Use 240V household type MCBs in low voltage circuit?  - Page 1
Forum section: electronics / general

author: Lomax
author_rank: Frequent Contributor
timestamp: April 28, 2017, 04:43:59 pm
content: 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:

author: Someone
author_rank: Super Contributor
timestamp: April 29, 2017, 04:07:32 am
content: NOThey wont be sized for breaking the current without the zero crossings of the AC mains, get a proper DC rated part.

author: Delta
author_rank: Super Contributor
timestamp: April 29, 2017, 06:09:53 am
content: Absolutely not.They will NOT be able to interrupt large DC currents.

author: Nusa
author_rank: Super Contributor
timestamp: April 29, 2017, 07:34:38 am
content: Quote from: wraper on April 29, 2017, 07:32:09 amThere 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.

author: wraper
author_rank: Unknown
timestamp: April 29, 2017, 07:34:48 am
content: Quote from: Delta on April 29, 2017, 06:09:53 amConsidering 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.

author: wraper
author_rank: Unknown
timestamp: April 29, 2017, 07:39:22 am
content: Quote from: Nusa on April 29, 2017, 07:34:38 amMy guess is, the main reason would be they are made for different market, therefore no point for DC specification.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 29, 2017, 07:46:33 am
content: 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.

author: DTJ
author_rank: Super Contributor
timestamp: April 29, 2017, 07:48:15 am
content: 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.

author: sibeen
author_rank: Frequent Contributor
timestamp: April 29, 2017, 08:12:41 am
content: 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.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 29, 2017, 08:23:05 am
content: This MCB has a DC ratinghttp://cpc.farnell.com/abb/s201-d10/circuit-breaker-thermal-mag-1/dp/MC02489?st=30vdc+circuit+breaker&pf=110141551&voltage-rating-v-dc=60vdc&ddkey=http%3Aen-CPC%2FCPC_United_Kingdom%2Fw%2Fc%2Felectrical-lighting%2Fwiring-accessories%2Fconsumer-units-accessories%2Fmcbs

author: Kjelt
author_rank: Super Contributor
timestamp: April 29, 2017, 08:25:38 am
content: 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.

author: electr_peter
author_rank: Unknown
timestamp: April 29, 2017, 08:26:08 am
content: 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).

author: sibeen
author_rank: Frequent Contributor
timestamp: April 29, 2017, 08:31:11 am
content: Quote from: mikeselectricstuff on April 29, 2017, 08:23:05 amABB & Schneider were the two manufacturers I was thinking of that do supply information on the DC rating of their MCBs. I'm just a tad surprised at the price you can get the ABB for.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 29, 2017, 08:35:48 am
content: Another option - automotive breakers designed for standard blade fuse holdershttp://www.vehicle-wiring-products.eu/product.php?xProd=1247

author: Lomax
author_rank: Frequent Contributor
timestamp: April 29, 2017, 09:07:45 am
content: Blimey, that put the cat amongst the pigeons - so many strong opinions, but what are thefacts?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.

author: Nusa
author_rank: Super Contributor
timestamp: April 29, 2017, 09:29:13 am
content: Safety Ratings are much closer to fact than opinion. Ignore them at your own peril.

author: SeanB
author_rank: Super Contributor
timestamp: April 29, 2017, 11:56:36 am
content: 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.

author: Lomax
author_rank: Frequent Contributor
timestamp: April 29, 2017, 12:52:39 pm
content: 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 abilityPrices are as predicted waaaaay below what you'd pay for similar breakers in the W67 style.

author: Lomax
author_rank: Frequent Contributor
timestamp: April 29, 2017, 12:57:00 pm
content: Quote from: SeanB on April 29, 2017, 11:56:36 amActually, 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?

author: madires
author_rank: Super Contributor
timestamp: April 29, 2017, 01:42:10 pm
content: Maybe Schurter T11 series (about €4.30)?

author: SeanB
author_rank: Super Contributor
timestamp: April 29, 2017, 01:57:22 pm
content: Quote from: Lomax on April 29, 2017, 12:57:00 pmIt 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.

author: Lomax
author_rank: Frequent Contributor
timestamp: April 29, 2017, 03:56:57 pm
content: How about the Kopp GE-2 series?http://www.kopp.eu/verteilereinbaugeraete-ge2.pdf60V DC rating, 10kA breaking ability, 12V min. voltage, "B" characteristic, plus:Quote

author: madires
author_rank: Super Contributor
timestamp: April 29, 2017, 05:11:27 pm
content: They seem fine too. The 16A is €2.50 and the other ones are around €4-5.

author: Lomax
author_rank: Frequent Contributor
timestamp: April 29, 2017, 06:28:21 pm
content: Quote from: madires on April 29, 2017, 05:11:27 pmGreat, I think we have a winnerI 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 25mm2cable, which is perfect.

author: bazza
author_rank: Contributor
timestamp: April 30, 2017, 02:30:52 am
content: Quote from: mikeselectricstuff on April 29, 2017, 08:35:48 amClose 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.

author: WA1ICI
author_rank: Contributor
timestamp: April 30, 2017, 05:15:53 am
content: I think this has been hashed-out successfully - use a breaker with a DC rating. DC is harder to break, since there are no zero crossings to extinguish the arc.However, I thought I would relate a relevant incident that happened at my first job at Intel, around 1978. I was a design engineer on the 8089 I/O processor which followed the 8086 design by about a year. (It never became very successful, but the 8086 - that's another story). At that time we prototyped microprocessors in a TTL breadboard to test & debug functionality before first silicon came back. This was in the days before logic simulation. I got called into the lab by the 8086 lab technician, who wanted me to look into a problem with the 8086 TTL breadboard. This was built with multiple Augat wire-wrap boards that were mounted in a tall rack cabinet. At the bottom of the cabinet was a shoe-box sized 5V 200A switching power supply with big (I guess about 8 gauge) insulated wires running to each Augat rack assembly. The complaint was that the breadboard had stopped working. Putting a scope on one of the bus lines showed a logic high of about 1.5V. The VCC was about 3V. As I was looking around for the problem, I started smelling something hot. I looked down towards the power supply at the bottom and saw that the +5V wires coming up had no insulation (it was burned off) and were glowing a deep red color. Tracing the problem, one of the +5V wires had shorted to the rack cabinet due to an over-enthusiastic cable clamp. At the lowest rack position was a panel with a single household circuit breaker in line with the +5V, rated at 30A, if I remember correctly. It was somewhat charred, but was otherwise happily passing hundreds of amps from the massively over-sized power supply. I asked the tech about this, and he thought that a 30 Amp breaker would work just fine. Two problems here: Don't use a power supply that is almost an order of magnitude bigger than needed (the bread board used less than 30 Amps), and no, a 60Hz 120VAC circuit breaker doesn't work on 5V DC. Putting in a smaller power supply with current limiting and fixing the short brought the breadboard back to life.- John Atwood

author: richard.cs
author_rank: Super Contributor
timestamp: May 07, 2017, 09:37:08 pm
content: It was buried deep in the documentation somewhere but I remember Hagar specified theirs for either 48 or 60 V d.c. - I have a feeling we had to ask them specifically, it wasn't in the device datasheets.One thing to note when using MCBs on DC - the magnetic trip operates on the peak of the AC waveform so a B type with the magnetic trip that is intended to be 3-5 times rated current but will actually be sqrt(2) higher on DC, almost turning it into a C type.

author: necessaryevil
author_rank: Regular Contributor
timestamp: May 09, 2017, 07:17:14 am
content: Wait... household style breakers are less than 15 UK pounds ($20 USD)? Or do you have them laying around?On topic:Look at a electronic wholesale such as Digikey, Farnell or Mouser. A quick look around tells me you can find some cheap ones there. Another solution might be the use of a PTC fuse.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 09, 2017, 07:23:16 am
content: Quote from: necessaryevil on May 09, 2017, 07:17:14 amMore like £2-3http://cpc.farnell.com/click/db216/mcb-c-curve-6a-16ka-sp-type-c/dp/PL10147http://cpc.farnell.com/pro-elec/aub1-1p-c40a/mcb-single-pole-c-curve-40a/dp/PL11596http://cpc.farnell.com/crabtree/s61-c16/starbreaker-16a-sp-type-c-curv/dp/PL12119