Neutral Distributed / Non Distributed

Explain me the difference between a neutral not distributed and with neutral distributed.

'neutral distributed' simply means you have a N conductor in your circuit. (For the moment I'll ignore the possibility of re-creating an artificial N downstream).


It's not always obvious from the nature of the load whether N will be present or not - e.g. a supply to a 3-phase motor might include a N not because the motor needs a N but because 230V is needed for some of the controls that drive it. Or someone might include a N just to allow for future changes. Even if it's not used, the fact it's there means it could be involved in a fault.


(I can't say I recognise your formulas though - is there an "S" missing from the last one?)


    - Andy.

You do not normally say 'neutral distributed' in the case of single phase circuit, but of course it more or less always is, except  in the cases like centre tapped 110V building site supplies which are really 55-0-55, and you sort of have 2 live conductors relative to earth.


In a 3 phase system it gets more complex, you may have a 2 wire 400V load - like floodlights on a large sports facility where volt drop on 230 would be undesirable, then the neutral may well be distributed part way round the wiring for the odd 13A socket or other low load, even though it is not taken all the way to, nor is used by, the main load.

Things like delta connected motors and transformers do not need a neutral so 3 lives and an earth may be all that is provided, that is another example of  'non distributed' past a certain point.


The question really hinges on credible fault conditions - phase 1 to phase 2 or one phase to ground or neutral, or indeed the 3 phases bolted fault.

It is really important to be clear which you are considering, and which you are testing directly and which you are just deducing from the test readings. Your meter may do 3 phase to neutral tests, and estimate a phase to phase fault figure for example.

mike,

The question asks for the minimum short circuit current.  If we use the BS7671 definition of short circuit that implies that we are dealing with faults between live conductors.


For a 3 wire 3 phase system the minimum short circuit current is produced by a line to line fault (2 phase fault) and for a 3 phase 4 wire system the minimum short circuit current is produced by a line to neutral fault.


The equations are basically just I = V/R.


The numerators are the same apart from the value of  voltage i.e Ur = Rated voltage line to line and U0 = line to earth or in this case line to neutral.

The 0.8 maybe something to do with voltage regulation and the K terms are generally correction factors for impedance (probably referring to the source)  (in IEC 60909 terms).


The denominators represent an approximation of line impedance.  For the first case this is line to line and for the second it is line to neutral.


p.2L/S and p.(1 + m).L/S are both dc resistance (resistivity formula) and the 1.5 is probably a factor to approximate impedance Z (for cables over 300mm^2 1.5 is in the ball park)


p.(1 + m).L/S probably allows for reduced neutrals i.e if the neutral is the same size as the line m = 1.


Regards

Geoff Blackwell

I tend to think that if the breaker is of current limiting type / fuses, the maximum length of the cable need not be verified. Cable size based on let-through energy is not dependent on the length of the cable. Indeed as the fault location is further away from the breaker, the let-through energy decreases which improves the situation.