The ordinary wall sockets around the house are normally connected to a ring circuit also referred to as a ring main.
Socket ring main fuse.
This video looks at the difference between a metal and plast.
The ring main must be protected by a 30amp fuse if it is on a cartridge fuse board or a 32amp mcb fuse.
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Id take a rcd fused spur from that socket make sure its on the ring 13amp fuse then run a twin and earth through some conduit to the shed to a socket and one more fused spur for the lighting with a 3amp fuse.
These outlets require the correct fuse rating for the appliance and are connected to the appliance by a cable or flex.
The ring is formed by the cable going from the consumer unit to the first socket then on to the second socket and then the next socket etc.
Clip cables properly with cable clips.
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Where connection to a fixed appliance is required a fused outlet unit may be fitted to the wall rather than a plug socket and connected into the ring main.
It could just be a faulty mcb if you want to test it then swap it for the one same rating should be 32a that controls the upstairs ring main.
Its advantages over radial circuits ar.
The reduced diameter conductors in the flexible cords connecting an appliance to the plug intended for use with sockets on a ring circuit are individually protected by a fuse in the plug.
Meter mains with safety socket bypass.
So as zax47 says it s most likely a general fault on the ring main that s tripping and causing your main rcd to trip.
The cable itself can be up to 60 meters long if it is protected by a cartridge fuse and 50 meters long if protected by an mcb.
Ring type nema type 3r ansi 61 gray e coat finish aluminum snap ring included standards ul 67 listed ansi c12 7.
The ring circuits of a domestic property supply the socket outlets and fixed appliances in the premises.
Until the cable returns to the consumer unit.