Stand by your guns Beaver - if you`re doing the job & signing the paperwork (& it is notifiable of course) then it`s perfectly reasonable for you to make the call.
I always 30mA protect my outside installs & as with here, it`s usually stated in the manufacturers instructions (& only a fool goes against them in such circumstances)
Having said that, there actually isn`t any statute (amazingly) that says you have to RCD exterior lighting per se
(i never let that stop me tho - my right as the competent person to dictate what is appropriate)
Of course there are many situations where RCD protection would INDIRECTLY be required - feed from S/O cct (RFC or radial) likewise supplied from lighting cct also feeding a bathroom or even if its on its own dedicated cct, cables concealed in walls <50mm
Perhaps the customer has experienced nuisance tripping of RCDs in the past - if you do the job right - as i`m sure you would, that won`t be an issue. Or is it the usual question of saving a few quid? And if she realised just what a lifesaver RCDs can be, she`d think differently.
Tell her this. In a recent study (by the ECA i seem to recall) it`s been ascertained that NO-ONE has been fatally shocked by an operable 30mA RCD protected cct . Apparent fact. Compelling or what.
Good luck with it mate
