Significance of neutral in 3 phase residential setup

Hardware
1

Hey guys,

I’ve been looking at some 3 phase monitoring systems, and notice some do and some do not monitor the neutral with CT coils.

When looking at ‘common european 3 phase houses’ what is often done, is use each phase on a RCBO and then 4 breakers on each rcbo, so 12 ‘circuits’ for a 3 phase system. Very sometimes, a 3 phase (all 4 wires) goes to one of those bigger red socktes for true 3 phase hook-ups.

When monitoring 3 phases with just 3 parallel branches, I think it’s fair to say, the Neutral carries all those return signals, and since it’s out-of-phase of eachother, the max current should be manageable for the neutral’s thickness?

Secondly, when monitoring these 3 phases, what value would monitoring the neutral bring, other to help detect some inconsistencies? especially if one where to also run a 3 phase motor (with and without neutral)?

Thanks,

Olliver

2

None.

The neutral current is the phasor sum of the three line currents.

Summing the neutral current and the phase currents will detect a residual leakage current to earth, which is the job of protection, not of monitoring. This is what your RCBO is for. And a long-standing principle is you don’t mix monitoring and protection. Each system should stand alone, and not be capable of interfering with the other.

3

Thanks @Robert.Wall

What brought me to the question, is that years ago, I was reading TI’s Application notes (I still think their MSP430 series are great MCU’s for exactly this application :slight_smile: But see this https://www.ti.com/lit/pdf/slaa577 pdf for example, they explicitly monitor the Neutral, and it is an AN for Watt-Hour metering, not protection or anything like it? So this is what confused me a lot …

4

That is for metering - almost certainly (without reading the note) it’s concerning fraud detection.

1 Like
5

Thanks! That was probably the ‘missing link’ for me!

6

What is the likeliest way someone will attempt to steal electricity - connect a parallel wire around the meter. What happens then, the phasor sum of the 4 wires is non-zero - the fraud (or a fault to earth) is detected.

7

Sure, but why not bypass neutral AND the phases in that case?

8

Would the average cheat/criminal think of that?

9

Lol, idk; to me it would be obvious ‘my extension cord has 2 wires, so I must connect both’ I think most people aren’t even aware you only need 1 wire? But I can understand from a design pov; you’d want to monitor neutral at least for half the cases …