If you are talking about PNEU/ELEC switch on the altimeter gage, then the answer is the following - if you have lost either electrics, or CADC, to retain altimeter operation you have to switch it to PNEU operation mode. In this mode altimeter is directly fed by wing mounted pitot-static probe. There will be a small error in indication, but not that big as it can be IRL because so far we don't account for non-standard atmosphere effects. In ELEC mode altimeter is servoed by CADC data.