Using AC MCB for DC applications

The rule of thumb that I use is to pick up the phone and talk to the equipment manufacturer directly. Then, I'm funny that way.

You do know that the breaker's job is to protect the wiring, don't you?

Only if the AC MCB is rated by the manufacturer for DC use and only if used within the OEM application constraints.

If there is not an OEM label on the device stating it can be used for DC applications I would contact the manufacturer and obtain written specifications on the breaker as well as permission from the OEM to use the MCB in a DC application.

Failure to follow these recommendations will cause unsafe personnel exposure and can result in catastrophic equipment failure. (Explosion, Fire, Injury, and/or Death)

There is not enough information here to say this is a good "off design" application or almost a good application that may lead to a problem down the road. My first concern is the maximum surge current your DC supply can produce. If this supply cannot produce more than 12A of surge current then it will be fine for powering your 8A load but your breaker will be just as good as a piece of wire. There are several other attributes that may make this a poor choice.

Notice: If you properly sized your wires to be able to safely handle the 16A the breaker will sustain and your DC supply current limits at 12A, a short circuit will not damage these wires. Breakers and fuses protect wires not loads.

...so it might be better to use a fuse instead. If you're not sure what rating, then ask an Electrician to work it out given the cable size, its length, and the method of installation. It's all in your local electrical standards book, and Electricians do it every day.

There is no formula that can safely determine a DC rating for a device that is designed and rated for AC, the DC ratings can only be determined via empirical methods, meaning testing by the mfr under known conditions designed to emulate worst-case scenarios. This is because from the standpoint of interrupting the flow of current, which is the JOB of a circuit breaker, DC sustains an arc for much longer than AC, so the total amount of energy in a fault must be significantly lower in order for the MCB to survive and reliably interrupt it. I know that when I was at Siemens, the DC current ratings of many AC designed devices was a tiny fraction of the AC current ratings. So the ONLY viable method of determining the DC rating of an MCB that only shows AC ratings is to contact the mfr directly and obtain a specification sheet that tells you the DC ratings, if any.

That said, I checked a couple of brands that I use and both of them are rated for at least 48VDC with no change in the current rating.

GA

When I worked for Reliance Electric we used "AC breakers" in the DC legs feeding the DC motors. Now the breaker was significantly de-rated, all 3 poles were used in series, the manufacturer was involved in the selection. testing, and certification process for this application. The inductance, current, and voltage all provided upper limits for the breakers. In the end the breakers were certified and labelled with both AC and DC ratings.

What many people are not aware of is that many fuses are also only AC rated. They also have problems breaking DC current. The sand in them can turn to plasma and keep on conducting. Fuses also must have a DC rating, and are limited to current, voltage and inductance.

If your wiring is not rated for 16A then no, that's a fire waiting to happen.

The MCB datasheet or application sheet should have DC ratings on it you can check against.

This is the very reason I use ANN up to 80VDC and JJN fuses up to 144VDC type fuses for all of our DC propulsion systems on the DC side. I do not trust breakers. I may be old school, but no boat or propulsion system I build will burn down a marina, Ever. We operate from 36-144VDC input to 25-102VAC 3 Phase. Do not trust DC breakers.

If you look in the technical catalogues of the major MCB manufacturers (Schneider, Siemens, GE ...) you will find that they all give DC ratings for some of their MCB ranges, but not necessarily all.

For instance GE (the one I'm most familiar with, usual disclaimers apply) has published DC rating for most of the Redline series, but does not for the economy Domus series.

You are typically limited to low voltages such a 48VDC per pole, and need to wire multiple poles in series to protect higher-voltage circuits, but if you look carefully it is possible to find higher ratings. GE's EP110UCR range are rated for 250VDC per pole.

The trip current ratings are often the same between AC and DC, but it does pay to check the technical catalogues to make sure of this and the other usual important details that you check when speccing MCBs.

Basically DC is much much harder to interrupt than AC. If you look at switches it is not unusual to see a small switch rated AC 240 volts, 15 amp but the DC rating if it even has one might be 12 Volts, 0.5A. Breakers are even more different. There is a reason for this but I will not bore you with Physics or EE facts. Never use a device that does not have the rating that is needed. If it is good for DC it will state it. The manufacturer would like to sell as much product as possible. If it lacks a DC rating it will fail to interrupt, so you should never use it. DC breakers of the same rating may cost up to 10X as much as AC and be much bigger. As stated by others, fuses must have DC ratings also.

There is a famous picture of the first GE lighting installation in New York. It shows a large electrical panel with one man standing with his hand on a large knife switch and another man standing behind him with a broom on his shoulder. Many assume the man with the broom is an interested janitor. Not so. They were both engineers. The second man is standing there to "sweep" away the arc that forms when the DC switch is opened to turn off the lights. Kinda crude at first, but the few hundred volts caused a sustained arc whenever the switches were opened. The use of a straw broom was standard operating procedure on DC circuits.

I use standard 240 vac mcb's on my boat for 12 vdc. They work fine. Although reading the posts here, I might check out a couple of things to be on the safe side. ie, do they actually trip at the rated current.

My first recollection of practical problems with ac versus dc arose when I first started work back in 1951 as an apprentice electrician. 240 vac was gradually being introduced but there were still houses in London with dc (110v I think). Older folk might recall the tumbler switches used in those days; they big chunky clunky long throw toggle action; soon to be phased out in favour of new silent micro-switches.

What was not widely known was the distinction between ac and dc, even amongst qualified electricians, as was obvious, because as an apprentice, you asked endless questions, and whilst you 'learnt' your trade, some of the explanations I recall, with hindsight and 'school' learning, would not have stood up to close scrutiny.

Lack of appreciation of the finer points led to some early fires, because unwittingly micro switches were used to replace toggle switches (either faulty ones or as a fashion statement) in dc houses.

What was soon learnt was the micro switch used on dc drew a continuous arc that burnt out the contacts, and probably the cabinet casing as well (remember that was the wooden trunking then used). In fact, an electrician was basically a carpenter with 'wiring knowledge' until cabinet casing was replaced by metal conduit or trunking, sometimes lead covered, or multi-core rubber cable - and soon to have PVC insulation appear on the scene.

So back to AD 2015, I would say the clearance gap of the contacts is a critical factor, although the gap might be difficult to see in a fully enclosed breaker.

Having said that, I cannot say what the gap should be. But I do know in the old days the gap in (some of) the early manual micro-switches was small enough to wedge a fag paper in.

I guess in my case, for my boat, a de-rating from 240 vac to 12 vdc is enough.

It would be interesting to know if the ac magnetic forces in an mcb are related to peak, average or rms voltage, compared to the equivalent voltage when used on dc.

While the AC breakers might be able to interrupt an overload, say twice normal or less, would they interupt a dead short with the batteries full capability. You will not know that, and batteries have a very large short circuit capability. It is not the 12 volt system voltage on DC that you must interrupt, but the Inductive voltage on DC caused by (L di/dt) where time tries to go as close to zero as it can and the current, however small, is still trying to flow through any inductance L that exists in the circuit. It is almost impossible to build a normal circuit without inductance. The inductive voltage on a 12V system can easily be well above the system voltage and can result in failure of your CB's. From a design basis it is possible to use reverse diodes of appropriate size but that would require some calculations and the values (R, C, L, and max I) that exist in the system. Bottom line, your CB might work or might not work on a direct short and if they don't and you are miles from others and shore you had better have a good fire extinguisher and/or a good flotation device. A relatively small problem could be a very big life threatening problem due to incorrect CB's.

The difference between A/C and DC MCB's is basically that for A/C MCB's have to be rated for the Interrupting Capacity (symmetric or Asymmetric) of the Voltage in use and the utility or secondary transformer available current during a short circuit condition. In DC applications, the MCB is subject to an inductive response from the source and thus, the interrupting capacity is not affected by the impedance of the circuit. In theory, a DC rated MCB has to be built with a full available short circuit current from the source since the impedance attenuation factor is very low. But, then again, since most DC sources have limited current capacity or are fed by a limited capacity transformer/rectifier sources, short circuit currents fall within the limited available current capacity of the source. In utility supplied A/C sources, the momentary short circuit current inrush is very high and the available power from the power company is usually measured in MVA's (Million Volt Amperes). I believe that the risk of using A/C MCB's for DC applications should be lower than using D/C MCB's for A/C applications, unless the DC source short circuit capacity excedes the typical utility available short circuit power of your application. In general, AC MCB's come 25KASIC, 65KASIC or 100KASIC interrupting capacities which should accommodate most of your common day DC applications. Also, because of the absence of reactive response from a DC circuit, arc occurrences are practically not a concern with DC MCB's. That's why they require less insulation and usually smaller in size than their AC counterparts. Finally, I suggest you consult the manufacturer of your MCB for the ultimate recommendation.

I don't know where you get your information from, but this differs from my experience. I worked in design and field engineering for one of world's largest electrical equipment manufacturers and have had a lot of experience with large DC supplies in Boats, RV's, Utility Battery DC power for controls, industrial DC battery banks, and large DC supplies for electric vehicles. DC is always harder to interrupt than AC. In AC you have current zeros, and resistance and inductance limits current. In DC the only limit to current is resistance except in the brief instant where the arc starts. Once the arc starts, the resistance of air is for all practical purposes zero and once the arc starts it is very hard to extinguish unless it is thrown out to a larger distance through arc chutes. The DC breakers must be carefully designed and built to work with specific energy levels.

In AC circuits, although the line power may be MVA's the impedance to the user is much more limited and should be determined by a short circuit study. In a residence, it is rarely high enough not to use the standard line breaker panels which are only good for 10,000 amps. Breakers tied directly to the grid may be even see less than the full available short circuit, but that is determined by study and they are usually SF6, Vacuum, or if old oil filled or ATB.

To 4wsilver #13 (and to jbourass #12 for an alternative view)

The advanced design theory is fine, so when everything is installed and ready to run in a dc system (my 12 volt 440AH batteries for instance) what practical tests do you suggest can be carried out to check if mcb's provide the protection expected.

My own simple test was a primitive short circuit one for mcb's in the domestic circuits fed by the consumer unit, but there is no automatic protection (but there is a manual isolating switch) for the main power cables from he batteries feeding the consumer unit.

The risk here I assume would be from violent external damage to the mains equipment and system rather than failure of an electrical item connected to it, itself protected by local mcb's.

The bit that is missing it seems, is a test to see if the mcb's in the consumer unit protect the circuit cables from sustained gradual overload.

I would not test the equipment with any type of test I could dream because you probably don't have the test equipment to properly perform the test. I would suggest that you do the following. Install a "Mega Fuse" manufactured by Buss in series with the battery. On my Vixen Motorhome I installed one between the Negative battery terminal and ground. These are designed for DC application, isolate faults quickly, and I have never had one fail except for it's intended purpose. If you search for them, the competitors products will also come up and I have no reason to believe that other manufacturers are not as good. If you have an AC breaker in series with this and it fails the only damage will be to the breaker, the fuse will clear the fault if the breaker does not. Since I put these in my Vixen, I have had 3 occurrences of shorts which were cleared immediately by the fuse. This resulted in the need for a replacement fuse and improving the insulation of the cable affected. In each case, if the fuse had not worked, the only thing to clear the fault would have been 2/0 copper cable or failure of the battery. Many vehicle fires (including among Vixens) have been caused by shorts like these plus I would have been upset if my $2500 Lithium batteries had been damaged. (360 amp hours)

I had a vehicle fire once when a wiring harness shorted out in the middle on a sedan I was driving. Luckily a Highway Patrolman was near me and pulled out a pair of cable cutters he used for many similar situations and cut the ground wire to the battery. The only loss was a ground wire and wiring harness but the results were not pretty due to fire and smoke damage to clean up.

key words - he believes!

It doesn't really seem necessary to test your MCBs for trip current provided you have followed the manufacturer's published data/recommendations and done your design homework correctly in the first place. The manufacturer has already done all the testing.

I have never seen MCBs (that is miniature circuit breakers) with interrupt ratings of '25KASIC, 65KASIC or 100KASIC interrupting capacities' (post 13), these rating are usually only seen on molded-case cbs. Usually MCBs come in 6KA and 10KA ratings, with some going to 15KA.

I guess "M" could be miniature. I assumed it was magnetic.

Actually if you look up mcb or MCB you will usually have to look at the second or third page depending on your search engine before you find anything regarding circuit breakers and in the electrical industry, MCB does stand for Magnetic Circuit Breaker unless you are looking in the area of electronics. No doubt some manufacturer has used it incorectly for minature.

MCB does stand for Magnetic Circuit Breaker

Are you sure, it doesn't down here in this part of the world for standard electrical and electronic industries. Magnetic and miniature CBs (MCB) are very different.

Re Redfred #19.

When I first came across them in the early 1960's for use instead of porcelain fuses they were made in plastic and called 'moulded' circuit breakers. Later, as they got smaller, to be called 'miniature'. Now I've got to learn to call then 'magnetic' - at my age!

I guess this begs the question : did the OP, in asking about MCBs, mean Magnetic Circuit Breakers, or did he mean Miniature Circuit Breakers?

I must admit, I never thought to question this, but it is nevertheless highly pertinent to the present discussion.

Could Sheererm please enlighten us?

Realizing that my own experience was somewhat tainted because I was used to thinking about "small" circuit breakers as 1200 amps and 15 KV, I had looked up on google and several others the terms MCB and mcb as I have previously stated. Many other items came up first, before any mention of circuit breakers and when circuit breakers came up about page 3 the terms went to magnetic first, then miniaturized and finally minature circuit breakers. As with anytime we use acronyms, the first thing you think of is based on your own personal experience and that is why they tend to confuse the world.

I was always taught in engineering writing to put in a list of definitions so anyone reading the report would be on the same wavelengths. I live in a town of one of the highest populations of scientists and other technical people in the world. It sometimes is funny over our weekly coffee when 2 people get into a heated discussion because they each think a particular acronym means different things. No one is really wrong or right.

What if we were at a Marine Corps Base discussing an article we read in the Molecular and Cellular Biology journal about research funded by the Mauritus Commercial Bank involving a Master Cell Bank being exposed to Monochlorobenzene and we were suddenly interrupted by somebody with a volt ohm meter in their hand. They then ask us politely "Sirs, I'm here to work on the MCB. Do you know where it is?"

Agreed.

I was under the impression MCCB stood for Moulded Case Circuit Breaker.

25ka to 100ka AC interrupting is easy in an MCCB

Did something get shortened to MCB?

MCCB does stand for Moulded Case Circuit breaker.

In my country, and in Australia and a fair bit or Europe, MCB stand for Miniature Circuit Breaker. Elsewhere, it might instead stand for Magnetic Circuit Breaker.

Nothing I do is miniature. Only the blower motors on the motors I use are less than 5 hp, and due to the power behind the supply we have significant interrupting capacity issues. So no miniature anything can be used.

So you must use Massive Circuit Breakers (MCB) instead for all of your designs.

I hate undefined acronyms.

and occasionally vacuum breakers.

MCB and MCCB are definitely different. This link may help ease the confusion.

http://en.wikipedia.org/wiki/Circuit_breaker

It really depends on your fault current level as to which one you use (among other things).