The over current part will only sense on one pole. A truly double pole device, like one for 3 phases has two sets of detection and can be used in split phase (L-0-L) and polarity unknown situations, This is incredibly rarely needed in the UK, and many RCBOS sold as double pole are in fact not - they just have two sets of breaking contacts.

So, we are not yet ready to declare reverse polarity a non-fault !!
Mike

I note Wylex were claiming them to be safer in a recent advert in PE. 
They are useful when downstream of an RCD as once they trip, they won’t leave a detected neutral to earth fault to pull out the upstream RCD. 

There are a couple of points with SP+switched N devices - the lack of overcurrent protection on the N is one but also that the N contacts are typically more akin to a simple switch than the arc breaking capabilities of an MCB. In normal single phase earthed N circuits and the N contact arranged to open after the main L contacts, it's normally not an issue. Reverse polarity (e.g. in a caravan taken abroad) things could get interesting though - on a couple of counts - on L-PE faults the fault loop might be through the N side of the device rather than L - so the switching contacts don't have a suitable breaking capacity, also in that mode you're relying on the RCD element to provide fault detection - which for high PFCs could be relatively slow - (MCBs are faster for higher fault currents) - so much greater likelihood of excess energy let-through and unnecessary damage to equipment (esp cables).

In most situations where polarity is reliable, I don't see any major issues though (unless we start considering faults from L of one circuit to N of another).

   - Andy.

"SP+N" is a bit ambiguous, but I tend to assume it refers to a device which only switches L, with a solid N - rather than one which switches both poles.

Actually, confusingly not always - there are 3 types in common use and a 4th rare one.-

1) Un-switched neutral, sometimes called SPSN = single pole solid neutral, - until a few years ago all RCBO were like this, and it makes true isolation and fault finding quite hard, especially for N-E faults.

and 

2) With a full 6kA blow arc chute on the live but just late breaking early making  contacts on N also sometimes shortened to SPSN = Singe pole switched neutral. Doh!
These are the SP+N switching, as the newer from Wylex Crabtree etc

and

3) Full rated simultaneous contacts on both L and N, called true double pole, usually two module width, but still sensing over-currents on the live only

and

4) The full monty double pole sensing which is the really rare one.

I am not sure that even the folk who sell them are that sure of which type is needed when, and the abbreviation SPSN confusion is frankly ludicrous.

Mike.

I agree.

If you take a 3-phase RCBO, there are 4 poles, which take up 4 x 18 mm ways. Presumably the switch inside the N pole is the same as the L poles - at the very least, the casing is the same. If the switch in the L of a single-phase is similar to that of an MCB, it is difficult to see how you could fit another one for the N beside it.

9mmsingle pole  MCBs exist, with 6kA breaking and CE marking.  and 13mm as well Just not from the normal suppliers.  They do make for very compact boxes but the wiring is tight, and you cannot escape the feeling it must in some way be weaker, though I am not really sure. i presume the same techniques are used to make the normal width double pole ones.
Also double pole MCBs  in single module width - again rare.
Mike.

Thanks all for the input. Confirmed my basic understanding and provided some more depth, which is always useful.