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.
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:
MK short link http://preview.tinyurl.com/38zzfhd
Whereas, ABB are suitable for d.c.:
Moeller standard FAZ-type MCB's are good for 48V d.c.
Moeller make special MCBs for d.c. upto 250V d.c.
(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.
Merlin Gerin C60-type are suitable for d.c. and give a little treatise
on the subject here: (cat. page 166, actual page 6)
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!)
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.