Earthing SWA

[KeenPensioner]

As an adult learner, I have lots of theory but not enough practical experience so I'm hoping I can get some help here (not abuse). I'm trying to get my head round earthing SWA to an outbuilding with no extraneous conductive parts. I understand the problems of exporting a PME supply etc. If I have a 3 core 6mm2 SWA and use one of the cores as the CPC and the house CU has RCD protection why do I need to earth the SWA? My thinking (probably wrong) is that if a spade goes through the cable it will cause a short between phase conductor and earth conductor and trip the RCD?

 

[SJD]

If the armour is not earthed, and your spade happens to dig into the armour and the live (L, line) only, ... the armour goes live, so does your spade, ... and anyone touching the metal parts.

 

[KeenPensioner]

If the armour is not earthed, and your spade happens to dig into the armour and the live (L, line) only, ... the armour goes live, so does your spade, ... and anyone touching the metal parts.

Thanks for taking the time to reply. Makes perfect sense - why didn't I figure that out!