I'm in need of a number of magnetic (or thermal) circuit breakers for a 12V electrical system, but the cost of these seems quite high - easily £15 a pop. This made me wonder, would it be ok to use household style MCBs at 12 volts? You know, the type commonly fitted on DIN rails in domestic fuseboxes:
NO
They wont be sized for breaking the current without the zero crossings of the AC mains, get a proper DC rated part.
Absolutely not.
They will NOT be able to interrupt large DC currents.
Absolutely not.
They will NOT be able to interrupt large DC currents.
Considering it's only 12VDC vs 230VAC, I would not be so sure in judgments. It's already almost a zero crossing at such voltage. For example, relays often have something like 250VAC and 125VDC ratings.
There are breakers that have a dual AC/DC rating. Those are fine. But if the DC rating isn't there, it's not there for a reason. Don't use an AC rating for DC applications.
Absolutely not.
They will NOT be able to interrupt large DC currents.
Considering it's only 12VDC vs 230VAC, I would not be so sure in judgments. It's already almost a zero crossing at such voltage. For example, relays often have something like 250VAC and 125VDC ratings. Also it would be much more difficult to achieve currents as large as at 230VAC.
But if the DC rating isn't there, it's not there for a reason.
My guess is, the main reason would be they are made for different market, therefore no point for DC specification.
Depends what you mean by "OK".
For a quick lash-up, probably fine. If failure means someone gets hurt, probably not.
These tend to have both thermal and magnetic characteristics - the latter may be polarity-dependent.
As they are designed to prevent arcing from sources capable of maybe 1000A, breaking 12v will probably be OK.
Problems with breaking a dc current aside...
I wonder what voltage drop you will see across these breakers at close to the operating current.
At mains voltages the drop would not matter but it could be significant at lower DC voltages.
Most MCBs, at least in the IEC world, have a breaking capacity of 6 kA, although you can get cheaper ones which have a 3 kA fault current rating. This is for an AC current [alternating current current
] at a specified maximum vboltage. Many manufacturers also give a DC fault current rating which will also be specified at a reduced voltage compared to the maximum AC voltage.
So I would have absolutely no issue in using a MCB as long as there is supporting documentation to show that it is suitable for the job. The one issue I think you will find is that it is normally only the major respectable manufacturers who will give this information and you may find that their MCBs come in at more than 15 quid anyway.
Go visit a camping caravan shop, caravans (house on a trailer behind your car) have fuseboxes with circuitbreakers for 6A and 10A perhaps even higher.
They are not that expensive.
I had myself terrible experience with using 230VAC wallswitches in a 24V installation (switching a 24V relais) after a year or so it would not turn off anymore. They are definitely not suited for low voltage esp. DC applications and I would not gamble that a normal breaker will. If it fails it might cause fire if it is your last failsafe then please add a normal fuse with higher amps in series just in case it will fail on a short that will protect you.
Regular AC circuit breakers are not rated for DC operation. Main reason - DC arcs are much harder to extinguish. In video below teardown of AC and DC breakers are shown (DC breaker from 9:30). DC breakers have magnets inside to help break the arc. DC breakers are polarity sensitive and may fail with wrong polarity applied!
Switches are another example in AC/DC rating difference. Typical switch may have 250 V AC 10A and 24V DC 10A rating (lower voltage with DC).
Blimey, that put the cat amongst the pigeons - so many strong opinions, but what are the
facts?
http://www.ebay.co.uk/itm/272265686326I am already using W58 series breakers for some sub-assemblies, but for the main circuit board I want something more substantial, e.g. W67, I'm just surprised by the high cost. Considering I will have something like 10-15 circuits, if I can get away with £8 household type MCBs instead of the £20 "marine/aerospace" W67s there are considerable savings to be made. DIN rail mounting would also be a bonus since you can easily find nice cabinets for mounting them in.
Safety Ratings are much closer to fact than opinion. Ignore them at your own peril.
If it is rated for DC use it will work, and if it has a 100V breaking rating on DC it will work on 12V DC as well. Problem with them comes with ones AC only rated, as the arc separation speed, the arc breaking ability and the arc diverters are them possibly not capable of breaking a DC arc. Too slow, too poor a spreader and it might arc internally with a low enough supply impedance.
Use the DC rated ABB or Schneier devices, but add an upstream ( likely at the battery input with the currents you are talking about) DC rated fuse to provide incoming supply protection as well. A 60A automobile style fuse is not too expensive, and a good thing to have as well, as it can also serve as a battery isolator as well.
DC rated (48-250V) MCBs with 4-6A trip limit:
http://uk.rs-online.com/web/c/automation-control-gear/circuit-protection-circuit-breakers/mcbs/#esid=4294959274&applied-dimensions=4293154660,4293304659,4293452075,4293498199,4293582065,4293610385,4294308571,4294308579,4294562531,4294569625,4294569966,4294573285,4294573291,4294812480,4294853422,4294854596&sort-by=P_breakPrice1&sort-order=ascLooking at these because ~5A is the lowest branch current I would need to trip on, and some models only seem to be available >10A. Interestingly many of these have a 10kA breaking ability
Prices are as predicted waaaaay below what you'd pay for similar breakers in the W67 style.
if it has a 100V breaking rating on DC it will work on 12V DC as well
Actually, having looked at a few datasheets, some DC rated models also list a "minimum voltage", which sometimes is higher than 12V. Still curious about power losses; I expect these will have a coil inside which the current passes through, and if the magnetic field thus produced exceeds a certain limit, it will trip. What kind of losses can I expect from these coils? Insignificant?
Maybe Schurter T11 series (about €4.30)?
if it has a 100V breaking rating on DC it will work on 12V DC as well
Actually, having looked at a few datasheets, some DC rated models also list a "minimum voltage", which sometimes is higher than 12V. Still curious about power losses; I expect these will have a coil inside which the current passes through, and if the magnetic field thus produced exceeds a certain limit, it will trip. What kind of losses can I expect from these coils? Insignificant?
It will be in the datasheets, losses at rated load, and the coil resistance is only going to be an issue with the lower current ones, anything over 6A will be using a 20 turn or less coil, and it will be 1mm wire or thicker.
How about the Kopp GE-2 series?
http://www.kopp.eu/verteilereinbaugeraete-ge2.pdf 60V DC rating, 10kA breaking ability, 12V min. voltage, "B" characteristic, plus:
The inner life of the KOPP MCB has changed significantly compared with previous automatic cut-out technology. The thermal actuator consists of a bimetal snap disc that is integrated into the magnetic actuator. This has reduced the number of welded connections in the device. The result: Less power dissipation and thus less device intrinsic heating.
They seem fine too. The 16A is €2.50 and the other ones are around €4-5.
They seem fine too. The 16A is €2.50 and the other ones are around €4-5.
Great, I think we have a winner
I love the idea of being able to use regular a DIN type enclosure, busbars, etc - so easy to find things that fit, at prices way below typical marine/aeronautical gear. I do use something similar to the Schurter T11 breakers you suggested for sub-distribution panels, but for the main distribution I want something a little more heavy duty. I see that the Kopp GE-2 accepts up to 25mm
2 cable, which is perfect.
Another option - automotive breakers designed for standard blade fuse holders
http://www.vehicle-wiring-products.eu/product.php?xProd=1247
Close to useless, these are.
You are much better off with regular fuses. And they take too much height, so they won't fit in places they should (in addition to not tripping when they should). The price on this website is several times more expensive than what you can get them on ebay. At that price you can get a DIN rail-mounted circuit breaker that actually works.
For the OP, I would consider Chinese DIN-rail mounted DC circuit breakers from ebay. I have tried the TOMZN brand & they work fine.