DC circuit breakers

Knowing the electrical characteristics of direct current and its differences in comparison with alternating current is fundamental to understand how to employ direct current. Direct current presents different problems than alternating current with a regard to the phenomena associated to the interruption of high value currents since the arc extinction results to be particularly difficult.

Usually DC sources are low voltage like batteries and such, so you don’t associate DC with sparking, but it will, especially when voltages get considerably higher than 12 or 24V (Common DC voltage levels to get idea of different DC voltage levels in use). Take a look at this AC versus DC load breaking comparison with a knife switch video that clearly shows the difference in electrical contact by alternating current to direct current:

Yes, DC sparks much more aggressively than AC. When a switch carrying DC is opened, the current being interrupted is always equal to the current flowing in the circuit. When interrupting an AC current, the current varies with time, and, on average, is less than the peak current (it normally goes to zero during mains power zero crossings). For this reason, a given switch normally has 2 current; one for AC current, and a lower one for DC current. It can also have different voltage ratings for AC and DC. This applies to practically all components that switch electrical current: switches, relays, fuses and circuit breakers.

Switching resistive DC load is hard, and switching off inductive load is even harder. Why does a switch spark when disconnecting a coil carrying high dc current? article tells that the reason is that an inductor resists a change in current. If you have a steady state current going through an inductor and you attempt to suddenly decrease the current to zero by opening the circuit, the inductor will respond by attempting to maintain the current, but that current has nowhere to go. This creates a large negative voltage spike across the inductor. It is very common to see transients of several hundred or thousand volts. This is why you need often to have some kind of suppression circuit in place. Breaking DC is considerably harder than AC. For example many small relays that have 250VAC 10A rating have only 30VDC rating for the same current!

The next question is can I use AC circuit breaker for DC? Can I use an AC circuit breaker in a DC circuit discussion says that most AC breakers are rated at least for 120VAC. Some of them will also mention a DC rating which is usually like one-fifth of the AC voltage rating (24V DC). The reason for the downgrading is that with AC the voltage dips down to zero every 120th of a second, giving the spark across the opening contacts a chance to snuff out. With DC there is no such helpful time and the sparks can continue to jump across a much longer distance than AC sparks. Assuming your batteries are 24V or less, then a 120VAC breaker that also says “24VDC” should work.

In Finland the mains voltage is 230V like it is all over Europe. Here the circuit breakers are rated for 230V or higher (400V for three phase power). I have seen several 230V breakers with 48VDC voltage ratings. So with such would be suitable for 48VDC. When you check the DC voltage rating, check also if other data is different for DC (for example maximum current breaking capacity and operation current of magnetic trip device). For typical battery powered systems (up to 24 VDC), those standard 230 or 400 VAC circuit breakers will probably work, but it is best to check that from the manufacturer data sheet. If your breakers are rated for 48VDC, then battery voltages up to 48V are OK with them. I have also seen some dual circuit breakers that are rated for 415VAC and 110V DC.

AC DC Circuit Breaker – Can DC trip an AC breaker? video shows how AC circuit breaker being used in a DC circuit and how it trips when overloaded. It gives good introduction to using AC circuit breaker for DC, but I do not agree on the the conclusions on the trip current difference on AC and DC (thermal trip can take quite a bit of time with double the rated current on both AC and DC):

Generally AC circuit breakers will trip on DC and but at higher currents and voltages extinguishing of the arc may be an issue and a potential fire hazard. Typically the thermal trip remains typically the same but magnetic tripping can need higher current to operate than with AC (note that the magnetic trip on most AC breakers is set approximately 5-10 times the breaker rating on AC operation). You need to consult the manufacturers specifications for thermal trip times (for example a 10 amp breaker can take 30 secs to trip at 20 amps, 1 sec at 60 A, 10ms at 100A).

ABB circuit-breakers for direct current applications document says that the thermal magnetic trip units fitted to a.c. circuit-breakers are also suitable to be used with direct current. The part relevant to the thermal protection does not change with reference to its tripping characteristic since the bimetal strips of the trip units are influenced by the heating caused by the current flow, it does not matter whether alternating or direct: in fact the bimetal strips are sensitive to the r.m.s. value. As regards the instantaneous protection against short-circuit, due to ferromagnetic phenomena, the instantaneous tripping occurs at a different value in comparison with the analogous case in alternating current.

Really bad things can happen if the breaker does not trip when it should and if it can’t break the DC current you feed to it. DO NOT try this with an AC breaker that does not have a DC rating! A 6 Amp AC breaker trips on DC at 240 Volts 30 Amps video does not recommend an AC breaker for DC applications because of potential fire hazard when extinguishing of the arc has issues with DC:

There are special circuit breakers for DC applications. The bigger DC breakers even have a magnet near the contacts to try to bend the arc away from the shortest path. You need special DC breakers especially in applications where you handle few hundreds of volts DC (for example large solar power systems and DC power distribution as used in some data centers). For information on DC breakers check ABB circuit-breakers for direct current applications document. It says that Miniature circuit-breakers series S280 UC comply with Standard IEC 60947-2 and differ from the standard versions in that they are equipped with permanent magnetic elements on the internal arcing chambers (allow the electric arc to be broken up to voltages equal to 440VDC). The presence of these permanent magnetic elements establishes the circuit-breaker polarity (positive or negative); as a consequence, their connection shall be carried out in compliance with the polarity indicated on the circuit-breakers. An incorrect connection of the polarities could damage the circuit-breaker.

So there are special cautions related to use of circuit breakers designed for DC applications in mind: There are types need to be installed carefully in the right way (plus/minus connection and input/output sides). DC Circuit Breaker Fires video shows the difference of right and wrong installation:

For details on correct wiring check Correct Wiring Of Double Pole DC Breakers – Clean Energy Council. A number of the breakers on the market (e.g. ABB, GE and Terasaki) only have the ‘+’ and ‘–‘ symbols on one side and a number of the breakers on the market (e.g. Clipsal, Klockner Moeller)) have the + and – symbols on both the top and bottom terminals. These can cause confusion. The positive and negative outputs of power source must be connected to the respective ‘+’ and ‘–‘ terminals on the circuit breaker. Non polarised circuit breakers operate safely as load breaking isolators and for fault current protection regardless of the direction of current flow through them.

So when working with DC circuits, read carefully the technical specifications of the circuit breakers and other components you plan to use to make sure they are suitable for the task and you connect them to the circuit in the right way. Always use a direct current rated fuse or circuit breaker in a direct current system.


  1. Newport Brass Nb3 230b 56 Diverter says:

    Simply citing this blog post everyone will agree with it because its correct so its pleasant to see a poster thats stating snippets such as this publically to review

  2. Pro Pac says:

    Evidently reading the above topic readers will agree with this because it is correct and it’s good spotting an op thats showing this online to look at.

  3. Renovations says:

    Excellent blog! I’ll keep keeping an eye out for more of your posts in future.

  4. Tomi Engdahl says:

    ABB Advance Makes Renewable-Energy Supergrids Practical

    A high-power circuit breaker makes it possible to create highly efficient DC power grids.

    ABB, the large power and automation company, has developed technology that could provide an efficient way to transmit power from widely distributed solar panels, wind turbines, and other sources of renewable energy. The new technology is a fast and efficient circuit breaker for high-voltage direct-current (DC) power lines, a device that has eluded technologists for 100 years. The breaker makes it possible to join high-voltage DC transmission lines to form a resilient power grid.

    If renewable energy is ever to account for a large part of the total energy supply, countries will need to install new, large-scale transmission grids, both to get power to cities from remote areas such as deserts that often have the best renewable resources, and to combine power from widely distributed wind turbines and solar panels, which can help average out fluctuations in their output. In Europe, there’s been talk for years of a supergrid that would pull together power from hydroelectric dams in Scandinavia with wind farms in Germany and large solar farms in Spain and even North Africa

    But such a supergrid has faced serious technical hurdles.

    The transmission lines that make up conventional power grids use alternating current (AC), which loses large amounts of power over long distances unless complicated and expensive measures are taken. DC is more efficient over long distances, and it offers the additional benefit of working well underground and underwater, reducing or eliminating the need for the unsightly transmission towers that can make it difficult to site new transmission lines.

    ABB’s circuit breaker changes that. Within five milliseconds it can stop the flow of a huge amount of power—equal to the entire output of a nuclear power plant, ABB says.

    Researchers have been trying to develop high-voltage DC circuit breakers for a century (see “Edison’s Revenge: The Rise of DC Power”). Mechanical switches alone didn’t work—they shut off power too slowly. Power electronics made of transistors that can switch on and off large amounts of power offered a possible solution, but they proved far too inefficient. ABB’s solution combines power electronics with a mechanical switch to create a hybrid system that’s both fast and efficient.

    “The hybrid breaker should be less costly.”

  5. Karl says:

    Hi there
    Can anyone explain to me why DC voltage should be switched in series, i.e. if you have a 2-pole breaker that can handle 400VDC then it appears that by installing a link across the two poles the breaker can safely isolate 800V through the 2 switches. Is this possible to do in parralel as well or does the DC voltage require series?

    • tomi says:

      From http://www04.abb.com/global/seitp/seitp202.nsf/0/6b16aa3f34983211c125761f004fd7f9/$file/Vol.5.pdf

      Direct current presents different problems than alternating
      current with aregard to the phenomena associated
      to the interruption of high value currents since the arc
      extinction results to be particularly difficult.

      This extinction involves
      energetic phenomena which depend on the voltage level
      of the plant (Un) and lead to install circuit-breakers according
      to connection diagrams in series to the advantage
      of the performances under short-circuit conditions
      (as a matter of fact, the higher is the number of contacts
      opening the circuit, the higher is the breaking capacity
      of the circuit-breaker).

      This means that, when the voltage rises, it is necessary
      to increase the number of current interruptions in
      series, so that a rise in the arc voltage is obtained and
      consequently a number of poles for breaking operation
      suitable to the fault level.

  6. katrina says:

    Good job! Full of information, everyone that will read this blog will get so much to learn. Thanks! very helpful :)

    Please visit us: http://www.thegreenbook.com/products/circuit-breakers

  7. John Delanski says:

    Relectric is a national supplier of new and reconditioned used and obsolete circuit breakers, electrical distribution and control equipment. http://www.relectric.com

  8. Amy Parsons says:

    Great explanation. Electrical currents are so dangerous and so many precautions need to be taken when working with electrical currents. If you need a UPS backup for your equipment then I suggest you check out http://switchon.eaton.com.

  9. Tomi Engdahl says:

    Tripping Out: A Field Guide to Circuit Protection

    Residential breakers in North America come in a couple of different flavors. The branch circuit breakers are used to protect each branch circuit – the outlets, light fixtures, and appliances that are connected in parallel back to the main panel. Each branch circuit is typically rated for either 15 or 20 amps, and unless it’s running a large load like an electric dryer or a well pump, it’ll be a 120-volt circuit. In addition to the branch breakers, a panel will have a main breaker to catch any faults that a defective branch breaker misses

  10. Tomi Engdahl says:

    AC versus DC load breaking comparison with a knife switch

    AC Circuit Breaker 230V DC Test

  11. Tomi Engdahl says:

    Breakers AC, DC & AC/DC – Solar Safety Part 2 – 12v Solar Shed

  12. Tomi Engdahl says:

    DC Circuit Breakers at Your Local Hardware Store

    Great topic post. You can use SQ D QO series breakers single pole or 2 pole up to 48 VDC. (Bought from home depot, lowes, electrical supply)You must derate the AIR rating from 10,000 to 5,000. The one major difference between AC breakers and DC is AC breakers do not have a high arc rating for the contacts like DC breakers do. The DC arc is much stronger. A non rated DC breaker could fuse together upon this arcing. The SQ D QO series is good from 10-70VDC. They are also the only ones i know of to buy off the shelf. if your using a 12 or 24v system there are other automotive fuses that can be used that are cheaper and just as effective.

    The Square D ckt breakers (QOs) are ok for the lower voltage DC Solar Feeds, however for those (like myself) that want the higher voltage and lower current, Solar Feeds (IE: for those long runs from the PV Panels to the Charge Controller) the Square D “Heavy Duty” Fused Disconnect (they are DC rated) are a better choice! If properly configured!!! …much mis-information on the WEB about configuration/wiring and/or what components to use in your Solar Powered System!

    Great Bit if info for those one a tight budget. I run a 48 volt system and I have solar volts up to 80 volts and later I will have up to 130 volts DC from solar, so I have to spend more on the more costly breakers, I use Din rail breakers on my system, Your right they are costly that is for sure. Seems the higher the amperage the higher the breakers cost.

    Square D QO, Square D Home Line Difference

  13. John Advent says:

    Very interesting reading about dc and breakers, I have learnt something new

  14. adam says:

    This is very good reading i learnt a lot

  15. Tomi Engdahl says:

    AC Circuit Breaker 230V DC Test

    Arc died only because contacts melted away.
    This is what will happen if you install AC breaker into DC circuit, for example PV circuit.
    And don’t try this at home kids.

    MCB Popping (or why you don’t protect DC loads with an AC breaker)

    This is why you don’t protect high-voltage or high-current DC loads with an AC breaker. If this was high current (instead of high voltage) the outcome would have been similar, just much more rapidly

  16. Tomi Engdahl says:

    AC Circuit Breaker 230V DC Test

    Arc died only because contacts melted away.
    This is what will happen if you install AC breaker into DC circuit, for example PV circuit.

    MCB short circuit test by dc voltage

    here i show u MCB short circuit protection test with 150Ah 12 volt lead acid battery the ratting of mcb is 16 amp 240,450 volt

  17. James Adams says:

    Thanks for the information about DC breakers i have learnt alot from this post

  18. 247 Electrical Services says:

    Very interesting reading about the DC breakers it was very educational.

  19. Tomi Engdahl says:

    MCB Popping (or why you don’t protect DC loads with an AC breaker)

    This is why you don’t protect high-voltage or high-current DC loads with an AC breaker.

    Most 230V AC breakers are ok for up to 50VDC

  20. Tomi Engdahl says:

    A Dramatic Demo of AC Versus DC Switching

    Switches seem to be the simplest of electrical components – just two pieces of metal that can be positioned to either touch each other or not. As such it would seem that it shouldn’t matter whether a switch is used for AC or DC. While that’s an easy and understandable assumption, it can also be a dangerous one, as this demo of AC and DC switching dramatically reveals.


    As dramatic as the demo is, it doesn’t mean we won’t ever be seeing DC in the home. It just means that a little extra engineering is needed to make sure that all the components are up to snuff.

  21. Tomi Engdahl says:

    AC vs DC Switching Demonstration with Arcs

    The difference between switching AC and DC.
    With DC, the arc formed as the switch contacts open is far more difficult to extinguish, and can cause significant damage.

    Explains why relays have different voltage rating for AC and DC.

  22. Tomi Engdahl says:

    Power Supplies and Circuit Breakers Keep Faults in Check

    In the last decade or so, significant advances have been made in the design of industrial power supplies and dc-dc converters, from the materials and device levels to size and weight reduction, thermal management, and package design. However, one often-overlooked category is protection of circuits and systems provided by the power supply and accompanying circuit breakers. These advances have contributed greatly to reliability and system availability while maintaining safety as well.

    One of the most far-reaching is selective fuse breaking (SFB) or selective shutdown, which when enabled in both power supply and thermomagnetic, as well as other types of circuit breakers, provides significant benefits.

    The magnetic trip mechanism
    Response time of this type is much faster than its counterpart, typically 3 to 5 ms

    Thermomagnetic circuit breakers are available with one of three different characteristic response curves, M, SFB, and F

    Phoenix Contact was the first to introduce thermomagnetic circuit breakers that follow the SFB curve, and are designed for use with power supplies that also are based on SFB technology. When combined, the two provide exceptionally reliable tripping, even with long cable lengths between the power supply and the devices it serves.

    The power supply also delivers the large amount of power reserve required in systems like this one that have long power cable runs, in which the amount of current available for tripping the breaker is limited.

    By delivering a higher level of current than is normally required to trip the breaker (up to 10 times normal for 12 ms in Phoenix Contact QUINT SFB power supplies), such situations are prevented.

    Without SFB, faults become a detriment to system availability, as they take an entire block of functions offline, even though only a single circuit has failed. The Phoenix Contact QUINT power supplies also complement SFB

  23. Tomi Engdahl says:

    Solar Breakers and Solar Fuses for your solar System

    Solar breakers and solar fuses are highly recommended to protect your charge controller from the solar panels and battery bank. Use the correct fuse or breaker for your situation.

  24. Tomi Engdahl says:

    Breakers AC, DC & AC/DC – Solar Safety Part 2 – 12v Solar Shed

    In a follow up to part one ( ) I’m looking at breakers. I talk briefly about how they work and then I take look inside an AC breaker, a AC & DC Breaker and a DC only breaker. It turns out they have some similarities as well as some differences.

    Remember, please use the correctly designed breaker for your circuit. Do not use AC breakers on DC circuits (or vice versa).

    MidNite Solar’s Secrets to DC Breakers

  25. Tomi Engdahl says:

    Type of Fuse │ Different Types of Fuse │ How Many Types of Fuse│ Various Types of Fuse │

    There are different types of fuses available in the market and they can be categories on the basis of Different aspects. Good to know: Fuses are used in AC as well as DC circuits.

  26. Tomi Engdahl says:

    Things you should know about fuses. (including a 15kV one)

    A fuse is designed to be the weakest point in a circuit so that it fails first and breaks the circuit protecting other components and cables.

    A 1A fuse does not blow at 1A. It will blow quickly at very high fault currents, but take progressively longer to open the circuit as the current is lowered to nearer its rating.

    Glass fuses are only rated to break a low fault current.
    Ceramic sand filled fuses can break much higher fault current.

    Time delay or slow blow fuses (T) can withstand surges.
    Quick blow or fast blow fuses (F) will blow faster in a fault.
    Semiconductor fuses (FF) are very sensitive and expensive.

    The voltage rating of a fuse indicates its ability to work safely up to that voltage. 3A is 3A so a 250V fuse will work fine on 12V.

    HRC (High Rupturing Capacity) fuses have the fuse wire running through a fine powder that quenches the arc quickly and forms a solid glass-like substance internally in the process.

  27. Tomi Engdahl says:

    Dc Power InLine Breakers

    Ordered These Online. They Look Like Nice Breakers To Use. Breakers Are Good For 80AMPS.

  28. Tomi Engdahl says:

    Installed: Blue Sea Systems 200 amp circuit breaker

    Blue Sea Systems Thermal Breakers

    Chuck Hawley talks about the 187, 285 & Klixon thermal circuit breakers

  29. Tomi Engdahl says:

    Breakers AC, DC & AC/DC – Solar Safety Part 2 – 12v Solar Shed

    In a follow up to part one ( ) I’m looking at breakers. I talk briefly about how they work and then I take look inside an AC breaker, a AC & DC Breaker and a DC only breaker. It turns out they have some similarities as well as some differences.

  30. Tomi Engdahl says:

    DC Circuit Breaker Fires

    Toasty… thanks for posting this, I’d always wondered just what happens when the polarity is wrong.

    Good to know polarity is so important !

  31. Tomi Engdahl says:

    300AMP DC Circuit Breaker and Test

    Received this circuit breaker today in the post all the way from Hong Kong!
    Rated at 300 Amps, I put it to the test using 4kw!

    300Amp DC Circuit Breaker Teardown Troubleshooting

    A quick look inside a a cheap DC breaker from overseas as it had become unreliable and faulty. quite interesting once I got it’s lid off!

  32. Tomi Engdahl says:

    Dc Power InLine Breakers

    Ordered These Online. They Look Like Nice Breakers To Use. Breakers Are Good For 80AMPS.

    question IS…. will they work..??

    Some are good and others are not good. Need to watch where there bought.

  33. Tomi Engdahl says:

    Off Grid Solar Power – Fuses and Circuit Breakers are Important.

    Off Grid Solar Power – Fuses and Circuit Breakers are Important.

    I’m just showing here the importance of fuses and circuit breakers and this will also give SOME idea of the importance of correctly sizing your cabling. The smaller the cable, the higher the resistance with lower voltages. If you want to run smaller cable, you have to increase the voltage. If you want to run high current at low voltage, you have to get yourself some FAT cabling. Fat cabling also means high capacity fuses.

    These circuit breakers are cheap chinese jobbies and you can see that they’re just not up to the task, holding a much higher current than what they’re rated for.

  34. Tomi Engdahl says:

    So another failure and again it’s the DC Breaker

  35. Tomi Engdahl says:

    Solar Combiner Box with Square D QO Breakers

    Using Square D QO Breakers (rated to 48VDC).. for my solar panel combiner box.. using a QO ‘load center’.. QO 15 amp Breakers are $6 each and the indoor QO612L100 ‘load center’ was $20.. if you want an outdoor (3R rated) they are about $36 from HD, Lowes, or MillsFleetFarm..

  36. Tomi Engdahl says:

    AC DC Circuit Breaker – Can DC trip an AC breaker?

    In this video I demonstrate an AC circuit breaker being used in a DC circuit and how it trips when overloaded. A bit of research is still required to understand if they are safe to use in this application and when they would trip.


    3 vuotta sitten (muokattu)
    Looks like Type C MCB, It will trip within 20seconds at 20amps charted for a 230Vac supply british regulations

    Within a fairly small margin both AC & DC will trip at the same time. The device is a very basic bimetal trip element, so operates on heat, so DC & AC RMS will create the same amount of heating.

    What you need to look at are the trip curves. MCB’s do not trip until after around 110% of their rating, even then it takes minutes to trip, your 6A unit could easily carry 7A on AC or DC for a long time (if not indefinitely)

    The comment below about B&C curves would not make any difference to this amount of current. The curves only come in at much higher currents

    B = 300~500%
    C = 500~1000%
    D = 1000~2000% of the rated current of the device

    This is a good couple of video’s to watch I do not recommend an AC breaker for DC applications it is dangerous even as a switch. and it is a good reason for insurance companies not to pay up in the event of disaster and fire cause investigators are seeing this all to often

  37. Tomi Engdahl says:

    Don’t use polarised DC breakers in an off grid system

    DC Circuit Breaker Fires


    AC breakers don’t work well with DC, DC is crazy on arcing compared to AC, DC breakers have special arc cutoff mechanism that cut the arc

    Ive seen this happening with several Solar PV insallations in Indonesia…Installers are not aware of this.

  38. Tomi Engdahl says:

    A 6 Amp AC breaker trips on DC at 240 Volts 30 Amps

    This experiment proves that an AC circuit breaker should not be used at mains voltage levels on DC. You may get away switching 12 or 24 Volts DC but the time delay is too much to maintain safe operating of the MCB at higher voltages. Severe contact burn in will happen as a DC arc is more difficult to break. Under the test conditions of 30 Amps AC. the 6 Amp breaker trips immediately. Under test conditions of 30 Amps DC, It takes 11.5 seconds for a trip. the later trip of about 6 seconds was due to the bimetallic strip being warm.

    D.C. arcing in circuit breakers

    Correct selection of circuit breakers for d.c. currents is vital to avoid fire and arcing

  39. Tomi Engdahl says:

    AC versus DC load breaking comparison with a knife switch

    Difference in electrical contact by alternating current to direct current. The four resistors are wire chrome 650×4 = 2600 watts. Note: The tension of 220V rectified is about 311V, but under load it drops to approximately 213-209 volts.


    Reminds me of those videos where they switch off high voltage AC transmission line switches in the kilovolts range. (I mean hundreds of kilovolts!)

    AC lets you break the load without arcing by voltages of 400-500V maximum. Above that level it becomes a problem again.

    And this proves that Tesla’s idea of Alternating Current was far better than Edison’s idea of Direct Current. If it wasn’t for Tesla, light switches and contactors would have to be way more beefy thus being much more expensive. Thank goodness he came along.

    DC is better than AC in various aspects (more efficient,safer,no reactive power,no capacitance, no inductance,no skin effect etc) with the exception that arcs more easily (which is good for welding) and is more difficult to convert at large scales, so Edison was right in some aspects the only problem is that AC was much more practical back then, now DC is becoming more practical and replacing AC in Datacenters (380vdc) and in long distance transmission (HVDC 500kv+)

    The older wall switches and to this day, many heavy duty inline cord switches and some others, are snap type switches with both an very fast and loud snapping make and break, and are also double break with the movable contact swinging wide to make a long arc path to extinguish arcs instantly, making them AC/DC switches.

    Despite the voltage discrepancy, I think this is a great quick video to showcase AC having a lower tendency to arc.

    What’s most impressive is that such a big arc can be sustained on a relatively low voltage such as 220Vdc.

    Nice setup and probably the best demo of the difference between AC and DC I’ve seen (30 or so years in Electronics!) Thanks!

  40. Tomi Engdahl says:

    Solar System Circuit Breaker DC Isolator – Know The Dangers!

    Solar Training Videos – RATING DC ISOLATORS

    This short video explains how to correctly rate DC isolators for installation in a solar PV system.

  41. Tomi Engdahl says:


    Standard dual-rated ac/dc circuit breakers may be used in dc system applications and will provide thermal (overload) protection as shown on the ac time-current characteristics curve. These circuit breakers are UL Listed for application in dc systems using the instantaneous trip multiplier for the ac time-current characteristic curve. Standard ac/dc circuit breakers may be used on grounded or ungrounded dc systems.

    QO/QOB – Available with dc voltage ratings from 48 Vdc to 125 Vdc.
    QOU – Available with dc voltage ratings from 48 Vdc to 125 Vdc
    Multi 9 – Available with dc voltage ratings from 60 Vdc to 125 Vdc
    FA and LA – Available with dc voltage ratings from 125 Vdc to 250 Vdc
    PowerPact® H-frame and J-frame – UL/CSA labeled for applications up to 250 Vdc

    Crane & Hoist
    Solar Installations
    OEM Commercial
    Healthcare Facilities
    Data Centers

  42. Tomi Engdahl says:

    DC Circuit Breaker Theory and Uses You Never Know

    DC circuit breakers are frequently used in applications such as:

    Battery powered electric circuits, such as those found in homes with solar panels. Battery powered circuits are also found in homes located in rural zones without an electric grid.
    Electrical components of vehicles, which are present in both gas cars and electric cars. Every car has a fuse box with DC circuit breakers.
    Charging stations for electric vehicles.
    Uninterruptible Power Supply (UPS) systems, which typically uses batteries. Even if the UPS supplies power for AC devices, it must store energy as DC in the battery bank.
    Photovoltaic solar panel installations, their control system and their battery banks.
    Direct current electric motors.
    Some types of electric arc welding machines.
    High efficiency LED lamps.

    A very important difference when interrupting alternating current and direct current is that the arc extinguishing point is higher for a DC circuit breaker.

    How Does a DC Circuit Breaker Work

    DC circuit breakers work with the same principle of thermal protection and magnetic protection which is found in AC circuit breakers:

    Thermal protection trips the DC circuit breaker when electric current above the rated value is present. This protection mechanism is based on a bimetallic contact that heats, expands and trips the circuit breaker. The thermal protection works faster as the current grows larger because more heat is generated to expand and open the electric contact. Thermal protection in a DC circuit breaker protects against overload current, which is only slightly larger than normal operating current.
    Magnetic protection trips the DC circuit breaker when high fault currents are present, and the response is always instantaneous. DC circuit breakers have a rated breaking capacity that represents the maximum fault current that can be interrupted, just like AC circuit breakers. An important consideration with DC circuit breakers is that the current being interrupted is constant, so the circuit breaker must open the electric contact further in order to interrupt the fault current. Magnetic protection in a DC circuit breaker protects against short circuits and faults, which are drastically larger than an overload.

    Since the protection mechanism is virtually the same for AC and DC, some models of circuit breakers are designed to work with either type of current. However, it is always very important to verify that the current type of the electric supply and the circuit breaker are the same. If a circuit breaker of the wrong type is installed, it will not be able to protect the installation effectively and electric accidents might occur!

    Miniature DC circuit breakers typically work with currents below 100 amperes and they are classified into Type B (trips at 3-5 times rated current), type C (trips at 5-10 times rated current) and type D (trips at 10-20 times rated current).

    DC circuit breaker installation is very important if the user has a solar PV system. It requires protection at several points:

    Solar PV panels are connected in series circuits, and one installation may have one or more circuits depending on its capacity. All of the circuits are connected to a PV combiner box, where each is protected by a DC circuit breaker. Protecting the solar panel circuits is critical, since they are the most expensive part of the system.
    Once the power supplied by all solar panels is combined into a single direct current output, a main DC circuit breaker will be needed.
    If the system uses a battery bank, it must also be protected by a DC circuit breaker.
    The inverter, which converts direct current to alternating current, requires a DC circuit breaker at the source.
    Finally, if the user has an exclusive electric panel for DC loads, an array of DC circuit breakers will be required.

    DC circuit breakers are less common than AC circuit breakers, but equally important since they have plenty of applications in both homes and businesses. For home owners, DC circuit breakers can be a relatively new technology since most home devices work with alternating current. However, DC circuit breakers play an important role in electrical protection of expensive energy efficient technologies such as LED lamps, photovoltaic solar panels and electric vehicles.

  43. Tomi Engdahl says:

    AC Circuit Breaker 230V DC Test

    Arc died only because contacts melted away.
    This is what will happen if you install AC breaker into DC circuit, for example PV circuit.
    And don’t try this at home kids.

  44. Tomi Engdahl says:

    12V Circuit Breakers, Good or Bad? – Quality Testing

    An in depth quality test of pretty standard 12V dc circuit breakers you can buy. Unfortunately these where brought directly from china through a business contact so i cannot provide a sale link however examples like this can be brought from Ebay, Amazon, ect…


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