Short Circuit Current at Transformer Calculation

Everyone seems to have a different answer, or no answer.

Here's my answer assuming
3 phase and no offset current

I=1000000/(415X3^1/2)=1391 amp rated

Isc=1391/0.06=23,190 amp

What i learnt is school about '5% impedance' means that: with the secondary shorted, one requires 5% of the rated voltage at the primary to pass rated current through the secondary.

Here is the calculation :

Guest,

Your question is fair, and understandable for when a person is first learning about electricity. The answer you have received is correct. A transformer is capable of delivering its nameplate current forever. In doing its work, it is storing and transforming the incoming power in a magnetic field to convert the voltage from the primary to the secondary voltage. If you short out the secondary, either at the terminals of the transformer or further downstream, the impedance (AC resistance) of the load circuit is greatly reduced. This stored energy now is being output very quickly into the resulting short, and it results in a much higher current being possible.

Every electrical wire has a current rating for how much it can carry continuously in a safe manner. As the current is raised above this level, the wire will overheat--initially destroying the insulation (as almost every electrician has seen at one time or another), and eventually so hot it can melt. At very high current levels, this melting can occur in milliseconds or less. Then, as the current is flowing where the wire was, vaporized particles of the metal form a conductive pathway, called plasma. The temperature of this plasma can be as high as 30,000 °C. Because of the high current, this arc also has a very strong magnetic field. The vaporization of the metal and the strong magnetic field combine into a violent explosion. This has injured and killed many unfortunate electricians and maintenance workers, and is the reason for use of protective clothing and safety distances.

If the equipment is designed so it includes devices that can safely interrupt this high current quickly, the energy liberated in this arc can be reduced to a level where it does little or even no harm to the surrounding equipment. If not, it can blow heavy sheet metal enclosures apart, lift people into the air as they are blown back as much as 3-10 meters, and light fires.

Take a simple circuit breaker, and look at the back end of it--you will see some slots. When the circuit breaker is switched off, the arc between the contacts is forced out through these slots, which serve to separate the arc until it can quench itself (by being cooled and lengthened, as well as the time lapse until the AC voltage crosses through zero). This design is capable of interrupting a current much higher than the breaker's labeled "size". In the USA, a 20A (or 15A, 40A, etc.) breaker must be able to safely interrupt a short circuit of 5000A. If it is exposed to a short circuit current that is higher, it will probably be unable to quench this arc, and will fly apart--if the current is much higher, this will be very violent as described above (an arc that has not been safely interrupted becomes a ground-fault arc and/or a phase-phase fault, and can vaporize multiple sections of switchgear in seconds--I have seen the results of this!). For more money, you can buy and install breakers capable of interrupting 10,000A, or 22,000A, and more; you can buy fuses capable of interrupting as much as 300,000A.

Engineers, code enforcement personnel, electricians, and many others need to know that the electrical equipment is chosen to have components that can safely interrupt the available fault current--no explosion, no fire, no equipment damage. Therefore, calculations are done to determine what the available short circuit fault current is, at the main equipment, and further downstream (if it is over 5000A or sometimes 10,000A). After these calculations are done, then the proper equipment can be selected for the wires, wireways, breakers, fuses, motor controllers, and everything else.

Its just not nice when things go boom.

--JMM