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 Post subject: SHORT CIRCUIT STUDIES
PostPosted: Tue Nov 12, 2019 6:39 am 

Joined: Sat Aug 03, 2019 5:33 am
Posts: 2
WHICH kA SHOULD I CONSIDER FOR GIVING INPUT DATA TO PROTECTIVE DEVICE IN SKM SOFTWARE FROM FOLLOWING DATA. CLIENT OPERATING VOLTAGE IS 0.433KV BUT IN THE FOLLOWING DATA KA IS NOT THERE FOR 0.433KV OR 433V?


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 Post subject: Re: SHORT CIRCUIT STUDIES
PostPosted: Tue Nov 12, 2019 3:19 pm 
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Joined: Wed May 07, 2008 5:00 pm
Posts: 819
Location: Rutland, VT
The chart provided looks like a chart for equipment ratings not the available short circuit fault current needed from the utility. Did you request this from the utility?

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 Post subject: Re: SHORT CIRCUIT STUDIES
PostPosted: Mon Nov 25, 2019 7:53 am 

Joined: Wed Jul 18, 2018 12:35 am
Posts: 3
Location: UK
In the UK 415V transformers are specified as 11000/433 where 433V is the off-load voltage and 415V is the nominal system voltage (Un) therefore you should look for 415V ratings.

Once rated kVA load is applied the transformer impedance will regulation will give 415V on load.


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 Post subject: Re: SHORT CIRCUIT STUDIES
PostPosted: Mon Nov 25, 2019 2:11 pm 

Joined: Wed Jun 29, 2011 3:33 pm
Posts: 14
cflatters wrote:
In the UK 415V transformers are specified as 11000/433 where 433V is the off-load voltage and 415V is the nominal system voltage (Un) therefore you should look for 415V ratings.

Once rated kVA load is applied the transformer impedance will regulation will give 415V on load.


Similar in Aus, although the 415V supply level is now 400V to line up internationally.

The 433V secondary on transformers is common to allow for voltage drop at the end of long cables. Use the 415V ratings in your table.


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 Post subject: Re: SHORT CIRCUIT STUDIES
PostPosted: Mon Dec 02, 2019 9:26 am 

Joined: Tue Oct 29, 2019 6:23 am
Posts: 19
AF calculations are not all that exact. I would suggest that different inputs that result in a small Ei variation are not particularly important. However,s mall variations in Iarc my be important if they cause protection to operate radically different! Particularly if the lower Iarc is less than a specific fast protection threshold such as instantaneous protection.

As far as system voltage, not aware if your system has automatic voltage regulation but your voltage could vary, as others indicated, based on load, time of day, etc.

Assuming you are conservative about PPE assignations, i.e. for calculated 7.999 calories you go to 10 or 12 calorie protection, not 8. I would suggest calculating at the lowest V so you get the lowest possible Iarc and ensure protection is properly set for those lower arcing currents. If you wish to be thorough calculate at the higher V as well.

Also when estimating gaps, always use the largest practical gap for any one exposure. That is more conservative as it gives you lower Iarc and higher Ei/time. I did a small comparison to test some assumptions. If you calculated Iarc at 415V/36mm you get a lower arcing current than at 430V/32mm... that is obvious. But the EI for the two calculations at 18" WD and 100mS protection is essentially the same!

If it worked I added some graphs. Otherwise you may contact me at


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Last edited by wbd on Tue Dec 03, 2019 6:08 am, edited 1 time in total.
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