How to Differentiate Between AC and DC MCBs

Both AC MCBs and DC MCBs appear the same when seen on a DIN rail, having the same body, toggle and sometimes even the same breaking capacity of 6 kA shown on the front panel. However, putting an AC device on a 400 V DC battery circuit may let it go on for a week before it gets welded shut in case of an actual fault.

The difference is the result of physics, which can be spotted by carefully looking at the label.

In brief: A direct current (DC) miniature circuit breaker (MCB) has a certain level of DC voltage and polarity (example 500 V DC, + / −), normally rated with a known time constant, and has magnetic blowout plates in the arc chute. An alternating current (AC) MCB is labeled 230/400 V AC, it has no polarity, and relies on the current crossing the zero 100 or 120 times per second to put out the arc. If the label states AC only, it is AC only, irrespective of its ampere rating.

The One Difference That Decides Everything: The Arc

An alternating current runs through zero two times per one complete cycle. For example, when alternating current is running in a 50 Hz frequency, it corresponds to 100 times per second where the arc is possible to extinguish. The principle of the traditional miniature circuit breaker is based on this very fact: the arc can be split up between several steel plates, cooled, and after the zero crossing, extinguished by itself.

On the contrary, direct current has no zero crossing. When the arc appears in the DC circuit, it keeps burning until the distance is long enough to break the plasma or until the plasma is cooled down so that the voltage is unable to sustain it. In a case of 500 V DC string, the arc may be stable for dozens of milliseconds, and any device with contact gap which equals 10 mm and with plastic arc chamber cannot withstand such an arc.

That is why DC circuit breakers are designed differently. It uses a permanent magnet or blowout coil for pushing the arc to the side into the chute, it stacks more plates and distances between them becomes significantly less.

Reading the Label: What a DC Rating Looks Like

Every manufacturer publishes these figures on the datasheet as well as on the moulding, and the datasheet is where the poling diagram lives. Our own technical download library holds the DC characteristics and the series-connection tables for the KNB2-63S6Z range, because the series arrangement is the part installers get wrong most often.

Four Ways to Tell Them Apart Without a Datasheet

Search for polarity signs. A DC circuit breaker has + and − symbols by the terminals. An AC breaker does not have anything like it as it is not concerned with the current flow direction.

Examine the arc chamber on the side. A DC breaker has a row of closely placed plates and a small magnet placed on them in the case of higher current ratings. An AC breaker has fewer plates that are more spaced out.

Verify the voltage rating with size. A one-pole breaker with 18 mm width and a voltage rating of 500 V AC is common. The same type with voltage rating of 500 V DC is not possible without multiple poles in series connection or special chambers, hence a compact DC breaker with 500 V rating should be examined additionally during analysis.

Count the number of poles. Two-pole DC MCBs are often connected in series connection on the same wire. A compact example is this two-pole 6 to 25 A DC miniature circuit breaker, where the series connection is shown on the side label. If the wiring diagram shows the supply going in one pole and returning through the other to the same line, you are looking at a DC application.

Why an AC Breaker Fails on DC

The failure is not normally a failure to trip but rather insufficiency in dealing following the trip.

There are three things that go wrong at once. The first is that there is no zero crossing, which means the chute has to perform all the operations. The second problem is the lack of magnetic blowout, which is supposed to push the arc into the chute. Therefore, the arc remains intact in the contact gap and burns the contacts. The final problem is that the thermal element cannot see a reverse current, meaning that the DC currents accumulate heat in only one contact, and it does not distribute it at all.

As a result of the three failures, the breaker trips, arcs for about 20 to 50 milliseconds, turns the chamber into carbon, and either welds shut or fails next time it is turned on. It has to be pointed out that the ampere rating is irrelevant because ampere rating alone expresses only half of the breaker specification.

Another trap in the area of PV work involves string currents which do not exceed the limits of the modules. Hence, the fault current is only 1.25 times greater than the regular current. In this regard, a breaker does not expect a large fault and thus can function normally for a long time but eventually fails because of that fault. The selection logic for that case is set out in this guide to specifying DC MCBs for photovoltaic and storage.

Why a DC Breaker Is Not a Drop-In for AC Either

It does trip properly on AC circuits, which leads people to believe it works properly. There are two arguments against this.

The first argument is certification – a DC device has to have a DC mark on it, and, therefore, the inspector can reject such device when used on an AC circuit as it cannot be used for that purpose, the insurance rules are the same.

The second argument against that is the so-called magnetic blowout. On AC, the arc moves depending on the direction of current, which changes after every half cycle. Therefore, the magnet that is designed to force DC arc into the chute can actually give some force against AC arc and stop it for half the cycle.

Where DC MCBs Actually Get Used

Four applications mainly capture the bulk: protection of PV strings, storage of batteries, distribution of DC energy in telecommunication and data cabinets, and circuits for controlling industrial panels.

In terms of PV strings, the requirements are rather specific. The device should be rated for the open circuit voltage of the array at the lowest temperature that can be expected. In addition, it should match the short circuit current of the string, with some margin. The rating should also consider temperatures inside the combiner box, which can be easily 60 °C in summer.

This kind of device is not the usual MCB with another sticker. Our two-pole DC miniature circuit breaker for solar PV strings is built for string duty rather than general distribution: DC arc control, a 500 V DC rating at the array’s time constant, and a breaking capacity sized to the fault level of a string circuit.

Choosing a DC Device for a PV or Battery Circuit

Complete five steps in the following order:

• Maximum system voltage, which shall be estimated based on the recorded minimum ambient temperature (not derived from the value indicated on the inverter’s label).
• String or rack short-circuit current has to be calculated and then multiplied by the 1.25 factor used in the standard.
• DC voltage rating of the device has to be calculated using the time constant at the moment.
• Breaking capacity higher than the fault current at the moment should be calculated (the fault current should include the contribution of the parallel strings worked with).
• The temperature should be calculated because the device can be rated differently (e.g. 25 A at the temperature of 30°C is not equal to that at the temperature of 60°C).

The consumer application is the one that is usually mistaken by the majority of purchasers, as their garage roof with a single string looks negligible. However, it still remains a DC circuit with an open circuit voltage of 600 V in the cold morning and it is still necessary to break a DC arc.That is the context in which a properly rated 2-pole DC MCB, such as the 32 to 63 A PV DC MCB rated 15 kA, belongs in the array rather than in the inverter’s own enclosure.

FAQ

How to know if the circuit breaker is AC or DC?

Check out the upper side of the device. The alternating current breakers have the sine wave logo and write down the voltage, which is typically where the voltages of 230/400 volts AC are given. The DC breaker shows the DC symbol, the DC voltage of about 500V DC, and usually has symbols of polarity at the terminals. If the marking is not clear, see what the datasheet says.

Can I use an AC MCB for DC?

No, unless the manufacturer provides a DC rating for the unit. The AC rating applies to an arc that disappears at an AC zero current. DC does not have a zero. If manufacturers provide DC ratings on AC machines, the voltage is usually lower than the AC rating, and the device has poles wired in series and the accompanying diagrams.

Can I use a DC circuit breaker for AC?

Typically, the device will trip, yet it is not classified for such purpose, and magnetic arc control isn’t meant to reverse currents. An inspector can reject it in case of residential and commercial application, while insurance can also deny the claim made.

Is there a difference between AC and DC circuit breakers?

Yes, the arc extinction system contains several components such as the contact gap, arc chute, magnet, voltage rating, and certification. Both thermal and magnetic trip elements work on similar principles and therefore make the two devices appear to be identical.

References

• c3controls — MCB: AC vs DC
• ETEK Electric — Difference Between AC and DC MCB
• GEYA — AC and DC Circuit Breaker Differences
• NEMA — Low Voltage Distribution Equipment
• UL Solutions — Molded Case Circuit Breakers and DC Ratings

Conclusion

It is the label which makes the choice. The presence of a sine wave plus 230/400 V AC indicates that which directly identifies it as alternating current. The presence of DC voltage, time constant and polarity identification signifies that the equipment is designed for extinguishing a permanent arc.

If you’re working on specifying for a photovoltaic system or a battery storage solution, please share with us the system voltage at the lowest temperatures recorded and the string fault current readings and we shall advise you on pole design and ratings.