It depends on how safe you want it to be.It's the old lights argument again and it's in at least one other active thread here too. Yes, the lights will go out if your RCD trips. How is the different from having the RCD in the garage? Yes there are fewer final circuits that could trigger a garage shutdown but it you'll still having a spinning down angle grinder in your hand. Would you put the lighting on a non-RCD final circuit/own RCBO? Consider this scenario:
Then if you want, design it so that that doesn't happen.House with a split load dual-RCD board. Upstairs lights on one side and downstairs on the other, because heaven forbid that all lights should go out if the RCD tripped. No complains about that setup, I take it, it's fairly standard after all. I'm standing in the kitchen, top side of beef on the bottom shelf of the oven and a baking tray full of goose fat heating on the shelf above. I take the fat out to add the par boiled potatoes and just as I'm half way across the kitchen, that old immersion heater trips the RCD and the kitchen plunges into darkness.
Nothing is foolproof.With my one one remaining hand I mop the floor, decide to mash the spuds instead and replace the lighting with it's own RCBO. The spuds are now finishing off in boiling water, in a pan on the oven. Just as I'm carrying the pan to the sink to pour off the water, the YL comes home and turns the hall lights on. One of those GU10s reaches the end of its life and pops the B10 RCBO. Damn. Shall we have one more iteration? OK. Without any remaining hands, I get someone else to rework that lighting circuit to accommodate a C10 breaker. Just as I go to drain off the peas, I'm half way across the kitchen when a gas working in the street puts a drill through the service cable. From my hospital bed I call that mate back and get him to fit multiple, battery backed up lights to the kitchen and also get me a number for the local takeway.
People do all sorts of stupid things.In summary, regardless of where the RCD should/shouldn't go, I'm not sure the lights going out in a given configuration is a very valid justification as there's plenty more configurations that can give rise to exactly the same result. I've also used the angle grinder in the house a few times...
However, we are suppose to install systems with regard to what may happen and make them as safe as possible within the constraints of how much the customer wants to pay.
We do/should not install systems which are less safe than they could be with the same parts - i.e. with all the lights on the same RCD or the lights on the same RCD as the room sockets.
Are you calm enough, when the power goes off, to stand still while the angle grinder spins down or do you become startled and jump turning into someone stood next to you?