Need to know more about your assignment as to type of supply Tns etc. I think you are getting confused between protective bonding & supplementary bonding. Study Part 4 Onsite guide.
Take a look at Part 2 DEFINITIONS & look at MET . Also, all the earthing definitions, CPC sizes are related to the Type of Supply & Line conductor sizes aswell.
Practice Exam Questions for Electrical Science and Principles - https://www.efixx.co.uk/Apprentice%20Hub/science-and-principles-practice-exam-questions
The transformer is probably double insulated & no earth need for earth at the plug. Take a look or post a picture with any information on the transformer. I would expect it to be low DC output.
Now you are adding other issues as to the IP rating of Trunking which you will need to ask the manufacturer as not all may comply with Standards. At least IPX4 , now you are talking coolant, is this corrosive?
Fix it to the wall perimeter for ease of installing circuits & future maintenance & access. You wouldn't want to be working on steps or ladders if can be avoided.
Wait and see what your electrician intends to do, you are giving the impression that you are doing it yourself, so at least please do not fit all your accessories to the trunking lid.
I'm with SJD What about the discrimination of protective devices. If a socket trips a 16 A mcb it will more likely be the one in the Mainboard. What are you planning on load-wise in the garage.?
As timhoward mentions, it is load related with an imbalance, which might be telling you your drill is on the way out. There is a lot of information regarding how RCD's Work online with pictures too.
Is this any help https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/solution-center/bus-ele-tech-lib-dual-element-fuses.pdf
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