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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi all,
I need some help, and will try to remain simple - which of these images is right? A 3 phase mains source is powering a symmetric load ins tar configuration. I have came to conflict, whether I should draw all the currents (a) as going through the neutral line and their sum being zero, or not draw them at all (b). In reality obviously it's the same as there'll be no voltage between midpoints - but is the (a) really necessary to satisfy KCL? Should I, mathematically, view the whole neutral wire as one node and not remove currents from it even if their sum is 0?
I feel this is purely philosophical and with no meaning in reality, but I came to some contradictions in books and argument that I'd like to clear up.
Registered Member #53
Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638
In trade school we only wrote in the sum current of each wire. If all 3 loads are balanced then I would write I=0 over the neutral. If the loads are not all the same then I would write in the difference. So that's the "Canadian electrician answer" to your question.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
It's only philosophical in the nice theoretical world of a balanced source and balanced load. In the messy real world, there's always an imbalance, whether it's the small error in balance of a nominally balanced load, or a large imbalance resulting from one phase going down. In my street, every 3rd house is connected to the same phase, so by diversity, on a long enough road, the load will be sort of balanced. Just write the net current on the Neutral, it's tidier than writing sum(I.p1 I.p2 I.p3).
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Neutral line in a 3 phase system
