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We use 3 core for single phase installations, the cable is glanded and earthed at both ends so it just becomes a parallel path for earth, with your 3 cores as phase, neutral and CPC.
It’s nice to have it in single phase colours because it saves you faffing with tape to sheath them in the correct colours is all.
4 core gets used for transformers that get moved around - 3 phases and the neutral gets sheathed in green/yellow and used as an extra earth because if the tranny gets bashed or the cable pulled and the armouring comes loose from the gland you lose earth, which is no bueno (and the tranny doesn’t need a neutral).
But we regularly use the armour as the sole earthing. Chucked in 90m of 70mm2 x 4 core for a crane supply on Friday 3P+N, . Definitely makes you yearn for twin and earth when you’re wrestling that through a series of bog filled ducts, lol.
Nef-
4 core gets used for transformers that get moved around - 3 phases and the neutral gets sheathed in green/yellow and used as an extra earth because if the tranny gets bashed or the cable pulled and the armouring comes loose from the gland you lose earth, which is no bueno (and the tranny doesn’t need a neutral).
Not sure about not needing a neutral. Balanced loads such as motors don't need neutrals as there is no return current. Site transformers (presumably 110V) aren't going to be balanced across the three phases as each phase has a different load.
It's all about Phasor Diagrams.
Mr_Sworld
Just brown, blue and P/E? The armour isn't always enough cross section to provide an earth.
Best just to treat it as armour but not earth both sides of it sometimes. There are some tables that tell you exactly how to deal with the various sizes.
But Sunday and a bit drunk...