Without even going into the regs, common sense would tell me installing an ELO below a DB would create restricted access to the main switch and the earth leakage unit, make sure you put a clip on the side of the ELO so that you can use a broom stick to switch on the breakers (seems to be the norm).

The height would need to be taken into consideration.

A IP rating would have been considered, however the ELO is contained in a cupboard or exhausted out the front of the unit, there shouldn't be a moisture problem directly above the ELO