wrote:
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?


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/Document...%20Breaker.pdf

MK short link http://preview.tinyurl.com/38zzfhd

===

Whereas, ABB are suitable for d.c.:

http://docs-europe.origin.electrocom...6b80b15fa5.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.electrocom...6b8069247e.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.electrocom...6b80afb159.pdf

===

Merlin Gerin C60-type are suitable for d.c. and give a little treatise
on the subject he (cat. page 166, actual page 6)

http://docs-europe.origin.electrocom...6b80166362.pdf