# 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.