We have an Industrial plant with primery voltage 33KV and secondry 6KV and
th-ry voltage 400V .
I want to calculate short circuit current.
It means that i should to know equal Impedance of all circuits.
all of consumers loads totaly is 4MVA.
i Want an Algorythm or program or sample sheet calculation.
You need to know more about the transformer. They typically include
synchronous reactance, as well as transient and sub-transient reactances.
And the line feeding the transformer primary.
This really is an involved calculation, it is best to hire a pro rather than
get advice on USENET for this.
Ah, you got me. I slipped up. I was thinking he had an industrial
generator feeding a transformer (I work mostly with power plants, and that
is the common configuration we work with for such calcs). Then the
generator is feeding the fault through the transformer.
But as I look again, I guess he's talking about a primary line feeding a
substation into his 'industrial plant'. The other factor he would need then
is the line impedance and its short-circuit capability.
The impedance of the short circuit itself is zero. But this would mean
you have an infinitely high short circuit current, which is of course
You still have the impedance of the power lines, the transformer,...
Take these into account, and then you can calculate the short circuit
current by dividing the voltage by the total impedance.
True, but in fault analysis and when determining interrupting rating of
protective devices, generally only the source impedance and intervening
lines are used, the fault is assumed 'bolted' with zero impedance. Slightly
conservative, but there you are.
True. We use bolted faults because we can't know what will actiually occur.
I was just taking exception to the statements that the short-circuit
impedance is zero. A more accurate statement is "we assume for calculations
that it is zero".
In customer fault level calculations here we are generally required to
assume that the Utility impedance is zero as well. Plus we are forced to
use low impedance assumptions for utility transformers. I wonder if they
will lighten up for calculating arc flash hazards when that becomes
mandatory. I don't think I'll hold my breath.
I recall a distributor mentioning a new product they have, high speed
relaying or something solid state, that they use for tough arc flash
applications. He used 600V MCCs as the example. They knocked 100cal/cm^2
down to 8cal/cm^2 in one instance.
So far I haven't had to do any arc flash calculations. What are the
dominant factors in a calculation? I? I^2t? SCMVA? V? Got any advice to
share for situations that look like a high cal/cm^2 value is likely?
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