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.

77 Comments

  1. Tomi Engdahl says:

    Inside a 125A DC circuit breaker. Unexpected arc quenching feature.
    https://www.youtube.com/watch?v=w0ntxqTEqyM

    Initially I thought this was going to be quite a complex breaker compared to a traditional AC circuit breaker, as it’s a lot harder to break a DC arc.
    After initial disappointment at what appeared to be a fairly standard breaker inside with the usual bimetallic strip for detecting low level overloads and a solenoid for instant trip on very high current faults, I spotted the secret feature….

    Reply
  2. Radu Coriu says:

    But can a DC breaker be used in AC? Why?

    Reply
    • Tomi Engdahl says:

      Some can and some can’t.
      There are breakers rated for AC, DC and AC+DC operation.

      Some DC breakers are designed for just to work on specific current direction (for example trip mechanism or electronics for switching) and those do not work correctly with AC (do not work at all, might be damaged etc..).
      If you want to use a breaker with AC, best is to check that it’s printed specifications and/or data sheet mentions AC operation for that breaker.

      Reply
  3. Abhinav Kumar says:

    We are need 5 amp to 100 amp DC Circuit breakers.
    ( Low voltage and low current circuit breaker)
    1. DC voltage rating 12V to 120V and current 5 amp to 100 amp.

    Thanks
    Abhinav Kumar

    Reply
  4. John says:

    Very interesting reading this about the circuit breakers. we are always learning as electricians.

    Reply
  5. Tomi Engdahl says:

    New Circuit Breakers for Electrical Devices
    https://www.electropages.com/blog/2020/06/new-circuit-breakers-electrical-devices?utm_campaign=2020-06-17-Latest-Product-News&utm_source=newsletter&utm_medium=email&utm_term=article&utm_content=New+Circuit+Breakers+for+Electrical+Devices

    The race is on to develop a new range of mechanical Direct Current (DC) circuit breakers. While the significant power users depend on alternating current (AC), which cycles on and off 60 times per second, DC is used to power devices such as flashlights, smartphones and electric cars. DC has inherent advantages over AC, among them higher efficiency and the ability to carry more power over longer distances. However, unlike alternating current that can be safely shut down when the power level hits zero during a cycle – the basis for breakers that protect modern power systems – switching off the DC for even just a few microseconds or milliseconds, is fraught with difficulty. Several projects are now underway that seeks to address this problem.

    TU Ilmenau Researching New Circuit Breakers

    In May, the German Technical University of Ilmenau commenced a large scale collaborative project to develop groundbreaking mechanical circuit breakers for electrical devices and systems that work based on direct current. Project delivery is scheduled for 2023. The three-year research project, ‘FASS (Fast and selective Switching)’

    ‘Hybrid Breakers’ to make DC Practical in High Power Applications

    Meanwhile, researchers from the Georgia Institute of Technology and Florida State University (FSU) are looking to develop a ‘hybrid’ system that could enable breaker switching speeds up to ten times faster than existing equipment. They are expecting to commercialise the technology through a consortium of industry partners. Lukas Graber, an assistant professor in the School of Electrical and Computer Engineering at Georgia Tech, explained, “We are proposing a hybrid DC circuit breaker in which the current will have two paths. One path will be through the semiconductors, which can interrupt the current when needed. The second path will be through mechanical switches, which will provide a much less resistive path that will be more efficient for normal operations.”

    The hybrid circuit breaker under development by the research team will use stacks of very large transistors to switch off the DC when necessary. Semiconductors are less efficient at conducting current than conventional mechanical switches, so under ordinary conditions, the current will flow through mechanical switches. But when the power must be turned off, the current will be briefly routed through the power electronics until the mechanical breakers can be opened. “We need to be extremely fast,” Graber said. “We have to separate the contacts within 250 microseconds and to completely break the current within 500 microseconds — just half a millisecond. For that reason, we cannot use spring-loaded or hydraulic actuators common to AC breakers. Devices that rely on the piezoelectric effect can do that for us.”

    ABB Reinvents the Circuit Breaker – Breakthrough Digital Technology

    Last year ABB announced a technological breakthrough in the form of a solid-state circuit breaker, which the company claims will enhance the performance of renewable energy solutions, industrial battery storage solutions and so-called edge grids. With power losses 70 percent lower than comparable solutions, the technology is said to be ‘vital’ for the electrification of sustainable transport. It will enable savings of up to $200,000 in a ferry and up to $1 million in a cruise liner over ten years. The product is to be made available this year. The ABB solid-state breaker concept works by replacing the traditional moving parts of an electro-mechanical circuit breaker with power electronics and advanced software algorithms that control the power and can interrupt extreme currents faster than ever before.

    Reply
  6. satnilesatnews says:

    We are need 5 amp to 100 amp DC Circuit breakers.
    ( Low voltage and low current circuit breaker)
    1. DC voltage rating 12V to 120V and current 5 amp to 100 amp.

    Thanks

    Reply
  7. Tomi Engdahl says:

    Inside a Circuit Breaker for Solar Inverter to Battery
    https://www.youtube.com/watch?v=Woigo3sTp_E

    this was a lot of fun to look inside and see how it works. This circuit breaker might be different from an ordinary house breaker. This one is sold by “Midnite Solar” and is made for DC (direct current) applications. This would include inverter to batteries, parallel battery strings, charge controllers, etc. I’m a big fan of the circuit breakers from Midnite Solar.

    Reply
  8. Tomi Engdahl says:

    D.C. arcing in circuit breakers
    https://www.youtube.com/watch?v=Sz0ksWWfJyM

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

    Fuse switch arcing with d.c.
    https://www.youtube.com/watch?v=z0QhkuGClMw

    Fuse switches are not designed as load breaking for d.c. currents

    Reply
  9. Tomi Engdahl says:

    DC Circuit Breaker Fires
    https://www.youtube.com/watch?v=Cup5fMGaE2g

    You let out the magic smoke. That always makes things stop working properly.

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

    Good to know polarity is so important !

    Reply
  10. Tomi Engdahl says:

    HVDC Circuit Breaker – Types, Working and Applications
    https://www.electricaltechnology.org/2020/08/hvdc-circuit-breaker-types-working-and-applications.html

    Requirement for HVDC Circuit Breaker
    The HVDC circuit breaker operation & design is complex as compared to AC circuit breaker due to the absence of natural zero crossing. The arc generated in HVDC will never extinguish & it will heat up the contacts of the breaker & eventually destroy the contacts rendering the whole CB useless.

    Not to mention, the circuit will still be complete & the equipment connected will get damaged due to the fault current. Therefore, the following requirement must be completed to ensure safe circuit breaking in HVDC circuit breaker,

    Creation of artificial zero crossing
    Dissipation of the stored energy inside the LC circuit
    Withstanding the voltage between its contact
    Prevention of arc restriking

    In order to fulfill the above requirements, an LC circuit is introduced with the circuit breaker in parallel which will generate artificial zero current across the line to safely break open the circuit. The strength of the arc is directly proportional to the voltage level & the current. Therefore the fault current must be brought down to zero using an external circuit before breaking it.

    Reply
  11. Tomi Engdahl says:

    GFPA400 passive and active
    GIGAVAC continues leadership in sealed switching technology with the introduction of sealed fuse products. GIGAVAC fuse products now include the PyroTactor™, the world’s first contactor with integrated pyro fuse, and the GigaFuse™, the world’s only hermetically sealed electromechanical fuse. Both of these series are designed for high voltage and high power fuse application requirements.
    https://www.scnnordic.com/fi/fi-components/tehokontaktorit/fuses/gfpa400-passive-and-active

    Reply
  12. Tomi Engdahl says:

    Fuse switch arcing with d.c.
    https://www.youtube.com/watch?v=z0QhkuGClMw

    Fuse switches are not designed as load breaking for d.c. currents

    Reply
  13. Tomi Engdahl says:

    AC DC Circuit Breaker – Can DC trip an AC breaker?
    https://www.youtube.com/watch?v=kRB7Z66brO0

    Breakers AC, DC & AC/DC – Solar Safety Part 2 – 12v Solar Shed
    https://www.youtube.com/watch?v=S0diAeysxVo

    Reply
  14. Tomi Engdahl says:

    The Ultimate Guide to DIY Off-Grid Solar Systems – 04 – DC switches
    https://www.youtube.com/watch?v=gmi-vrGOFyk

    Reply
  15. Tomi Engdahl says:

    10 amp AC Switch used on 7 amps DC
    https://www.youtube.com/watch?v=CUFVSc5ll4s

    A 10 amp AC switch is used with 7 amps DC. Failure is inevitable.

    I always wondered why these switches are marked ‘AC only’ – never seen it demonstrated so well.

    Reply
  16. Tomi Engdahl says:

    Fuse switch arcing with d.c.
    https://www.youtube.com/watch?v=z0QhkuGClMw

    Fuse switches are not designed as load breaking for d.c. currents

    A 6 Amp AC breaker trips on DC at 240 Volts 30 Amps
    https://www.youtube.com/watch?v=S6BRo5fjtVk

    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.

    Comment from video:
    Generally 230VAC MCB are rated to break up to 50VDC Higher DC voltages need special DC rated breakers. AC breakers and switches will fail to break a circuit and clear the arc on hundreds of volts DC even when breaking just a few amps.

    Reply
  17. Tomi Engdahl says:

    The biggest problem with AC breakers being used on DC is the fact that they cannot suppress the DC arc, thus posting a fire risk.

    Reply
  18. Tomi Engdahl says:

    Power switches at their core offer a simple way to turn on and off your voltage rail or protect your power path. Load switches offer a compelling replacement for discrete power MOSFETs, and eFuses include additional integration for complete system power protection. However, both ICs go beyond just offering significant PCB space savings. By joining this webinar, power engineers will be shown how load switches and eFuses offer substantial performance improvements to comparable discrete circuits – all at an approachable price point

    Reply
  19. Tomi Engdahl says:

    Can AC circuit breaker be used for DC?
    http://waterheatertimer.org/Can-AC-circuit-breaker-be-used-for-DC.html

    1) Can AC circuit breaker be used for DC?
    It will “work” but if the breaker is not UL listed for DC operation then it becomes a safety / reliability thing. I would never recommend it.

    DC roasts relays and contacts faster than AC

    Each circuit breaker has specific rating to extinguish arc. DC has greater arc than AC.
    This means arc extinction capability of breaker is challenged by DC voltage.
    So the breaker rating is reduced for DC voltages, when compared with AC voltages
    “QOU miniature circuit breakers are rated in the following UL 489 voltages:
    •120/240 Vac •240 Vac
    •48 Vdc •60 Vdc
    Circuit breakers must be chosen with interrupting ratings equal to or greater than the maximum available short-circuit current at the point where the circuit breaker is applied in the system”

    the higher the number of contacts opening the circuit, the higher 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.”
    What this quote means: A DC circuit breaker or any DC switchgear must have multiple contacts that disconnect as illustrated in diagrams below
    You want to increase the number of contact points, so the arc from DC power can be extinguished

    Switchgear for DC::

    Extinguish arc using contacts in series

    Reply
  20. Tomi Engdahl says:

    MCB – Miniature Circuit Breaker tested on AC and DC overloads
    https://www.youtube.com/watch?v=PRT6lv8RKvA

    Just to see what is inside a 6 Amp MCB.
    Overload tests at 25 Amps at 50 Hertz
    Overload tests at around 40 Amps pure DC.
    The breaker was tested beyond it’s design ratings of 230 Volts AC.
    The MCB breaker ended up in a meltdown.
    It is very important that the correct type MCB’s are fitted for the equipment it has to protect. Especially for DC, as there are many roof fires with solar systems where the wrong type MCB’s are fitted.

    Comments:

    Generally most 230VAC MCB will break up to 50~60VDC they can be used on 48V dc systems provided the fault current is low. Manufactures theses days have deliberately been omitting these low DC ratings on their datasheets so they can sell unnecessary expensive DC circuit breakers, that usual will work up to a few hundred volts dc. This is not needed for ELV below 50V. Its also piss poor to see that the breaker caught fire and kept burning, it supposed to be self extinguishing plastic.

    Exactly .
    Siemens Betaguard series breakers are rated for 60V DC per pole as per their datasheet..

    Pointless exercise, AC switches and breakers are not designed to open DC loads, that said, it’s a good demonstration of why you use HRC fuses in solar strings.

    Reply
  21. Tomi Engdahl says:

    Sähköautojen teholataus vaatii paljon elektroniikalta
    Julkaistu: 09.07.2021
    https://etn.fi/index.php/13-news/12353-sahkoautojen-teholataus-vaatii-paljon-elektroniikalta

    Sähköautojen latausjärjestelmissä ollaan siirtymässä 400 voltin jännitteistä kilovolttiin, ja lataustehoissa 50 kilowatin tehosta 350 kilowattiin. Tämä on ainoa keino lyhentää käytännön latausajat alle 20 minuuttiin. Suuremmat jännite- ja virtatasot lyhentävät latausaikoja, mutta lisäävät samalla järjestelmien turvallisuusriskejä ja suunnittelun haasteita.

    Suurjännitekontaktorit tarjoavat turvallisen piirin jatkuvuuden, kun taas sulakkeita tarvitaan rinnakkain piirin suojaamiseksi vaarallisen oikosulkutapahtuman sattuessa. Jännite- ja virtatasojen nousu ei vain vaadi kontaktoreita, joilla on korkeammat katkaisuominaisuudet, vaan tekee kontaktorin ja sulakeparin tekemisestä teknisesti haastavamman.

    Sensatan GigaFuse tarjoaa useita etuja verrattuna tavalliseen tasavirtalämpösulakkeeseen. Sulake kykenee esimerkiksi avaamaan piirin 3 millisekunnin sisällä eli nopeammin kuin tavanomainen DC-lämpösulake.

    ”Gigasulakkeen” yhdistäminen kontaktorin kanssa on helpompaa sen ainutlaatuisen sähkömekaanisen laukaisumekanismin ansiosta. Kontaktorin suorituskyky paranee, mikä estää ylikuormittumista ja pienentää lämpövastusta (tyypillisesti 0,15 milliohmin tasolle). Lisäksi ratkaisu eliminoi lämmöntuotosta aiheutuvaa komponenttien ikääntymistä.

    Sensata Technologies’ Power Disconnect Solution Enables Faster and Safer DC Fast Charging
    https://www.sensata.com/contact/newsroom/sensata-technologies-plug-and-play-power-disconnect-solution-enables-faster-and-safer-dc-fast-charging?utm_source=pr&utm_medium=ms&utm_campaign=dcfc-wins&utm_content=pr-page&utm_source=pr&utm_medium=ms&utm_content=pr-page&utm_campaign=dcfc-wins

    Reply

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