Choosing the right DC breaker to stay in the limits of battery bank

OntheWay · June 2015

Ok, first, I read all info I was able to find, on DC breakers. Mainly blue sea systems breakers and visually similar items of other brands in the market.

My inverter is able to pull 2400W continuous, 4800W peak. In order to handle 4800, my breaker should be over 200Amp, something like 250, @24V. This is from Inverter side. From battery bank side, I have a couple of serialized 110ah -not so deep cycle- 12V batteries and I simply do not want to push them to the limits. If anyone does, the breaker should control power hunger of the ppl at home.

Also, do not like the idea of allowing over 100Amp through the wires in home, for safety reasons.

My DC cables are 6AWG, and I need the breaker trip before 6AWGs become redhot (or, say at least before they heat up over touch temperature).

What I do not understand with DC breakers is, they are rated like 50A, 100A, 150A etc.. To make breaker trip somewhere like 2000-2200W band, which is will be the correct choice?

The reason why I could not calculate it is, they are not rated to a fixed voltage, some item description says 12/24V, others says 42V max. Lets take 100A one, what Watts will be the trip point? 12x100? or 24x100? or, 42x100?

(fyi, this system does not involve solar panels, batteries are charged from grid, as long as its available)

CALLD · June 2015

For starters please just replace your battery cables with something that can handle the current your inverter can pull - it's worth the peace of mind it really is...

Secondly a breaker will rarely trip at its rated current immediately. I've pulled 100amps through a 63Amp breaker and had to wait nearly 2mins for it to trip.

Breakers are designed to prevent your cables from overheating and should be sized according to the cable thickness.

It's really hard to control what people are going to plug in, I've tried and gave up on that.

Your batteries are also on the small side for an inverter like that - you will probably find a 2400w load will pull the voltage down below the inverter's shut down voltage within about 10-20mins anyway.

BB. · June 2015

The voltage rating for a circuit breaker/fuse is the maximum (working?) voltage that the breaker can safely trip (contacts open) and extinguish the internal arc. If you have a working voltage >> than the breaker's rated voltage, the arc will sustain and possibly catch the breaker on fire:

Here is an example of a ~120 VAC rated circuit breaker on 290 VDC solar array just being "turned off" under load:

https://www.youtube.com/watch?v=HLX9cdB4TFQ

Otherwise, the trip current is all the circuit breaker "cares" about... I.e., if it is rated at 15 amps, it will trip at 15 amps and 120 VAC, 240 VAC, 10 VAC, etc...

You can look at the rated current for 6 AWG wiring... You will find that there are many different rating requirements. Based on ambient temperature, insulation type (how hot it can get), exposed wiring vs in conduit, wet/dry, etc. Here are a couple simple tables to start with. NEC is pretty conservative, the marine is not:

http://lugsdirect.com/WireCurrentAmp...ble-301-16.htm
http://www.boatus.com/boattech/artic...-terminals.asp (see Table #3)

Depending "who you believe", you can "safely" run ~55 to 120 amps through a #6 AWG cable.

And remember, that the NEC and circuit breaker specs should be derated for continuous current flow (basically, the breaker will not trip below 80% of rated current, and will trip at 100% or greater of rated current--and that can take minutes to hours to trip):

55 amps * 0.80 NEC derating = 44 amps max continuous
120 amps * 0.80 NEC derating = 96 amps max continuous

The maximum surge current (say to start a motor) for AC circuits is usually around 5x rated current (55 amps * 5x surge = 275 amp surge for a few cycles to a few seconds).

A #6 cable in free air will fuse at ~600 amps.

If you want a 100 amp breaker on your 6 AWG cable [note: this is the 24 VDC input side of the AC inverter], the "max continuous" current would give you a "useful/reliable" AC inverter output of:

100 amp breaker * 0.80 NEC derating * 0.85 AC inverter eff * 21 volts minimum inverter voltage = 1,428 Watts into your load

My suggestion is not to scrimp on your DC wiring... Yes copper is expensive, but usually the distance to run those cables is only a hand full of feet--Spend the money and don't lose sleep at nigh that your system is waiting to fail you (Mr. Murphy will find a way--At the worst possible time in the worst possible way).

We are not used to circuits in our home that run near full rated current for hours at a time... However, when running solar panels and/or AC charging of your DC battery bank, you may be running rated current through your DC wiring for 5 hours or more, multiple times a week (if the battery bank is well discharged and if you have loads too). Even if you design to rated loads, the electrical wiring/connections will still get warm. Design with less margin, and the wiring will get hotter. Heat and Hot/Cold thermal cycling is really hard on mechanical (and electronic) devices. If you can keep them cool (don't run "small wire", have good air circulation, don't have stuffed in a hot closet, etc.). -- Your system will be much more reliable in the long term.

A handy engineering rule of thumb--For every 10C (18F) increase in operating temperature, you will have 1/2 the operational life.

-Bill

OntheWay · June 2015

Well, I am scared

Too bad that the inverter unit is able to adjust max. utility charging current, but theres no limit or alarm for output.

I will increase the cable thickness. Another good lesson I learned from the above and similar videos available is the following: If you do not place the DC breaker on a firewall or something, it can burn the house.

I do not care about copper costs, but worrying about diameter of the hole on the wall drilled for DC cables' access to batteries behind the wall. The wall ise solid concrete with metal skeleton within. Enlarging hole, drill will possibly hit to a metal skeleton piece and stop.

As far as I understand from videos, unless you incorrectly wire the breakers, they will trip over rated current. For extra protection, I think I can also add a regular single use fuse (called blade fuses I think). For the moment, I can get thicker (2AWG seems fit to hole) and use a fuse or two, around 50A.

Vic · June 2015

Hi OtW,

The Blue Sea breakers that you are probably looking at are almost certainly Thermal type units.

From what you noted, guess that this system is 24V. The Surface Mount breaker you are looking at may be in the 187 Series ? ? ??

IIRC, these breakers are limited to 3000 A Interrupt capability on a 24 V DC system. Many say that any breaker connected to a battery power system in the class that you are considering should have an Interrupt capability of a minimum of 5,000 A IR.

Just my opinion, but the Blue Sea breakers are convenient, but, being Thermal (my guess, as no part number of Link was provided) they may not be up to the rigors of a backup power system, especially if you choose later to upgrade battery capacity. Furthermore, being inside a residence, IMO you will want to take every precaution to make the system as safe as you possibly can.

How about looking at the Industry Standard Magnetic-Hydraulic circuit breakers from MidNite Solar, or, perhaps Outback Power (?):
Hi OtW,http://www.solar-electric.com/instal...-breakers.html

The major drawback of the shown Outback and MidNite breakers on the above page is that finding a quality, metal box to mount them is not commonly available at a low cost. And any hack that one might make would not be Code Compliant.

The MidNite Mini-DC boxes include a breaker (in varying sizes), are Code, and not too expensive (IMO):
http://www.solar-electric.com/instal...idcdipoce.html

If this system will be, or should be inspected, there are not too many options that one has to meet the probable Code in effect for your Jurisdiction. Because your system is connected to the Grid, it may well want/need to be Inspected, to help protect any Insurance that covers the Dwelling (I think that applies).

Do not want to overstate any Risks on DC battery-based Power Systems, but, as was noted previously, safety is very important, especially inside a dwelling.

Opinions, Good Luck, Vic

OntheWay · June 2015

Vic wrote: »

From what you noted, guess that this system is 24V. The Surface Mount breaker you are looking at may be in the 187 Series ? ? ??

Thanks for the opinions.

Yes my system is 24V.

Found this one online (185 series?):
http://www.denizdukkani.com/blue-sea-systems-otomatik-sigorta-/-siva-ustu-montaj

And also this one, but largest capacity is 50A:
http://www.denizdukkani.com/a-serisi-flat-rocker-tek-kutuplu-otomatik-sigorta

Also, do you advise using these ones:
http://www.denizdukkani.com/blue-sea-systems-midi-sigorta

I checked the the boxed ones you indicated, price for 125A ones are not so expensive, but its shipping will be

Vic · June 2015

Hi OtW,

Thanks for the added detail, and links.

OK, forgot, or did not know just where on the planet you are located ...

So, assume that you are not in North America. What I had mentioned about Codes will probably not directly apply, although, doing things as safely as you possibly can, will still serve you well.

The Blue Sea breakers are OK, and Thermal units are OK. They may not be rated for switching/breaking high current DC loads often, especially when manually switched, and so on.

It is not clear what is standard practice where you are located. There must be suppliers of Off Grid DC breakers where you are, that are compliant with standards, where you are.

Just my opinion, but believe that Blue Sea products, primarily, seem to be targeted to Marine applications. Nothing wrong with that, but things seem to be done differently in that environment, use different standards, etc.

You asked some questions, regarding Fuses, etc. The use of a Fuse and a circuit breaker in the same connection requires careful consideration, as one does not want a quality fuse to be blown at all often, as good fuses (like Class-T) are expensive, and FAST.

The first off-grid backup power system that I did, used a Blue Sea 187 style breaker, it was quick, and easy to mount and connect, and it was on the inverter leads, so I have used them, but just a personal bias, that I would try to avoid Thermal breakers if at all possible.

More later, Good Luck, Vic

OntheWay · June 2015

Vic wrote: »

OK, forgot, or did not know just where on the planet you are located ...

I am 10 hours ahead, in Turkey.

Right, The Code dont apply here, but of course, I am trying to take every measure which is applicable to my conditions. Therefore, unless I find magnetic types around, thermal breakers seem still a good option, then having no breakers at all.

Blue sea products are sold by marine suppliers here, other then that, Schneider brand available, although I do not know if they have something applicable.

Let me give some details on usage of power backup. Although its capable of, I did not allow it to automatically start providing power in case of electricity cut off. There are two things in front, first is a 1-0-2 crossover switch (completely isolates the grid), second is power (on-off) switch of inverters itself.

Planned use will be like this:

When inverter power is needed, I will first trip all unnecessary AC breakers (this will possibly left me lights and, sometimes, fridge). Then 1-0-2 switch to (2) position, then will turn on the inverter. I am not experiencing frequent electricity problems where I live. Therefore, I am not expecting to use this system, more then once a month, or two months. But, no one knows, its good to know I have backup.

Regards,

Vic · June 2015

Hi OntheWay,

Thanks for the info on your location, and detail on your anticipated use of this backup power system.

YES, absolutely, some circuit protection is far better than using a system without any protection while waiting to find the "perfect" protection.

Also, the Blue Sea products are generally good, from a reliable company, and should provide good protection.

In the USA almost all of our homes use circuit breakers that are Thermal/Magnetic types, which work quite well.

Schneider Electric does offer large-frame breakers that are suitable for use with inverters. These are usually rated at 175, and 250 Amps, and perhaps 125 A, also. But these are large, and normally mount in a large, expensive conduit box.

Your proposed use of the Blue Sea thermal breakers looks to be the best solution, as they are surface-mount.

This Forum has a broad footprint, into many countries (as you know). Sorry that I had guessed (primarily based on your use of the English language -- like a native speaker, which you may be) that you "must" be in the USA or, perhaps Canada. So much for assumptions!

Good Luck, Vic

OntheWay · June 2015

Vic wrote: »

Hi OntheWay,

Thanks for the info on your location, and detail on your anticipated use of this backup power system.

YES, absolutely, some circuit protection is far better than using a system without any protection while waiting to find the "perfect" protection.

Also, the Blue Sea products are generally good, from a reliable company, and should provide good protection.

In the USA almost all of our homes use circuit breakers that are Thermal/Magnetic types, which work quite well.

Schneider Electric does offer large-frame breakers that are suitable for use with inverters. These are usually rated at 175, and 250 Amps, and perhaps 125 A, also. But these are large, and normally mount in a large, expensive conduit box.

Your proposed use of the Blue Sea thermal breakers looks to be the best solution, as they are surface-mount.

This Forum has a broad footprint, into many countries (as you know). Sorry that I had guessed (primarily based on your use of the English language -- like a native speaker, which you may be) that you "must" be in the USA or, perhaps Canada. So much for assumptions!

Good Luck, Vic

Hi Vic and Bill,

At the end of the day, I finally have some good news. I managed to find a hydrolic-magnetic type breaker, a German brand (as far as I know). Here is a link to the product:

http://www.e-t-a.com/products/circuit_protection_devices/magnetic_hydraulic_magnetic_overcurrent_circuit_breakers/p/8345/

Its 8345 model I am planning to buy. In the phone call, they informed me there are three types of this product, one is tripping fast, immediately. Cannot remember the second one

and the last type is able to hold about 10 secs before tripping and can stand about three fold of rated amperage during this period (for refrigerator surge , etc). I am especially interested in with the last one. In this case, I can use something like 70A which translates to 1680Watts@24V, before derating. (Not sure if NEC derating applies to this type of breakers). But anyway, my maximum cont. load will be like 900-1100, only if I turn on all the lights, fridge, desktop computer, satellite receiver, TV and aquarium stuff. I am not planning to let let this happen, this much, of course. Normal load will be like 500-600W, may be even less, depending on the longevity of shortage.

Therefore a 70A breaker seemed fine to me, with added ability to handle motor surges, for a long time period then needed. (I am afraid there is no box for the breaker, I am planning to mount it naked to concrete wall, directly, this way, it will be in vicinity to access in emergency)

In short, I want it to operate hassle-free under loads mentioned above, on 2AWG wire, and -trip- if someone plug (or left it turned on) ie, the water boiler (2000W), without waiting for a long time. Am I missing anything?

And Vic thanks for kind words about language, never been in US or CA before. (I think I must thank to Bitnet (the one before Internet), It helped me a lot to develop written language skills, there were no graphs at all, just typing to communicate :cool:, more then a couple of decades ago)

Vic · June 2015

Hi OtW,

Great news on the breaker that you Linked to. It looks very good to me, and seems to have quite good ratings.

It is customary for most of us to size inverter breakers based on the maximum surge capabilities of the inverter, and size the cables to be safe with that breaker rating. It is the usual goad of seeing that the breaker never trips with essentially, and load on the inverter -- usually during normal operation, the inverter's electronic protection is so fast that neither the DC input breaker, nor any output AC breaker will ever trip (for an inverter that is fully functional, and operating normally). In the event of a catastrophic failure of the inverter, the input breaker may trip to prevent, a fire or messy residue from the inverter.

Your application for the breaker is a bit different, and therefore a lower current rating would apply. It is possible that you might need to choose a breaker with a different current rating, based on your experience with one with a 70 rating, but with a slow trip curve. These things are all in the "shades of grey" area.

Your experiences with bitnet appear to have served you well, as our planet shrinks with time.

Good Luck with your system. Have Fun, Thanks. Vic

2twisty · June 2015

I've got both fuses and breakers installed in my system. My fuses are rated a bit higher than my breakers so that the breakers should want to trip before the fuse melts. The fuse is there in case the load goes nuts and the breaker fails for some stupid reason. Yeah, it's overkill probably, but I'd rather not find out the hard way that I should have had fuses too.

I've got one large breaker (I grabbed an old 3-phase breaker and am using just the 2 outside poles for my pos and neg DC, and they are rated to handle 4/0 wire. It's a 100A (breaker rated for 240VAC) and it's also designed with the trip mechanism to extinguish arcs. I have 150A fuses at the end of each battery string and another right after the breaker heading into the inverter. That way, if the inverter goes bonko and the breaker doesn't catch it, the fuse will blow before my 4/0 wire gets too hot with 48V on it.

NOTE: I did not calculate ANY of these values, and you should. I was making do with what I have on hand since money is a scarce resource for me these days. Down the line, I plan to replace these things with properly calculated values so that I protect the wiring properly.

BB. · June 2015

I personally "trust" the breakers will be reliable (every bad breaker I have seen trips below rated current or cannot be turned on in the first place). Placing several "unreliable by design" components in series (fuse then breaker)--Just more costs and things that can go wrong.

It won't hurt anything though.

-Bill

2twisty · June 2015

Well, where I live, we have no fire insurance. I'd rather be paranoid and add unneeded fuses -- even at the risk of added complexity.

OntheWay · June 2015

BB. wrote: »

Placing several "unreliable by design" components in series (fuse then breaker)--Just more costs and things that can go wrong.

If one decides to put a fuse along with breaker, then what will be the correct order, counting from battery pole..?

2twisty · June 2015

I'm no EE, but if they are of differing sizes, I would not think that it would matter, the lower rated one should go first, and then the second. If I have a 100A breaker that for some reason fails to trip and then the system goes on to pull 150A, then the fuse should melt.

OntheWay · June 2015

Here is a qoute from a web page:
"So overall, electrons flow AROUND the circuit, toward the negative end inside the battery, pushed by the chemical reaction, and toward the positive end in the outside circuit, pushed by the electrical voltage."
https://van.physics.illinois.edu/qa/listing.php?id=583

If we are trying to stop electrons, and if I understand above information correctly, we must place a breaker after Negative pole of battery where electrons start their journey, followed by a slightly stronger fuse, then it goes to inverter.
But in real life, all fuses, breakers etc are on the Positive wire. What may be the reason for this?

BB. · June 2015

For basic safety issues... It would not matter which is first, the fuse or the breaker. As long as the "down stream" wiring is rated for that protective device.

In industrial buildings, sometimes because the AC distribution transformer is very close to the main panel, the electrician must use a very expensive fuse that has a higher Amp Interruption Current rating than the typical breaker (10,000 AIC is a typical rating for home AC breakers).

For DC Systems, there is a design requirement for + and - terminals... DC arc have a polarity, and if the breaker is installed "backwards", the arc may not be quenched--But continue (and destroy the breaker). Newer DC Breakers are supposed to are supposed to be polarity independent to avoid the polarity issues in battery powered systems (current flows "out" when battery is discharging and flows "in" when charging)... Which way to orientate the breakers....

Where to put the breaker(s)--It depends on the system design.

For Negative Grounded systems, the Breakers are placed in the positive leads.

For Positive Grounded systems, the Breakers are placed in the negative leads.

For floating systems, you need to put breakers in both the positive and negative leads (ideally the breaker pairs tied together so that one tripping will turn off its mate).

None of the above rules are based on direction electrons flow. It is based simply on fault protection. A short between the grounded conductor and ground--No short circuit current flow. Short on the other "hot lead", you can get over current flow and need a breaker/fuse.

-Bill

inetdog · June 2015

OntheWay wrote: »

If one decides to put a fuse along with breaker, then what will be the correct order, counting from battery pole..?

Since it is a series circuit it really does not matter.

OntheWay · June 2015

BB. wrote: »

For DC Systems, there is a design requirement for + and - terminals... DC arc have a polarity, and if the breaker is installed "backwards", the arc may not be quenched--But continue (and destroy the breaker).

I completely forgot about charging.

The magnetic hydrolic breaker I choose ( here is link for it) says nothing (to my eye) about whether its polarised or not, in the specs sheet. Found another document shows usage of 2-pole breakers

http://www.altechcorp.com/PDFS/DC%20wiring_white%20paper_Altech%20DC%20series_0813.pdf

May usage of 2-poles eliminate problems from this two-way functionality or breaker still needs to be non-polarised?

BB. · June 2015

I don't know--I would guess that if you put two polarized breakers in series (back to back) and ganged switch (both tied together so that if one is "turned off" or tripped, the second turns off at the same time) it would work properly.

From your second link:

If polarized breakers are wired incorrectly and turned off under load, the circuit breakers might
not be able to extinguish the arc and the circuit breaker will burn out. Polarized DC circuit breakers
use a small magnet to direct the arc away from the contacts and up into the arc de-ionization
chamber. If the direction of current flow through the unit is reversed, then the magnet directs the
arc away from the arc chute and into the mechanism of the unit, thus destroying it.

If your first link is correct, and does not use polarized contacts for DC operation, then those would seem to be the breaker for the job on a battery system with bi-directional current flow.

-Bill

vtmaps · June 2015

OntheWay wrote: »

I completely forgot about charging.

If you use polarized breakers, the breaker should be sized so that the charger cannot produce enough current to trip the breaker. The battery can produce much higher currents than the charger, therefore you install the polarized breaker close to the battery to protect the wiring between the battery and the charger, and the polarity of the breaker must be so the 'line' or 'plus' (+) marking is nearest the battery and the 'load' or 'minus' (-) marking is nearest to the charger.

--vtMaps

Dave Angelini · June 2015

Another point for someone not using the correct enclosure (From Outback or Schneider,others) is that the breaker or fuse's physical location should be a safe place. When you use equipment designed for the application some thought goes into what will happen in various failure modes. I have seen other peoples work with meted breakers and fuses that blew the fuse wiring on to battery terminals. The protection device must also be protected (metal grounded enclosure) if you want a safe system that can survive a failure or survive most nearby lightning.

BB. · June 2015

I thought was vtMaps did--That the arcing issue was only a problem for when the breakers trip... But reading a bit, I am not sure, but it appears the polarity of the current issues still even applies for "normal switching" too (such as if you manually turn off the breaker while the solar charge controller is charging).

I would do as vtMaps suggests and point the + terminal to the battery bank to protect against high current surge trips. And avoid turning off the breaker if there is charging current going through the wire (i.e., dark). In some cases, this is a good idea anyway... Turning off an alternator based charger (wind/water turbine, DC Alternator on a genset) when they are charging can cause problems (wind/water turbines can over speed, all alternators can over voltage and damage electronics directly connected to their output if the "load/battery" is switched off when they are at speed).

-Bill

vtmaps · June 2015

BB. wrote: »

I thought was vtMaps did--That the arcing issue was only a problem for when the breakers trip... But reading a bit, I am not sure, but it appears the polarity of the current issues still even applies for "normal switching" too (such as if you manually turn off the breaker while the solar charge controller is charging.

We've been through this before with a discussion of Midnite combiner boxes.

Midnite polarizes their string breakers as if they were switches. When used as a switch those breakers are interrupting a current that is less than their rating. When they trip (because of a fault), they are interrupting a current, of the wrong polarity, that is higher than their rating. Makes no sense to me.

--vtMaps

BB. · June 2015

Yeah, I disagreed with Midnite's interpretation too. If you want the breaker to protect against shorts, you need to point the "+" terminal towards the battery;. For pure loads, that is a no-brainer.

For Charging sources / bidirectional loads+charging sources (say an Inverter-charger unit)... Wiring the polarized breaker as a "switch"--there is no correct answer.

For pure charging sources (say a solar charge controller), one can make the case that most of the time the breaker is used as a switch and pointing the "+" to the charging device "works"... But in terms of safety, I would say that answer is wrong.

A charging source should never output more than its rated current, so the breaker would never trip anyway. The only time a breaker trips is if there is "down stream short circuit" (device or wiring), and I would want my breaker to open quickly/properly and not self destruct and possibly catch fire itself.

Finding non-polariized DC breakers is really the right (and only) answer. Or use a fuse + DC rated (non-polarized)switch instead.

-Bill

OntheWay · June 2015

BB. wrote: »

Finding non-polariized DC breakers is really the right (and only) answer. Or use a fuse + DC rated (non-polarized)switch instead.

A nice brainstorming is going on, thats great. On my side, I finally reach to people who distribute the breaker I mentioned a few msgs above, told me that they are not sure whether the unit is non-polarized, or not.

I spend days and nites to find something about this non-polarized breakers, and found very little. just realized that wind-sun site has some breakers. But I think non of them hydraulic magnetic type while some are certainly non-polarized.
Here what I found, including some other sites:

http://www.solar-electric.com/scel80ampdcb.html
http://www.solar-electric.com/quickshop/index/view/path/obdc-gfp2.html/?proid=891
http://www.solar-electric.com/80amp30pamoc.html

There is also a brand called noark, non-polarized;

http://direct.dksh.com.au/files/product_docs/88077_DS.pdf

And from outback brand;
http://www.amazon.com/Outback-Power-Systems-PNL-80-DC-Outback-PNL-80-DC/dp/B007IAC0WG/ref=sr_1_fkmr0_1?ie=UTF8&qid=1434649673&sr=8-1-fkmr0&keywords=OutBack+AC%2FDC+Circuit+Breaker-PNL-80

The last one is most promising since both mag-hydr and non-polarized, but it says item cannot be shipped to your location

Now I am looking for your advises for either one of these, or another unlisted...

BB. · June 2015

The non-polarized breakers are a "newish thing" as I recall... (NEC requirement over the last few years?) So they may be difficult to find at this time.

Personally, for standard run of the mill applications, I would not have an issue with thermal breakers. They are common in US homes and I help take care of a small apparement building and the Thermal Breakers on the electric water heaters lasted for ~40-60 years before I started to have to replace them (they would trip a less than rated loads--i.e., reset and a month to two later it would trip again).

If that is their biggest problem (cheaper thermal breakers), I can live with that.

Call or send an email to NAWS and ask them what they have in the non-polarized DC Breaker realm. They would be better able than I to tell you what they have. And ask if shipping to you is even possible (mixing US and European hardware together may be its own fun and games).

-Bill

OntheWay · June 2015

Thank you Bill, I will do that.

Let me understand one thing clear, with a non-polarized breaker, both ( battery-> inverter ) and ( inverter->battery ) scenarios are protected at the same time with a single unit, is this the case? (assuming breaker is placed on the positive wire).

Or twin breakers (tied-together) still needed for both neg. and pos. wires ?

Vic · June 2015

Per MidNite Solar, their MNEDC (150 V) breakers (at least for handle ratings up to 100 A) are non-polarized.

They are "switch-rated" at rated load for 10,000 cycles (at 6 per minute).

For the most part, these appear to be ClarlingSwitch breakers, mount from the front of the breaker, and have terminal studs at the rear of the breaker

Have also read that these breakers are rated for continuous current flow at rated current (to something like 40 C ambient, IIRC).

These breakers can mount in a MN DCQuad box -- spaces for 4 breakers, and are available from NAZWind&Sun in a wide range of ratings, inexpensively:
http://www.solar-electric.com/instal...amodccibr.html

Overview of MNEDC 150 V breakers with ratings form 5 - 100 ADC:
http://www.midnitesolar.com/productPhoto.php?product_ID=197&productCatName=Breakers&productCat_ID=16&sortOrder=13&act=p

FWIW, Vic

BB. · June 2015

It is a confusing subject....

First, the "bi-directional" current flow (such as an inverter-charger or a solar charge controller that gets a short circuit) is is better protected by a "bi-direction" single pole breaker.

Second, you can simulate a bi-directional breaker (maybe) by placing to uni-directional breakers back to back *i.e., two + terminals together, and connect load/sources to the two "-" terminals). And the two breakers have "ganged handles". That way when you turn off (or trip) a breaker, one of them has the current flowing the "correct way". You (perhaps) could still get damage as both contacts open at the same time--One "blows" the arc into the arc chute, the second "sucks" the arc into the breaker's mechanicals.

Third, If I was really parinoid (and you should be about safety). Use both a fuse (true bidirectional protection) and a breaker installed in the "operational current flow direction (or install a plain heavy duty DC switch).

Fourth--Completely different safety issue. In other countries, you do not see this setup very often. We have 120/240 VAC center tapped transformer power with the center tap "Neutral" being safety ground referenced.

For 120 VAC circuits, we put a single breaker on the "Hot lead" (not ground referenced neutral, or Brown wire--I think in Europe) only.

For 240 VAC circuits, we put a "ganged" two pole breaker on the "two hot wires" and the neutral may not even be connected to the load. If there is a short in one hot lead, the first breaker also trips (turns off) the second breaker. That way, when there is a trip, the branch circuit is now "safely" dead/turned off (of course, always check before grabbing 120/240 VAC wiring).

For "floating power systems" (A DC Battery bank connected to the DC input of an inverter, other loads, other charging sources,etc.). It turns out that there are various short circuit paths/faults that can cause excessive current to flow in some wiring if you only put a breaker in one of the battery wiring paths (say "+"). So, the proper thing to do is to put a two pole/ganged breaker on each pair of +/- wires that leave the battery bank such that is one wire is "over current", then both poles are turned off together.

If you are familiar with 3 phase current (US/Europe), 3 pole ganged breakers are the standard.

We can talk about it more--But it turns out, safety wise, if you DC Ground the battery bank negative bus to the cold water pipe/plumbing electrical green wire ground, etc. that you only need a breaker/fuse in the + lead (hot lead). Under all normal wiring conditions, the Neutral/Return/Negative lead can never become "hot" so does not need a fuse/breaker. (and there are postive and negative ground systems--For other reasons--But the issues are the same, just "reverse + and - in the discussion).

With floating power systems, the "first short" "ground references" one lead and the other becomes "hot". Then a second short (or person grabbing the "new hot lead") cause more problems (person gets electrocuted, excessive current flow in the wrong wiring, etc. if protected by one fuse/breaker in the "+" lead).

It is not an obvious problem and floating power systems are technically "safer" until the first fault--Then they can become "less safe" depending on what happens next (does the person find the first fault and fix it, or does a second fault now cause major problems, etc.).

We talk more or I can link to a white paper I wrote a few years ago about the problem of "floating ground" DC power systems...

-Bill

Choosing the right DC breaker to stay in the limits of battery bank

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