IEC60909 vs AS3851 in Australia

[healyx]

Hi,

Does anyone know where it is stated that it is OK to use IEC60909 to conduct short circuit analysis instead of AS3851 in Australia? I haven't found a software program that states compliance with AS3851 but have found plenty that do IEC60909, and these programs appear to be in use in Australia. I have also found software packages that comply with CENELEC R064-003 (NFC15-500), which I think is based on IEC60909? (can anyone confirm?). I have a copy of NFC15-500 (in french) and AS3851 and they seem to be quite different.

Most notable difference is the cable resistence used for minimum short circuit calc. AS3851 states that resistence should be 1.5x the 20degC resistence value (regardless of device/cable type), whereas NFC15-500 values vary for cable type and device type and are generally much lower (eg. 1.22x 20degC resistence for PVC). It looks like AS3851 will give far more conservative values so back to my main question - does anyone know where it officially says it is OK to use IEC60909 or similar in Australia?

 

[dpc]

I don't know anything about the Australian standard and not that much about IEC, but generally, you need to use the method that the equipment short circuit ratings are based on, not the country where the project is.

If the circuit breaker short circuit rating is based on IEC standards, the IEC method should be used.

As a side note, the differences in the various methods should not be too great. I also find it hard to imagine that Australia would have a standard method for calculating short circuit currents that varies greatly from IEC methods.

David Castor

http://www.cvoes.com

 

[7anoter4]

I know something about IEC60909, I don't know what's in the above Australian standard, but I agree with dpc.
Usually a cable presents low impedance with respect the other elements of the circuit as transformer or motors. But if there is a big difference between results for maximum short circuit you may take the rated temperature [for PVC 70oC] and for minimum short-circuit current the maximum permitted for short-time [for PVC 140oC].So for first case ktemp=(1+(70-20)*.004)=1.2 and for second (1+(140-20)*.004)=1.48.
By the way, as I understand NFC 15-500 is "harmonized" with IEC 60364 Electrical Installations for Buildings [in a way
similar to IEEE Std 241 [Graybook].

 

[healyx]

Thanks Guys.

I'll try and get my hands on IEC60909 to see what the differences are to AS3851.

7anoter4 - I pretty much work exclusively in LV installations, so the cables are the main contributer to different fault levels. In every standard I have seen, the maximum short circuit is taken at 20oC, it is just a question about minimum short circuit temp. NFC 15-500 states 70oC for breakers, and higher for fuses. AS3851 just states 1.5xR20. Not sure what IEC60909 says. Are you saying NFC 15-500 is specifically for buildings?

 

[7anoter4]

Correction: IEC 60909-0 takes for calculation of maximum short-circuit current the cable conductor resistance at 20oC.

 

[sibeen]

I've never seen where it is implicitly stated that you can use IEC60909 in calculations but it must be remembered that AS3851 is now a 20 year old document and was promulgated well before the Australian Standards were aligned closely with the equivalent IEC documents.

I'd be stunned if the new version of the AS document, whenever it's released, isn't basically a rewrite of IEC60909. If it came down to someone questioning my calculation methods I'd be claiming "world's best practice" and not a 20 year old document.

 

[7anoter4]

See also :

http://webfiles.portal.chalmers.se/et/MSc/NilssonMikaelMSc.pdf

 

[EddyWirbelstrom]

I have only found the following cases where AS 3851 is not a rewrite of IEC 60909-0. Is anyone aware of any other differences ?

Min Short-Circuit Current
AS3851 section 5.3 – For min short-circuit currents use RL of cables and OH lines of RL = 1.5*RL20
IEC 60909-0 section 2.5 – Use max permissible conduction temperature at the end of short-circuit duration.

Short-Circuits in Meshed Networks
AS3851 section 8.5.2.2 (b) – The peak short-circuit current is multiplied by 1.15.
IEC60909-0 section 4.3.1.2 (b) - The peak short-circuit current is multiplied by 1.15, however
“as long as R/X remains smaller than 0.3 in all branches it is not necessary to use the factor 1.15”.

Transformer Impedance Correction Factors
AS3851 – not found in AS3851
IEC60909-0 – Section 3.3.3

Synchronous Generators
AS3851 section 7.8.1 – Stator resistance RG
IEC60909-0 section 3.6.1 – Stator Fictitious resistance RGf
Comments – Both standards recommend the same value for stator resistance, however only IEC60909-0 mentions that the resistance is “fictitious” and should only be used for calculation of peak short-circuit current. The actual value of stator resistance – which is lower than RGf – should be used when calculating the dc component of short-circuit current.

Voltage Factors (c )
Small differences in c factor between the two standards.

 

[healyx]

Yes EddyW, despite its age AS3851 looks to be pretty close to IEC60909-0. I had noted your first and last points. I would use the IEC recommendation for max conductor temp (far more accurate) and the Australian c factors (more conservative).
I haven't really used the other things you noted - everything I do is LV, so I have been using more simplified methods for determining fault current.