Basic 3 phase question

Question

I've come across this question at work and it has stumped me... even though it is quite basic.

A distribution board is supplied by 3 phase 415 AC power. From the distribution board to a circuit breaker, only 2 of the three phases are used, and the other phase is grounded.

A RCD is to be installed after the circuit breaker. Is the power that the RCD to receive considered 3 phase with an asymmetric fault, with 415v between phases and 240 between phase and neutral?

Or is it split phase power? with 240 between phase and neutral and 480 between phases?

Where are you on the planet? Also, is the incoming service supposed to be a wye or a delta service? — ThreePhaseEel, Dec 10 '17 at 01:18
Should've known that, I've worked with enough Aussie sparks :D — Ian Bland, Dec 10 '17 at 01:21
"*... and the other phase is grounded.*" The other phase of what? The supply(!) or the load? Are you trying to run a 3-phase load on 2-phases + neutral of a wye/star supply? — Transistor, Dec 10 '17 at 08:39
Listen to Transistor. You can't ground a phase. Something is really off here. — winny, Dec 10 '17 at 09:09
I'm working off 40+ year old schematics..... three wires are shown inside a cable that supplies the circuit breaker - but the schematic shows that one is grounded. I don't know what to make of that — Liz_eE, Dec 10 '17 at 09:58

score 0 · Answer 1 · answered Dec 10 '17 at 01:19

You can't get 480V however hard you try out of a 240/415 (these days actually 230/400 nominal in Europe btw). If you have a load connected between phases and a fault occurs to Earth, it has to be on one side of the load (well, for simplicity for now) which means one side is a 240V short and the other side is just 240V through the load (so a lower current than normal), so the shorted side is your fault current to consider.

If it's somewhere inside the load (e.g. a primary transformer winding that grounds somewhere along its length) you're going to have to consider two 240V loads commoned to Earth, so 240V across each side. So if you think about it, shorting the middle of that hypothetical winding will overcurrent both sides by some relatively modest degree (in fact, 240/(415/2)) probably without tripping overcurrent protection but tripping an RCD due to the current to Earth. Which is one reason these days we have both overcurrent and RCD protection.

Forget about 480V. It just doesn't exist in a 3 phase system.

Follow up question... how would you wire a 2P+N source to a 4 pole RCD? — Liz_eE, Dec 10 '17 at 01:28
Just ignore one of the phase poles. It's the equivalent of a three phase connection with no load on one of the phases. — Ian Bland, Dec 10 '17 at 01:30

Basic 3 phase question

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