Generally speaking, unpolarized low-voltage DC MCBs and MCCBs (below 100VDC) are typically quite cheap, but once you get into higher voltage ratings you either have to spend a whole lot more money for an unpolarized DC breaker, or you have to use a (still cheap) polarized DC breaker. Polarized DC breakers contain magnets which direct the ARC into the arc chamber and must absolutely be wired correctly because the current direction matters in terms of directing the arc. The I.R. (interrupt rating) can be very low... solar panel strings produce very little current, so a breaker is perfectly acceptable *IF* it is wired correctly. There is a simple rule of thumb in terms of gauging DC breakers. You can use unpolarized DC breakers for low-voltage (e.g. battery DC busses) situations under 100VDC, and those breakers should generally be rated at least double your nominal maximum DC voltage. But any POLARIZED DC breaker you buy for the purposes of a solar disconnect had better be rated for 1000VDC or higher. Period, end of story. Do not use lower-rated polarized breakers for solar disconnects, ever. You can buy polarized 1000VDC breakers in MCB and MCCB formats for the purposes of implementing a solar string disconnect. Again, the interrupt rating (I.R.) is irrelevant for this application because solar panels cannot produce a whole lot of current. The best way to get it right is to buy a proper pre-wired combiner box that includes per-string fuses and a master breaker on the back-haul, even if you don't parallel any strings and even if the fuses are not needed. The "+" and "-" on polarized DC breakers are an indication of current direction, NOT POSITIVE OR NEGATIVE VOLTAGE. Current direction. The current direction goes from the "+" to the "-". There is no "line" or "load"... it might be marked, but it's not relevant. You must wire polarized DC breakers such that the current direction goes from "+" to "-". For battery DC wiring (under 100VDC), NEVER use a polarized breaker. The reason is because current can flow in either direction... there is no correct way to wire a polarized breaker in a battery system. Don't even use polarized breakers for situations, such as connecting up an inverter, where you know what the current direction will always be. Always use unpolarized DC breakers. -Matt
I have four 545 watt panels with a voc of 49.6 each. All in series going into my Ecoflow Delta pro ultra. Twice now I have had two inline 30 amp MC4 fuses fail. The fuse itself is fine. The contacts that touch the fuse fail. I feel safer with no fuse at all.
Was really hoping to see an example of a MCB with an inverter on the load side being opened under full PV load and then the same make & model MCB tested with a short circuit on the load side. For a polarised device the same testing both with correct and incorrect polarity. Still, it was good to see you did eventually get around to doing another test.
Ahh yes ok thats a valid point. It is however the same, an inverter at its peak would be close to the short circuit current of the solar array. I then did cross the polarity on the polarised circuit breaker, it was only then that it flashed over. However next time I come across one in the field I will do the test if conditions permit and record it 👍🏻
As an installer, you only see this when people buy the idea that they need to put as many panels as possible into series. Fact is, series is dangerous not matter what your told. Its safe operation depends on the fantasy that a every panel in the string being identical and NEVER changing, never getting bird crap on one cell, never having one leaf fall on it, never having one weak buss tab. I have seen so many disconnects and panels with burns in them, not one was just parallel strings, ALL were in series. Over 1/2 of all the jobs we are involved in have the panels in parallel ( mostly RV's and Small Cabins) , not series, yet only them in series even if same total watts, has this fire trend.
I don't see this trend myself at all. Generally speaking, series and parallel both have their warts, but series is significantly safer in practical application because. parallel configurations are often improperly constructed (people omit the required combiner box to reduce wiring). In the old days, people also paralleled multiple series strings together but that should not be done in modern times... each series string should have its own MPPT. With series strings you want to give each string its own MPPT these days. Paralleling multiple long series strings can be dangerous, even with a combiner box. For a single series string, current ganging on remaining conductors due to there being a few broken conductors or partial shading definitely happens, but modern panels have a lot of redundancy and the bypass diode takes over if too many conductors are blocked by shading or breakage. Brown spots are the worse that should ever happen with a modern panel that has been damaged. The backhaul cabling in a series configuration is cheaply overspecified (e.g. 10 AWG for 10A). High voltages are not present on individual series panels when you don't parallel with another series string. And there is no incentive to cheat on the wiring. Paralleling is safer only when each panel is paralleled through a proper combiner box, with individual fusing and a properly-sized backhaul. Broken and shared conductors in the panel will not gang to working conductors. BUT if you have a lot of panels this requires a multiplication of cabling up on the roof, so almost nobody does this. Or worse, people use MC4 paralleling connectors and create a far, far more dangerous fire hazard than any series configuration would ever have. Inline fuses up on the roof are also very dangerous. Since almost nobody wires paralleled panels correctly, series winds up being safer. Far safer. -Matt
String inverters require panels to be in series, this increases the voltage but keeps the amps relatively low. That would be why you see failures of this type as the voltage is the pushing power of the electrical current. We are allowed to have a Voc of 1000v DC now in Australian residential solar systems.
Great video, so please correct me if i'm wrong, is the key takeaway for diy offgrid guys to not use a breaker at all as even if correctly spec'd as it will most likely catch fire if broken under load. and just use a rotary isolator switch in a protected area?
The message is to make sure you choose the correct switchgear for your application, and that it is installed correctly. There is a link in the description to help with that or consult a professional. cheers jas
I prefer using a properly wired breaker over a DC disconnect. And you can't just use a rotary switch... a DC disconnect must be spring-loaded. In anycase, I like the breaker better because its failure condition from corrosion and carbon is generally a thermal trip via the bimetalic element whereas the failure mode for a DC disconnect under similar conditions is a fire. Listed breakers are required to fail in a tripped state. Environmental conditions are also a consideration. Generally speaking these days you should never have a switch of any kind up on the roof, or have a switch exposed to the elements. DC disconnects are often outdoor rated but I've seen plenty of failures of such devices. I still don't expose them to the elements no matter how they are rated. Breakers, of course, are not outdoor rated and must be in proper enclosure. So either way, I'm gonna be using an enclosure. -Matt
Interesting. I thought breakers like this would be symmetric in terms of polarity. Evidently not. I’m guessing the shape of the contact inside the breaker is such that negative charge building up on a pointed surface (resulting in a much stronger electric field gradient) is more likely to arc flash than negative charge building up on a flat surface. I’d be interested to read a little more about the physics of arc quenching. Previously I thought air gap separation was the main determining factor between one breaker working vs another failing.
High voltage DC breakers are almost always polarized, because an unpolarized high voltage DC breaker is very, very expensive. These are only applicable to the solar back-haul. Unpolarized DC breakers should be used everywhere else (on the battery bus, for example... DC voltages less than around 100VDC should always use unpolarized breakers rated for 100VDC or higher). Polarized DC breakers on solar backhauls should ALWAYS be rated for 1000VDC or higher, no matter what voltage the panels actually backhaul. Never use a polarized breaker rated below 1000VDC on a series string, even if the string voltage is below the rating. Polarized DC breakers have magnets which direct the ARC into the arc extinguishing chamber very quickly. But if wired incorrectly, the magnets direct the ARC away from the chamber instead and cause a fire. Unpolarized DC breakers do not have magnets and the ARC moves a lot more slowly, and are thus rated for far lower voltages in order to meet internal mechanical specs. -Matt
400vac does not equal 400vdc. AC breakers use the zero crossing point to extinguish the arc. DC does not have a zero crossing point a breaker or switch must be rated for or above the intended voltage and current. I think these are AC been re labeled as DC, I come to that conclusion as all relays and switches and relays I have come across always have a lower dc voltage rating.
Those small 2 pole breakers aren't good for high currents. There are way more robust breakers with 20kA rupture ratings, or a T class fuse and high amp isolator switch. Shutdown procedure is crucial, isolate the load first
Solar panels do not produce high currents under any circumstance. The roughly 2000A to 6000A IR of a MCB or MCCB is vast overkill in terms of current handling in this application. You have to be a bit more careful using DC breakers on battery busses as batteries can produce high short-circuit currents when ganged in parallel (depends what the BMS's instant-disconnect current configured to x N batteries). These are lower-voltage busses, though, so the breakers still won't be very expensive.
Such an important video merits a much better video with all the superfluous stuff cut out Sorry lad, I got the gist of your video, but it was very confusing.
Generally speaking, unpolarized low-voltage DC MCBs and MCCBs (below 100VDC) are typically quite cheap, but once you get into higher voltage ratings you either have to spend a whole lot more money for an unpolarized DC breaker, or you have to use a (still cheap) polarized DC breaker.
Polarized DC breakers contain magnets which direct the ARC into the arc chamber and must absolutely be wired correctly because the current direction matters in terms of directing the arc. The I.R. (interrupt rating) can be very low... solar panel strings produce very little current, so a breaker is perfectly acceptable *IF* it is wired correctly.
There is a simple rule of thumb in terms of gauging DC breakers. You can use unpolarized DC breakers for low-voltage (e.g. battery DC busses) situations under 100VDC, and those breakers should generally be rated at least double your nominal maximum DC voltage. But any POLARIZED DC breaker you buy for the purposes of a solar disconnect had better be rated for 1000VDC or higher. Period, end of story. Do not use lower-rated polarized breakers for solar disconnects, ever.
You can buy polarized 1000VDC breakers in MCB and MCCB formats for the purposes of implementing a solar string disconnect. Again, the interrupt rating (I.R.) is irrelevant for this application because solar panels cannot produce a whole lot of current.
The best way to get it right is to buy a proper pre-wired combiner box that includes per-string fuses and a master breaker on the back-haul, even if you don't parallel any strings and even if the fuses are not needed.
The "+" and "-" on polarized DC breakers are an indication of current direction, NOT POSITIVE OR NEGATIVE VOLTAGE. Current direction. The current direction goes from the "+" to the "-". There is no "line" or "load"... it might be marked, but it's not relevant. You must wire polarized DC breakers such that the current direction goes from "+" to "-".
For battery DC wiring (under 100VDC), NEVER use a polarized breaker. The reason is because current can flow in either direction... there is no correct way to wire a polarized breaker in a battery system. Don't even use polarized breakers for situations, such as connecting up an inverter, where you know what the current direction will always be. Always use unpolarized DC breakers.
-Matt
I have four 545 watt panels with a voc of 49.6 each. All in series going into my Ecoflow Delta pro ultra. Twice now I have had two inline 30 amp MC4 fuses fail. The fuse itself is fine. The contacts that touch the fuse fail. I feel safer with no fuse at all.
Was really hoping to see an example of a MCB with an inverter on the load side being opened under full PV load and then the same make & model MCB tested with a short circuit on the load side. For a polarised device the same testing both with correct and incorrect polarity. Still, it was good to see you did eventually get around to doing another test.
Ahh yes ok thats a valid point. It is however the same, an inverter at its peak would be close to the short circuit current of the solar array. I then did cross the polarity on the polarised circuit breaker, it was only then that it flashed over. However next time I come across one in the field I will do the test if conditions permit and record it 👍🏻
Thanks, good to see how things can go wrong.
As an installer, you only see this when people buy the idea that they need to put as many panels as possible into series. Fact is, series is dangerous not matter what your told. Its safe operation depends on the fantasy that a every panel in the string being identical and NEVER changing, never getting bird crap on one cell, never having one leaf fall on it, never having one weak buss tab. I have seen so many disconnects and panels with burns in them, not one was just parallel strings, ALL were in series. Over 1/2 of all the jobs we are involved in have the panels in parallel ( mostly RV's and Small Cabins) , not series, yet only them in series even if same total watts, has this fire trend.
I don't see this trend myself at all. Generally speaking, series and parallel both have their warts, but series is significantly safer in practical application because. parallel configurations are often improperly constructed (people omit the required combiner box to reduce wiring). In the old days, people also paralleled multiple series strings together but that should not be done in modern times... each series string should have its own MPPT.
With series strings you want to give each string its own MPPT these days. Paralleling multiple long series strings can be dangerous, even with a combiner box. For a single series string, current ganging on remaining conductors due to there being a few broken conductors or partial shading definitely happens, but modern panels have a lot of redundancy and the bypass diode takes over if too many conductors are blocked by shading or breakage. Brown spots are the worse that should ever happen with a modern panel that has been damaged. The backhaul cabling in a series configuration is cheaply overspecified (e.g. 10 AWG for 10A). High voltages are not present on individual series panels when you don't parallel with another series string. And there is no incentive to cheat on the wiring.
Paralleling is safer only when each panel is paralleled through a proper combiner box, with individual fusing and a properly-sized backhaul. Broken and shared conductors in the panel will not gang to working conductors. BUT if you have a lot of panels this requires a multiplication of cabling up on the roof, so almost nobody does this. Or worse, people use MC4 paralleling connectors and create a far, far more dangerous fire hazard than any series configuration would ever have. Inline fuses up on the roof are also very dangerous.
Since almost nobody wires paralleled panels correctly, series winds up being safer. Far safer.
-Matt
String inverters require panels to be in series, this increases the voltage but keeps the amps relatively low. That would be why you see failures of this type as the voltage is the pushing power of the electrical current. We are allowed to have a Voc of 1000v DC now in Australian residential solar systems.
Great video, so please correct me if i'm wrong, is the key takeaway for diy offgrid guys to not use a breaker at all as even if correctly spec'd as it will most likely catch fire if broken under load. and just use a rotary isolator switch in a protected area?
The message is to make sure you choose the correct switchgear for your application, and that it is installed correctly. There is a link in the description to help with that or consult a professional. cheers jas
if possible use mccb, they have better arc breaking capacity compare to mcb. but they are costly.
I prefer using a properly wired breaker over a DC disconnect. And you can't just use a rotary switch... a DC disconnect must be spring-loaded. In anycase, I like the breaker better because its failure condition from corrosion and carbon is generally a thermal trip via the bimetalic element whereas the failure mode for a DC disconnect under similar conditions is a fire. Listed breakers are required to fail in a tripped state.
Environmental conditions are also a consideration. Generally speaking these days you should never have a switch of any kind up on the roof, or have a switch exposed to the elements. DC disconnects are often outdoor rated but I've seen plenty of failures of such devices. I still don't expose them to the elements no matter how they are rated. Breakers, of course, are not outdoor rated and must be in proper enclosure. So either way, I'm gonna be using an enclosure.
-Matt
You mentioned something about a link to the specifications?
yes a link pops up on the screen at 3.36 or it is in the description. It is for how to select the correct DC Isolator.
noark non polarised are the ones for me
NOARK duel poll seem to be very good...
NOARK is a good brand, and also rather expensive.
Interesting. I thought breakers like this would be symmetric in terms of polarity. Evidently not. I’m guessing the shape of the contact inside the breaker is such that negative charge building up on a pointed surface (resulting in a much stronger electric field gradient) is more likely to arc flash than negative charge building up on a flat surface. I’d be interested to read a little more about the physics of arc quenching. Previously I thought air gap separation was the main determining factor between one breaker working vs another failing.
High voltage DC breakers are almost always polarized, because an unpolarized high voltage DC breaker is very, very expensive. These are only applicable to the solar back-haul. Unpolarized DC breakers should be used everywhere else (on the battery bus, for example... DC voltages less than around 100VDC should always use unpolarized breakers rated for 100VDC or higher).
Polarized DC breakers on solar backhauls should ALWAYS be rated for 1000VDC or higher, no matter what voltage the panels actually backhaul. Never use a polarized breaker rated below 1000VDC on a series string, even if the string voltage is below the rating.
Polarized DC breakers have magnets which direct the ARC into the arc extinguishing chamber very quickly. But if wired incorrectly, the magnets direct the ARC away from the chamber instead and cause a fire. Unpolarized DC breakers do not have magnets and the ARC moves a lot more slowly, and are thus rated for far lower voltages in order to meet internal mechanical specs.
-Matt
hope that wasn't a fluke you used - they are great tools but I don't think they are rated to toast marshmallows.
Much prefer built in isolators
400vac does not equal 400vdc.
AC breakers use the zero crossing point to extinguish the arc.
DC does not have a zero crossing point a breaker or switch must be rated for or above the intended voltage and current.
I think these are AC been re labeled as DC, I come to that conclusion as all relays and switches and relays I have come across always have a lower dc voltage rating.
Those small 2 pole breakers aren't good for high currents. There are way more robust breakers with 20kA rupture ratings, or a T class fuse and high amp isolator switch. Shutdown procedure is crucial, isolate the load first
Solar panels do not produce high currents under any circumstance. The roughly 2000A to 6000A IR of a MCB or MCCB is vast overkill in terms of current handling in this application.
You have to be a bit more careful using DC breakers on battery busses as batteries can produce high short-circuit currents when ganged in parallel (depends what the BMS's instant-disconnect current configured to x N batteries). These are lower-voltage busses, though, so the breakers still won't be very expensive.
Iam not sure but for me moral of story use electrical components according to its usage schema and check polarity?
Such an important video merits a much better video with all the superfluous stuff cut out
Sorry lad, I got the gist of your video, but it was very confusing.
Yes a couple have said that, but many complained my first video on the same subject lacked detail 🤷♂️ somewhere in the middle next time perhaps.
Isn't the top line and bottom load?
@@r3d3y3z either or so long as polarity is correct if it is a polarised device.
man you ramble a bit
Yup I do, sorry mate.