What current differences trip a 3-phase differential switch?

[Roger44]

Hello
To get 230V in 3-phase, you can connect between any of the 3 phases and a neutral if provided, or between any two phases. If you draw 230V using both approaches along a same row, I don't see how the 30mA diff switch at the left end can work. Can anybody help me on this one?

Thanks

**broken link removed**

 

[ronsimpson]

Are you talking about ground fault protection? Where you can run a 30A motor but if a person gets shocked by 30mA the whole thing shuts down.

 

[mab2]

Assuming (As Ron says) it's a 30mA GF /RCD type breaker, then it works by summing the currents in the four conductors - which should always add up to Zero; e.g. if you have a single 230v load between one phase and neutral and no loads on the other phases, and at a particular instant the current is, say, 3.2A, then the currents would be:

ph1: 3.2A
Ph2: 0A
Ph3: 0A
N: -3.2A
Total: 0A

if you have a single load (at a given instant, say, 4.0A) between two phases (incidentally it would be 400v between two phases) and no other loads:

ph1: 4.0A
ph4: -4.0A
ph3: 0A
N: 0A
Total: 0A

with both loads running at once:

ph1: 7.2A
ph2: -4.0A
ph3: 0A
N: -3.2A
Total: 0A

so as long as there's no 'leakage' (to earth) from the connected circuits the total always adds up to Zero; if the total (i.e. leakage) exceeds the 'trip' value (30mA) then it will disconnect.

NB: the breaker continuously sums the instantaneous currents not average currents or RMS currents, so if you tried measuring the conductor currents on the phases and neutral with a meter you wouldn't necessarily get the same results, depending on the power factors/ nature of the loads.
hope that makes sense.

 

[ronsimpson]

We are talking ground fault protection.

 

[mab2]

Yes that's what I'm talking about - over here we call them residual current Devices or residual current circuit breaker, but the OP seems to be referring to the 4 pole (i.e. 3ph +N) version, so that's what I described (or tried to)

 

[Roger44]

Yes, it is ground fault protection and "then it works by summing the currents in the four conductors" is the answer that I couldn't fathom out by myself. Many thanks.

And also thanks for reminding me that the RMS voltage between any two phases is not 230 but around 400V.

Roger

 

[KeepItSimpleStupid]

Phase to phase depends if you have a Y or Delta. In the US, we have a Delta - High leg.

 

[mab2]

Phase to phase depends if you have a Y or Delta. In the US, we have a Delta - High leg.

maybe - but with delta you would just have the 3 phases; you wouldn't have a neutral. Or of you do contrive a neutral, equivalent to the star point of Y then it couldn't be 230v between the phases AND 230v between ph and N.

Though I don't know much about the US setup beyond the split phase 115 - 0 - 115 supplies.

 

[Roger44]

Hello

Or of you do contrive a neutral, equivalent to the star point of Y then it couldn't be 230v between the phases AND 230v between ph and N.

I think we all agree on this point, but what arouses my curiosity is the mention of "legs". A quick Wiki made me realize there was more in this subject than I thought. I'll come back on this point next week when I've had a bit more time to think about it. By the way, what's the American terminology for a generally 30mA diffential 2, 3, or 4-pole circuit breaker.? A ground fault switch?

Secondly, you all seem very knowlegeable so I have other technical questions, all in the realm of mains voltage, like its reactance or its distortion, I'll post them next week but in separate threads.

 

[RODALCO]

Your differential breaker or RCD is a 4 pole one which measures the three Phases and the Neutral.
[mab2 post 3] explains it very well.
On the photo you don't show the differential breakers data, but I have seen 30 mA and 500 mA RCD breakers in Europe.
In Hospitals they are often rated at 10mA trip currents.
Any unbalance to Earth will be detected and the breaker will trip.

 

[Roger44]

RCD = residual current detector?

And here in France it is 30mA and 500mA