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Joined: Oct 2003
Posts: 289
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:andy: Offline OP
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this thing is what i am thinking about for longer time, there's a big discussion in another german elecrics forum, VDE has different meanings on this that dont give me clearness at all.

Following:
Using a 5x1,5mm² cable with L1 L2 L3 N PE, split for three 230V->N radials.
Now what if the load is asymmetric.
then the shared neutral, in case of 2 loads of 16A and one load of 0A will run 16A if i'm right.

Now what if there are phase shifts from inductive loads. Will the current on the neutral then raise above 16A (i think so), and how to calculate it?

lets say i have 16A inductive load with a phase shift of 90° and a 16A resistive load. is the current on neutral then nearly 32A?

tested with this animation: http://www.walter-fendt.de/ph11d/reskraft.htm
select "3" in the pulldown menu, then klick "gesamtkraft ermitteln".
just form the pointers to a triangle until the red one is in the middle. then move one phase to the middle, red pointer will show 16A between the two left over phases. now turn one left phase 90deg in direction of the other one. the red one's lenght will nearly double.
is this animation useable in this way for the current calculation?


[This message has been edited by :andy: (edited 12-05-2003).]

Joined: Jul 2002
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Gidday Andy!, [Linked Image]
Is this cabling to supply a Star or a Delta load?.
Because if it is to supply a Star(Wye) load, the Neutral will only carry the Out of Balance current, between the 3 Phases.
As a rule, the Neutral current is the Phasor sum of the Line currents.
A Delta load never has a Neutral, because it works on circulating line/phase currents.

Joined: Dec 2001
Posts: 2,498
T
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The idea was to supply three individual single-phase loads off a 3ph-circuit with a shared neutral, like a US multiwire circuit. I.e. 5x 2.5mm2 from the panel to a junction box and 3 3x2.5 wires to individual Schuko sockets.
From what I've gathered this wouldn'T be legal at all. In the case described above the junction box would be classified as a subpanel and would be required to have a main switch and individual breakers for each 1ph circuit. This is what I heard from some German sparkies @de.sci.ing.elektrotechnik.
I wouldn't like to to this at all because of the likelihood of a broken neutral in the branch circuit wiring, resulting in 220V loads being paralleled on 380V. Looks pretty ugly.

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:andy: Offline OP
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a broken neutral in this scenario doesnt trip the RCD, or does it?

Joined: Dec 2002
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djk Offline
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In Ireland (and I presume in the UK) 400V-3phase is rare in domestic and similar installations and where it is used it's exclusively for fixed appliences.

However, that would be totally illegal under the regs as you can't even have single phase sockets taken from different phases of a 3-phase supply in the same room.

We would also require pretty heafty stuff in terms of panels.

Normally 3-phase and single phase installations are kept completely seperate.

Also as 3-phase power is typically charged at a different rate sometimes both single and 3-phase metering may be employed. In this case the ESB have a completely sealed panel which provides the two meters with the correct phases and neutal arrangements.

Joined: Oct 2003
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:andy: Offline OP
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the problem is: the VDE allows this in principe, the only prescription is to make it shut down all 3 phases, for example with a 3-way mechanically bridged breaker.

but nobody thinks of or prescribes about the phase shift and N overloading.

Joined: Dec 2001
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Here in Austria you have to keep 220 and 380V wiring seperate outside the panel. You'll usually have a 380V feed and individual 220 and 380 V circuits in individual conduits. I don't think a multiwire would be acceptable without using a subpanel, and in any case it'S very uncommon.
No, I don't think the Summenstromwandler inside an RCD would notice a broken neutral since all power going in returns through one or another phase. Only some of the appliances would get rather low or high voltages. I once saw a case like this, the fridge would only work if the electric range was on and the light bulbs were very bright...

Joined: Aug 2001
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Low power-factor loads could certainly result in phase shifts leading to a higher neutral current. With, say, a 60-degree leading phase shift on one phase and 60-degree lagging on another, the currents would then be directly in phase and add in the neutral.

Obviously even at higher power factors the result could be more than 16A in the neutral with two 16A loads.

Whether such a situation is likely to occur in normal usage is debatable, but it's certainly possible.

And no, the RCD would not trip on an open neutral, because the currents flowing in the phases would still be balanced overall.

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C-H Offline
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The described setup is not uncommon in Sweden, although then with 2.5mm2 wire or 10A. Extension cords often start out as 3-ph but are tapped with 230V loads along the way. It's safe as long as the conductors don't break or come loose. Like the ring main. [Linked Image] Single phase radials aren't efficent, but there aren't many dangerous modes of failure.

With harmonics and power factor, current can be higher in the neutral than in the phase conductors, yes. With purely resistive loads, no.

The issue of neutral overload can be adressed by using a four pole breaker.

For the RCD to trip, it has to be on the load side end and have undervoltage or loss of neutral detection.

[This message has been edited by C-H (edited 12-07-2003).]


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