables take and how hot they get before the fuse blows. I think giving the BMS a chance to cut in for sustained > 100A loads is fine, if the MCB doesn't pop first. For higher fault currents like a short, the MCB's tardyness, especially if the voltage colapses will be backed by the 150A or 200A fastblow fuse. I have seen BMS's destroy themselves breaking high over current.Is a worse case to consider the "total available current" in the whole system and consider it could get "anywhere" during a fault. Such that in my design to get from teh battery to the panels, there are 3 MCBs and at least 2 fuses. So having 16A on the panels prevents them from taking some bizarre short from the battery bank.Does that make sense?

author: dmills
author_rank: Super Contributor
timestamp: April 14, 2023, 05:50:18 pm
content: All breakers have a tripping curve and will typically hold a couple of times rating for a long time, and trip instantly at 10 - 15 times nominal rating.Look up the part you have and you will find a curve for it in the datasheet, or at least an B,C,or D specification on the front along with the rated current which you can then refer to a standard to get the curve.The things exist to try to prevent fire and thus as long as the energy let thru is smaller then the energy required to heat the cables to ignition they are quite sufficient (And yes, they sort of suck if the available current does not suffice to trip them in reasonable time, this is actually a concern in some mains installations where the time to clear a fault can be excessive due to loop impedance).See some typical examples herehttps://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/molded-case-circuit-breakers/dc-series-time-current-curves-tc01215003e.pdf

author: paulca
author_rank: Super Contributor
timestamp: April 14, 2023, 06:11:08 pm
content: Of course. I do keep reminding myself that MCBs are protect wires.100A wire, 100A MCB.10A wire, 10A MCB.If you want to detect fault current within 10 times those, you probably need something else?If I want to limit "other" fault scenarios such as smaller over currents like 2x circuit expected, this I need other protection.A fuse is a pain in the ass to change, but if choosen correctly and balanced with the breaker, cable and specs, will respond much faster in a short? I do understand that at 100A rated fault current can be 10 times that, maybe 100 times that instantaneous. I would rather not have more than 2. So 100A MCB, 200A (T class is it? that gives a stated delay?) fuse... they aren't cheap.What about "service limits"? How do you correctly handle non-fault over-current (asides the BMS).Say I have a 100A cabling, but the load on the end of it shouldn't really draw more than 40A sustained over a minute. How to you get "soft" breakers like these, other than the afore mentioned BMS mosfet bridges?

author: dmills
author_rank: Super Contributor
timestamp: April 14, 2023, 06:33:38 pm
content: Z curve breakers as memory serves are the ones with close in fastish trip curves, and you can buy some HIDIOUSLY expensive semiconductor protection fuses, really made for inverter drives and such.Really if you have a 40A load on 100A cables (Not unreasonable sometimes, it keeps the voltage drop down) then surely a 40A protective device makes sense.Generally your run of the mill fuse is actually slower (and less accurate) then a breaker, but can sometimes handle stupidly large PSC which can be an issue in cheap DC breakers.Usually this stuff is really fire protection, you should design the gear with enough margin that you don't expect it to fail.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 15, 2023, 07:00:15 pm
content: Fusing of solar panels is nearly useless for the same reason as trying to fuse a current-limited lab supply - the fuse can't blow because of lack of current.The difference between maximum power point current and short-circuit current on solar panels is just some tens of %. It is impossible to physically manufacture a fuse or MCB which would blow at I_sc, but not blow at I_mpp. Even if it was possible, you would need optimum sunshine conditions coinciding with your failure situation to get the current needed.

author: rteodor
author_rank: Frequent Contributor
timestamp: April 15, 2023, 07:40:58 pm
content: Quote from: paulcaMy equipment distributor said that portfuses are enough for breaking (even if its written on them: Do not break under load). They delivered me the equipment and the connection diagram without DC breakers. 5+2 KW.I've added them myself for the same reason.Quote from: Siwastaja on April 15, 2023, 07:00:15 pmI was asking myself why put 16A fuse on panels with I_sc of 10A ?Well, an electrician explained to me that it makes sense if for example there are 3 strings in parallel and one has shorted panel(s). Then the two remaining strings may feed 20A into the faulty string. Other possible cases are faulty MPPT or the possible contact with other cables that are not part of the panels cabling.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 16, 2023, 05:27:25 am
content: Quote from: rteodor on April 15, 2023, 07:40:58 pmExcept that the 16A fuse will never blow at 20A (except by luck, after hours of said failure condition).

author: rteodor
author_rank: Frequent Contributor
timestamp: April 16, 2023, 05:43:54 am
content: Quote from: Siwastaja on April 16, 2023, 05:27:25 amThat or all 3 fuses will melt in the fire that will follow the fire started by the faulty string.I would better add those string diodes because these fuses can be better named FMOF: Fuses Mounted Out of Fear.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 16, 2023, 08:03:00 am
content: Parallel strings do not make a lot of sense, especially in the days of even the cheapest string inverters having multi-string inputs. Way better than diodes. And for weird enough corner case configurations, just use microinverters.

author: nctnico
author_rank: Super Contributor
timestamp: April 16, 2023, 10:37:54 am
content: Quote from: paulca on April 14, 2023, 06:11:08 pmIf you use the typical wires for solar panels (4 mm^2) then you won't need fuses to protect the wires. There is no way a string of solar panels can ever reach the current handling limit of a 4mm^2 wire. For example: the NL code allows to use a fuse up to 25A for 4mm^2 wires when used in conduit or cable.

author: paulca
author_rank: Super Contributor
timestamp: April 16, 2023, 11:03:36 am
content: Quote from: Siwastaja on April 16, 2023, 08:03:00 amIt's a DC system. It's touch safe voltage. I only have one MPPT. I want 3 panels. My roof suffers morning and evening shading. The MPPT will not take them all in a single string at 124+VoC. In series, IIUC they would not produce anything like full power until all 3 panels were fully exposed to the sun? That could be about an hour in the morning and an hour in the evening lost to partial shading of the series.I was planning on running 3 in parallel.The 16 MCBs provide, is nothing else, somewhere to isolate that panel/string for maintenance. The 40A provides a way to isolate the panels entirely from the MPPT and another 40A to isolate the MPPT completely from the DC bus.The panels breakers, in regards breaking fault currents, is only going to cover rare and bizarre cases and you might be better off with something else for those. However, you will still need an isolator somewhere and for the availability and cost, assuming it doesn't present too many mOhm to the system MCBs .. is there are reason NOT to have them?

author: Siwastaja
author_rank: Super Contributor
timestamp: April 16, 2023, 11:28:22 am
content: Series is better than parallel. Panels have bypass diodes integrated so that the shaded panels let the non-shaded panel still produce full power. Just the shaded panels go to zero.There is a lot of confusion about this because it's an ages old myth which was heavily pushed forward by companies selling microinverters or optimizers.If you have a lot of partial shading hours and want maximum possible (partial) power out of the shaded panels, the only way to get this is separate MPPT per each panel. For a DC system, this means getting three separate MPPT charge controllers.

author: Alti
author_rank: Frequent Contributor
timestamp: April 16, 2023, 01:37:37 pm
content: Quote from: paulca on April 14, 2023, 03:01:51 pmThe terms "short-circuit" and "overload" are well defined in power supply context. Both are different phenomena. Not interchangeable.If those are MCBs made according to IEC60898-1 then these typically dissipate 2.2W per pole, at nominal current. At 1.45In are guaranteed to trip within 1h, at 25oC, from cold state. So 4.63W is all you need to trip a typical circuit breaker, every time. So 0.2V is enough to trip a 16A version (approx).Your 192W seems like something went really off the scale.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 16, 2023, 05:03:53 pm
content: I'll say this just in case somebody have missed it: remember, if you EVER use a fuse in a DC system with voltage higher than some 30-40V, BE ABSOLUTELY SURE to use a fuse properly rated to breakDCat your voltage. Read the datasheet carefully. Trying to break high voltage DC with standard AC-only rated fuses leads to a disaster.