Online assistance for electrical trade people Login  |  Register  |   Forgot Password
 

 

 

 

 


Click here to send Ron a pdf document for publication on this Topic

Documents must be less than 200k in pdf format

Posted By Topic: AS/NZS 3008.1.1:2009 question

Rob
Mar 11 2013 15:48

I need to calculate the prospective fault current at the MSB for the below installation:
Supply Transformer 450kVA,230/400V,prospective fault current of Transformer 16kA.Distribution cable 70mm XLPE 4 core copper cable route length 35 metres.Consumer mains 25mm2 V75 mlticore sheathed cable copper cable route length 30 metres.
- To solve impedence of the transformer, i went Z=V/I, 230/14000.... this was wrong I am told. So tried with formula Fault(I)=kVAx1000/1.7321xVlxZ.....again told this value was wrong. Not sure how else I can do it.
- Same thing to find impedence of distribution cable got Xc & R from tables x length of cable. Z=Sqrt(Xc)^2+(R)^2... this came up wrong as well.
- Found some issues with finding table numbers and colum nos as well... I choose Table 30 column 10.
- Impedence of consumer mains = ?
- Table No. & Column No. = ?
- Finally prospective fault current at MSB (I think adding the Z together which I could not find and going I=230/Z I think ??
Can anyone please help.....thanking you in advance :-)

   

Rob
Mar 12 2013 22:00

I'm still learning this stuff but I'll have a crack.

Using cable temp 75 degrees and table 36 the AC resistance per km of the cables is 0.327 and 0.884 ohms for the XLPE and V75 respectively.

The total resistance of the cable from the transformer, through the consumer mains and back to the transformer is therefore:

2 x 0.327 x 35 / 1000 = 0.023 ohms, plus
2 x 0.884 x 30 / 1000 = 0.053 ohms, equals

Total AC resistance of cable is 0.076 ohms.

Impedance of the transformer under fault conditions is 230/14000 = 0.016 ohms.

So the impedance of the complete circuit under short circuit conditions is 0.092 ohms.

Sub this into Ohms law with a 230V supply and you get a prospective fault current at the MSB of around 2500A.

Could well be wrong of course. I haven't considered reactive and capacitive coupling between the conductors.
   

Rob
Mar 12 2013 22:36

Thanks for trying to help.
I think that you may have the wrong table and value EG: Dist. Cable 70mm2 4C Cu more likely to be circular (but could be shaped). T35 C5 Rc=0.343 ohm/km.
Did you do 2x value because they are 2 circuits meaning to & from the trany.
The other thing is to calculate cable impedence I was told that the formula was sqrtZ=sqrtR^2+Xc^2. Sorry I hope I am not confusing you.
   

Rob
Mar 12 2013 22:37

Thanks for trying to help.
I think that you may have the wrong table and value EG: Dist. Cable 70mm2 4C Cu more likely to be circular (but could be shaped). T35 C5 Rc=0.343 ohm/km.
Did you do 2x value because they are 2 circuits meaning to & from the trany.
The other thing is to calculate cable impedence I was told that the formula was sqrtZ=sqrtR^2+Xc^2. Sorry I hope I am not confusing you.
   

Donald
Mar 12 2013 22:43

If you are not competent to carry out this Calc, may i suggest that you engae the services of an electrical engineer to do it for you along with the DOC
   

Rob
Mar 12 2013 23:04

Thanks for trying to help.
I think that you may have the wrong table and value EG: Dist. Cable 70mm2 4C Cu more likely to be circular (but could be shaped). T35 C5 Rc=0.343 ohm/km.
Did you do 2x value because they are 2 circuits meaning to & from the trany.
The other thing is to calculate cable impedence I was told that the formula was sqrtZ=sqrtR^2+Xc^2. Sorry I hope I am not confusing you.
   

AlecK
Mar 13 2013 08:56

That calculation looks OK to me, though I admit I'm no expert at this - was never taught it and have never had to do it.

In practice, ask the network - usually a free service, and they are they only source of TX & distribution line data anyway, just as easy for them to run the calc as to provide the input data.

Reason you need this is to ensure main switch etc have adequate PSSC rating.

On the bigger jobs, make sure you get a certified design; new ESRs say we can rely on it.

Same for anything with a battery bank, even a small bank can have a big PSSC, and the switchgear has to have a good (DC) rating.
   

Rob
Mar 14 2013 00:58

Thanks Gents,

We will leave it here.
   

Rob
Mar 14 2013 01:01

Thanks Gents,

We will leave it here.
   

Rob
Mar 17 2013 21:16

Z=sqrt(R^2 + (Xl - Xc)^2)

As stated I've not considered inductance or capacitance in the cable so Z = R.