Spur feed to garden shed...

Hi
Comments, please. I've read history in this group, consulted the Wiring Regs and the OSG. I can find nothing that precludes me from implementing my preferred design for a power feed to the garden shed.
I intend to take feed from kitchen ring main through a DP switch and a 13A fused spur unit to a 4mm.sq SWA cable running to the shed. I intend to fit some RCD protected mains sockets in the shed, and take a feed through a 5A fused plate for lighting. The SWA cable is 3 core so I get a 4mm earth as well as the armour. The cable is burried at 400mm in an old metal pipe and plastic tape above. The house istallation is PME and I have test equipment to check earth loop impedence etc once the job is done. I will rely on the earth from the house (rather than treat the shed as a TT, float the earth at the shed end and drop an electrode). The load in the shed is minimal, including the use of power tools and garden tools and while I may install a small freezer I will accept a trip risk.
I have considerable DIY electrical experience and I would not as a matter of personal preference take a spur off the kitchen ring but I have no spare ways on the CU whuch is towards the front of the house which has concrete floors and there's no viable cable route. My only other possibility would be a feed from the Cooker Unit (this is a serious possibility if we put in a gas cooker when the kitchen is refurbed!).
Can anyone point me to any breach of regs or problems in the above design - I know it's not ideal, but it is a considered design.
Thanks John
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Wouldn't it be easier to take a two way consumer unit from your meter or doubled up with the mains switch in the existing CU, and then run the SWA supply from that. You say you have three cores in the SWA, so it would allow for the lighting and power to be drawn from the red and yellow cores and share the black as a neutral. You'll still have the armour as a decent earth to a metal clad junction box in the shed, or spike it at the shed end and reinforce the bonding to ground from there as it wouldn't hurt to give any extra. This way it is on its own and will not effect the house supply if a fault is detected in the shed.
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Personally I see no problem as long as the sockets that will be used for power tools are RCD protected.
Adam
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Rather than use RCD protected sockets in the shed, I'd use an RCD fused connection unit instead of the DP switch/fused connection unit you intend. This will mean that the cable run and lighting is also RCD protected (making low earth loop impedence a belt and braces approach, rather than critical) and be cheaper if you are installing more than one socket.
Christian.
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I forgot. If the shed has any structural metalwork or metal services, such as plumbing of any kind, they will need to be main bonded back to the main earthing terminal near the consumer unit using an unbroken cable of very large cross sectional area. As you say the consumer unit is inaccessible, this would preclude your technique and require a TT installation.
Christian.
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