How to wire a DC breaker?

[Hedges]

If that breaker was used with the "+" terminal connected to battery positive, and "-" terminal connected to battery negative, with load connected to the unlabeled terminals, it could interrupt current to shut off the load. In the event of a short circuit, it should do so for a battery able to deliver up to 10,000A, but wouldn't be suitable for one able to source more current (into a short.)

If used between battery and SCC, the issue is that massive current flows one direction if there is a short at the SCC, and moderate current flows from SCC to battery. There are some recommendations of which way to orient it, choosing the lesser of two evils.

If used for one PV string (or one PV array), it could have "+" terminal connected to PV positive lead, "-" to PV negative lead, other side goes to SCC. It shows ratings for up to 250Voc, and being a thermal-magnetic breaker, would be suitable for 50A continuous, 80% of its rating. Because PV panels can deliver an estimated 25% over Isc under some illumination conditions, good for up to 40A Isc array.

What I don't think it is good for is 3 or more PV strings (or arrays) each with this breaker, and connected in parallel. If one string shorts, the others could dump current through its breaker in reverse direction, and a polarized DC breaker can't interrupt that.

You can get some DC breakers that are non-polarized. Those are better if paralleling 3 or more PV strings, or for bidirectional applications like SCC to battery.

 

[HRTKD]

If used between battery and SCC, the issue is that massive current flows one direction if there is a short at the SCC, and moderate current flows from SCC to battery. There are some recommendations of which way to orient it, choosing the lesser of two evils.

What would be the situation where a massive current flows back toward the solar charge controller? My well of imagination has run dry today and I can't think how the situation would come about.

 

[Drizzt321]

@Hedges Thanks for all the recommendations. This is for a small trailer battery/solar setup. 1 (32a) between the ground-mount solar (for long duration, 2 old house panels, ~52v Vcc, 215W nominal, 2 total, either parallel or serial, not sure yet) through a Victron 150/45 MPPT. For shorter term camping, I'm going to get a 150W folding "suitcase" 12v panel. Either one will be ~8-9a at nominal, although it's possible depending on my real world power usage I find I don't really need/want the big house panels anymore and I'll get a 2nd suitcase style. I'm pretty overspecced on everything towards safety. I don't figure on running more than 30-40a in real-world usage at any given time, usually quite a bit less.

I plan on having a 63a one of these breakers between the battery and the busbar/rest of the system. I also have a 80a Class-T fuse as a final precaution which will be right at the battery terminal to everything else.

Battery is a Battleborn LiFePO4 100Ah. Possibly getting a 2nd for parallel in the future, but unlikely.

Other would be a 110v 15a Victron charger, and a Victron DC-DC 18a for charging from the tow vehicle.

I'll then have a blade fuse distribution block for the lights/fridge/etc on appropriately sized fuses for each circuit.

Was originally thinking of just having plain old single pole cut-off switches on the pos, but then happened across these style of DC breakers, and they aren't too expensive, so going to use them as both extra protection, as well as disconnect as-needed.

 

[Hedges]

What would be the situation where a massive current flows back toward the solar charge controller? My well of imagination has run dry today and I can't think how the situation would come about.

Short inside the SCC, or short in cables leading to SCC.

Maybe we put class T catastrophic fuse at battery, this 63A breaker at SCC. That reduces length of cable that could short and draw battery current through SCC. But a short by components inside SCC could well draw > 63A yet not enough to blow fuse.

If we orient breaker to protect against battery current flowing through breaker into a fault, it is used for reverse flow during charging. Good news is if you open breaker, SCC might go to 63V and battery might be 42V, only 21V across it. If a short between battery and SCC with breaker, then catastrophic use blows and breaker has to interrupt its own arc to 63V.

I'd rather have a bidirectional breaker (or fuse and switch. Of course most switches aren't meant to interrupt current flow.)

 

[Hedges]

@Hedges Thanks for all the recommendations.

Battery is a Battleborn LiFePO4 100Ah. Possibly getting a 2nd for parallel in the future, but unlikely.

Check out Midnight's offering; some breakers are non-polarized. And Blue Sea.

I assume lithium batteries could deliver 20,000A into a short (so would prefer 20k AIC not just 10k AIC like the 63A breaker) but these smaller 100 Ah batteries may be less. I haven't seen a manufacturer's published figure so I came up with 20,000A based on internal resistance figures people report.

 

[Drizzt321]

I assume lithium batteries could deliver 20,000A into a short (so would prefer 20k AIC not just 10k AIC like the 63A breaker) but these smaller 100 Ah batteries may be less. I haven't seen a manufacturer's published figure so I came up with 20,000A based on internal resistance figures people report.

You're probably accurate, but that's what the 80a Class-T fuse is for. Hopefully the breaker would trip first, but the 20k AIC at up to 125V DC.

 

[Hedges]

Yes, class T should be good. At least for one battery.
The 280 Ah cells had spec 0.25 milliohm, test results 0.17 milliohms. At 3.4V/cell that would be 20,000A.
I think somebody reported test results not quite that high. Things happening in chemistry or ion transport, vs. 1 kHz test or lower current loads.

Could be the 100 Ah are a fraction of that.

I've tried to "coordinate" trip curves of fuses vs. breakers. I don't have any lithium, but 400 Ah AGM. for that I have a couple 350 A class T at battery, fanned out to four Sunny Island (not sure their exact breaker rating). And a 200A main breaker at grid connection, selected 400 A class T for that. My intent is breaker trips for moderate overload, fuse blows first and hopefully saves breaker for bonded fault.

I did recently do a bonded fault when trying to apply an 80A load to a 100A breaker under test. About 50' 2/0 from utility transformer + 50' 2 awg to sub panel. Closed a 30A breaker into a dead short. It didn't trip, but 100A breaker tripped, 125A main breaker on sub panel tripped, 125A branch breaker on main panel tripped. Didn't have the class T in circuit yet. It didn't trip 200A main breaker or 200A breaker at meter. I figured if transformer was zero ohms about 4000A would have flowed, but including my guesstimate of transformer windings, probably only 1000A. So might have been just under the fast-trip of a 200A breaker.

Seeing one phase get crowbarred and then recover in maybe 8 milliseconds left my Sunny Islands on that phase thinking their relay was stuck. I had to reset them to recover.

 

[12VoltInstalls]

so I came up with

It’s been frustrating in the past knowing I have limited knowledge in some areas and yet being the most knowledgeable on a job. I certainly ain’t no EE.
Thank you for your posts here. I think I’ve confirmed or learned more from you in the last year about certain low-voltage elements than from anybody over my ~40-year career experience.

 

[Hedges]

You're welcome.

Mostly I've absorbed information on topics that interest me, then apply elsewhere. So mostly repeating what someone developed before. Occasionally something new. Utility transformer short-circuit current is calculated based on winding resistance, so I applied same to batteries.
At work there has been training on arc flash/arc blast and NEC which covered things I hadn't seen before.

It’s been frustrating in the past knowing I have limited knowledge in some areas and yet being the most knowledgeable on a job.

On the job, especially at start-ups, there tend to be few people for each discipline and often not up to normal industry competence. So things like thermal, mechanical, financial, safety compliance, etc. have problems and it's amazing the company has any success at all. Similarly big companies sometimes assign new staff who don't understand best practices from previous generation products.

 

[Drizzt321]

@Hedges, one last thing, just to confirm, there shouldn't be a problem with putting power through the breaker to the battery from the SCC/DC-DC charger/etc? As long as I arrange the battery in the input side of the breaker, correct?

Just super paranoid about this.

Or would this non-polarized one be better/safer?

 

[Hedges]

I think "+" side of breaker to battery positive, other side to SCC, is what some vendors have recommended. That give better interrupting if a short occurs at SCC. But it is running backwards during normal charging. If using that 250VDC rated breaker at 50V, should reduce the chance of arcing.

But I'd rather have a non-polarized one.

The specs for the one you found do seem better. Non-polarized. Also 500V. (Although a chance that means 500V for 2 poles, and the other one from Schneider might have been 250V per pole, same thing.)

Here are more non-polarized DC breakers (looks like the "300V" model is 2 poles in series)

Product - MidNite Solar

MidNite Solar Product Page.

www.midnitesolar.com

Product - MidNite Solar

MidNite Solar Product Page.

www.midnitesolar.com

 

[Samsonite801]

I think "+" side of breaker to battery positive, other side to SCC, is what some vendors have recommended. That give better interrupting if a short occurs at SCC. But it is running backwards during normal charging. If using that 250VDC rated breaker at 50V, should reduce the chance of arcing.

But I'd rather have a non-polarized one.

The specs for the one you found do seem better. Non-polarized. Also 500V. (Although a chance that means 500V for 2 poles, and the other one from Schneider might have been 250V per pole, same thing.)

Here are more non-polarized DC breakers (looks like the "300V" model is 2 poles in series)

Product - MidNite Solar

MidNite Solar Product Page.

www.midnitesolar.com

Product - MidNite Solar

MidNite Solar Product Page.

www.midnitesolar.com

I like the quality of the Midnite Solar breakers as well...

https://midnitesolar.com/pdfs/breakers_11x17.jpg

https://midnitesolar.com/pdfs/dc-gfp50-300.pdf

I went with 3 of the 30a version of this type for my PV home runs (installed inside a Midnite Solar DC combiner box):

Amazon.com: MidNite Solar Breaker 50A 300VDC MNEPV50 : Patio, Lawn & Garden

Amazon.com: MidNite Solar Breaker 50A 300VDC MNEPV50 : Patio, Lawn & Garden

www.amazon.com

 

[Hedges]

I went with 3 of the 30a version of this type for my PV home runs (installed inside a Midnite Solar DC combiner box):

Amazon.com: MidNite Solar Breaker 50A 300VDC MNEPV50 : Patio, Lawn & Garden

Amazon.com: MidNite Solar Breaker 50A 300VDC MNEPV50 : Patio, Lawn & Garden

www.amazon.com

Is that for three PV strings combined in parallel? (30A is rather large for a single string of PV panels)

That is the situation where polarized breakers concern me. If one string shorts (wire rubbed through or bypass diodes fail shorted), two strings dump their current through breaker of shorted string. That breaker trips, but if this is carrying current in the reverse direction, so it fails to interrupt the arc and burns.

I think if handles of all three (2-pole) breakers were ganged together, then when the breaker carrying reverse current tripped, the other two carrying forward current would interrupt he current. This assumes only interrupt capability cares about polarity, and tripping works either way.

 

[Samsonite801]

Is that for three PV strings combined in parallel? (30A is rather large for a single string of PV panels)

That is the situation where polarized breakers concern me. If one string shorts (wire rubbed through or bypass diodes fail shorted), two strings dump their current through breaker of shorted string. That breaker trips, but if this is carrying current in the reverse direction, so it fails to interrupt the arc and burns.

I think if handles of all three (2-pole) breakers were ganged together, then when the breaker carrying reverse current tripped, the other two carrying forward current would interrupt he current. This assumes only interrupt capability cares about polarity, and tripping works either way.

Each breaker handles 4s2p panels (8 panels, about 21.18a Isc)... 3 independent PV runs into the inverters (24 panels total).

These 300v 30a breakers I purchased from Midnite Solar are actually 2 single-pole breakers connected in series with each other (wire goes in on top-end, while bottom-end of breaker has a little bus bar connecting to the other breaker siamesed onto it, then the other wire connects to the adjacent breaker on the top-end.

So no wires hook to the bottom of the breaker, just has the copper bus bar.

 

[Phil.g00]

For a polarized DC MCB current direction is important. Incorrect orientation will not quench the arc when it should trip.

Therefore using this MCB as a battery isolator in a conventional manner would be unsafe. The MCB would only operate for external short circuits but not for internal short circuits or vice versa, depending on its orientation.

However, bear in mind, only the + or the - needs to be broken to clear a short circuit.
This presents several solutions for this application as a battery bank isolator and also resolves the orientation issue.
- Either the + or the - leg is passed through both poles of the MCB (once with the correct orientation once with the incorrect orientation.
-Or, alternatively, if both legs of the battery are to be isolated, the + can be correctly orientated and the - incorrectly orientated.
This will provide overload protection for both internal and external faults.

The orientation question is resolved because as long as 1 pole is wired right and one pole is wired wrong, the MCB will be bi-directional.

 

[Hedges]

Do the 3 independent PV runs go to three independent MPPT inputs?
If so, no problem because 2p doesn't require OCP.
If 3 runs to single MPPT (or two runs bring total of 4p to one MPPT), then backfeed through 30A breaker plus one 4s string dumping into a second shorted 4s string would exceed it's fault-current handling (see "max fuse" on PV panel label.)

This is where I suggest running a pin through all 6 poles to gang them together.

So you have 2 poles in series interrupting PV positive, and PV negative wires back without disconnect?
I would rather remove that little bar that came with the breaker. Feed PV positive through one pole, PV negative through the other pole. I like to be able to completely disconnect PV array before working on wiring.
Used as delivered, if you turn off PV breaker but a PV positive wire shorts to earth, PV negative carries current (if grounded), goes to high (negative) voltage if disconnected. That's a shock hazard.

My system (as presently built) has rotary DC switch on bottom of PV inverter, touch-safe fuse holders on one polarity of PV wires inside switch enclosure (PV positive for most brand panels, inverter negative-grounds array. PV negative for SunPower, inverter positive-grounds array.) For one inverter, PV positive and negative both go through touch-safe fuse holders in external combiner box.
I check voltages (confirm Vmp), turn off DC switch, confirm Voc (zero current), lift fuses, unplug MC connectors (color-coded so I can identify which string went to which fuse.) Wait for voltage reading on capacitors to discharge.
This way I can work on wires safely.

However, bear in mind, only the + or the - needs to be broken to clear a short circuit.
This presents several solutions for this application as a battery bank isolator and also resolves the orientation issue.
- Either the + or the - leg is passed through both poles of the MCB (once with the correct orientation once with the incorrect orientation.
-Or, alternatively, if both legs of the battery are to be isolated, the + can be correctly orientated and the - incorrectly orientated.
This will provide overload protection for both internal and external faults.

True - for this case of a 250V breaker and 48V nominal battery because each pole has sufficient voltage.
Even if the breaker is 125V per pole, that is sufficient for the much lower voltage battery.

Same approach could be used for PV, but voltage limit more likely to get in the way.

 

[12VoltInstalls]

2 single-pole breakers connected in series with each other (wire goes in on top-end, while bottom-end of breaker has a little bus bar connecting to the other breaker siamesed onto it,

The orientation question is resolved because as long as 1 pole is wired right and one pole is wired wrong, the MCB will be bi-directional.

Isn’t that basically what Mr Luggage’s Midnite breaker does?

 

[Hedges]

Isn’t that basically what Mr Luggage’s Midnite breaker does?

Don't think so.
I think Midnight wired them in series, both same polarity, so two 150V polarized poles serve as 300V polarized.
The suggestion is to reverse polarity of one, so it works as 150V non-polarized.

Note the two poles are physically oriented the same, but "+" and "-" marks on opposite ends.
Probably have a permanent magnet inside of different orientations.

 

[Samsonite801]

Do the 3 independent PV runs go to three independent MPPT inputs?

Yes, 3 separate PV inputs.

If so, no problem because 2p doesn't require OCP.

Yeah, I know I don't need protection, it's more of a disconnect, since I bought the Midnite Solar DC combiner box mainly for the big red switch on the outside which allows to shut off all breakers simultaneously, without opening the door on it.

If 3 runs to single MPPT (or two runs bring total of 4p to one MPPT), then backfeed through 30A breaker plus one 4s string dumping into a second shorted 4s string would exceed it's fault-current handling (see "max fuse" on PV panel label.)

This is where I suggest running a pin through all 6 poles to gang them together.

n/a

So you have 2 poles in series interrupting PV positive, and PV negative wires back without disconnect?

I guess that's how it will be, still not wired it all up yet. This breaker only has provisions for one conductor in > out... So assuming the ground will go to the bus bar inside the MS box.

I would rather remove that little bar that came with the breaker. Feed PV positive through one pole, PV negative through the other pole. I like to be able to completely disconnect PV array before working on wiring.
Used as delivered, if you turn off PV breaker but a PV positive wire shorts to earth, PV negative carries current (if grounded), goes to high (negative) voltage if disconnected. That's a shock hazard.

Problem, is, doing that may likely turn it into a 150v breaker again. The 150v breakers they sell are true single pole, bumping to the 300v breaker, they have the 2 in series. I do see your logic though. Midnite Solar does sell other non-polarized 300v breakers that are 4-poles though. My box is too small to put 3 of those in there though. I need 3x of 300v breakers for my PV runs (box is only 6 DIN wide).

My system (as presently built) has rotary DC switch on bottom of PV inverter, touch-safe fuse holders on one polarity of PV wires inside switch enclosure (PV positive for most brand panels, inverter negative-grounds array. PV negative for SunPower, inverter positive-grounds array.) For one inverter, PV positive and negative both go through touch-safe fuse holders in external combiner box.
I check voltages (confirm Vmp), turn off DC switch, confirm Voc (zero current), lift fuses, unplug MC connectors (color-coded so I can identify which string went to which fuse.) Wait for voltage reading on capacitors to discharge.
This way I can work on wires safely.

Yeah, I'm not really worried too much about it. I wear insulating gloves and use insulated tools, check voltages frequently when in doubt, and hopefully, I don't plan to work on this system much if at all once it's all deployed. I might just tie negative of PV circuits to Earth ground anyways so they can't float (since I'm switching off of the positive conductor only.

 

[Hedges]

I guess that's how it will be, still not wired it all up yet. This breaker only has provisions for one conductor in > out... So assuming the ground will go to the bus bar inside the MS box.

Does that mean 3x PV positive home-run wires but only one PV negative? For some SCC (Renogy, automobile application) PV negative is ground. But for most, it is required to be separate. So I think should be 3x PV positive and 3x PV negative, all isolated.

Problem, is, doing that may likely turn it into a 150v breaker again. The 150v breakers they sell are true single pole, bumping to the 300v breaker, they have the 2 in series. I do see your logic though. Midnite Solar does sell other non-polarized 300v breakers that are 4-poles though. My box is too small to put 3 of those in there though. I need 300v for my PV runs.

Two ganged 150V poles in series would interrupt 300V just the same, whether hardwired with a short jumper, or one pole for PV positive and one pole for PV negative, with home-run wires and SCC in place of jumper.

Getting polarity of one backwards would turn it into a 150V non-polarized breaker.


Midnight 4x 150V polarized for 300V non-polarized?
I didn't spot that with a quick scan. There was a 2-pole 300V non-polarized, assume that's made from 150V non-polarized poles.

here's 4x 150V polarized for 600V polarized:

Product - MidNite Solar

MidNite Solar Product Page.

www.midnitesolar.com