MCBs for low voltage (as in 12 volts) - practical?

Is it possible/sensible to use 'standard' mains DIN rail MCBs as protective devices on 12 volt DC systems or would the voltage drop across them be too great?
A small[ish] consumer unit would be a neat solution to use as a distribution box for the 12 volt wiring on our new (to us) boat, much cheaper and easier than a custom solution.
--
Chris Green


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snipped-for-privacy@isbd.co.uk wrote:

Generally not a good idea unless the manufacturer specifically says it's OK.
An MCB (as you'll know if you've ever taken one apart) is a complex device and has two operating principles. There is a magnetic part which responds quickly to short circuits and there is a thermal part which responds to small and moderate overloads.
The thermal part will not care whether the current is a.c. or d.c. (which is why thermal circuit breakers are pretty much universally rated rated for both a.c. and d.c.), but the magnetic part is optimised for a.c. short circuits (in particular in respect of arc-quenching) and may not be deemed suitable for d.c.
Consequently, if any given manufacturer does not rate the MCBs for d.c. supplies, I would not recommend that you use them.
== MK Sentry data sheet does not mention use on d.c. supplies:
http://www.mkelectric.co.uk/Documents/English/EN%20MK%20Technical%20Specifications/Circuit%20Protection/Miniatur%20Circuit%20Breaker.pdf
MK short link http://preview.tinyurl.com/38zzfhd
== Whereas, ABB are suitable for d.c.:
http://docs-europe.origin.electrocomponents.com/webdocs/0b15/0900766b80b15fa5.pdf
== Moeller standard FAZ-type MCB's are good for 48V d.c. Moeller make special MCBs for d.c. upto 250V d.c.
http://docs-europe.origin.electrocomponents.com/webdocs/0692/0900766b8069247e.pdf (catalogue page 12/13) (actual page 10)
See also catalogue page 12/58 (actual page 54)
== Hager 10kA-class MCBs don't mention d.c.
http://docs-europe.origin.electrocomponents.com/webdocs/0afb/0900766b80afb159.pdf
== Merlin Gerin C60-type are suitable for d.c. and give a little treatise on the subject here: (cat. page 166, actual page 6)
http://docs-europe.origin.electrocomponents.com/webdocs/0166/0900766b80166362.pdf
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[snip]
Thanks for the links, so I need to choose manufacturer, no problem there.
No indication of voltage drop though that I can see.
--
Chris Green


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On Jul 8, 10:45 am, snipped-for-privacy@isbd.co.uk wrote:

can always go to a local seller armed with a very good multimeter
NT
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     snipped-for-privacy@isbd.co.uk writes:

You need to check manufacturer's datasheets. Generally, they have a different range of MCB's for DC operation, and both the AC and DC breakers have a minimum operating voltage for reliable tripping.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
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On Wed, 7 Jul 2010 15:14:19 +0100, snipped-for-privacy@isbd.co.uk wrote:

I've used Polyswitches (polymeric positive temperature coefficient devices) for circuit protection on boats in the past.
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On Wed, 07 Jul 2010 15:14:19 +0100, tinnews wrote:

Shouldn't be a problem. There isn't much voltage drop as the magnetic trip uses a heavy wire coil. The DC current rating will almost certainly stay the same up to 30V. Above that and you get problems as there can be insufficient air gap to break the arc. Check your mcb spec though, you may have to use type D, especially if you have inductive loads.
There are some breakers intended for DC applications though, these are usually thermal releases only and don't give good short circuit protection. Usually low current ratings only.
--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
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