Arc Fault Duration

[pwolpert]

While calculating the arc fault on the secondary of a 150 kva transformer, I'm challenged with calculating the time duration until the fault will clear.

Scenario: Utility supplies 21,000 AIC , 208, 3 ? to the customers 400 amp fused disconnect. The time current curve for these fuses will clear at .01 seconds at 11,000 amps. Ten feet of Conduit to a 150 kva transformer.
208 Y to a 240 ▲ Z=4.8% The Secondary of the transformer is in conduit for about ten feet to a breaker.

Question: How to determine the clearing time of the upstream fuses (400 amp @ the main Disconnect) when a fault occurs on the secondary of the transformer, before the down stream breaker comes into play? Note: the calculated available fault current on the secondary of the transformer is about 8,600 or so. Will this available fault current produce enough current on the primary to open the main fuses in a timely matter?

Thanks

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[jim dungar]

? Note: the calculated available fault current on the secondary of the transformer is about 8,600 or so. Will this available fault current produce enough current on the primary to open the main fuses in a timely matter?

The transformer behaves the same whether it transformers load current or fault current.

The primary will see a balanced three phase secondary fault using the same ratio as it does any other secondary current.

 

[bob]

Scenario: Utility supplies 21,000 AIC , 208, 3 ? to the customers 400 amp fused disconnect. The time current curve for these fuses will clear at .01 seconds at 11,000 amps. Ten feet of Conduit to a 150 kva transformer.
208 Y to a 240 ▲ Z=4.8% The Secondary of the transformer is in conduit for about ten feet to a breaker.

Question: How to determine the clearing time of the upstream fuses (400 amp @ the main Disconnect) when a fault occurs on the secondary of the transformer .

You use the same curves you used to get the time 0.01 seconds at 11000 amps. Check the time for 8600 amps.

 

[charlie]

Question: How to determine the clearing time of the upstream fuses when a fault occurs on the secondary of the transformer . . .

This thread sort of scares me with the original question. It appears that you are looking at selecting PPE with your calculations. If you are indeed selecting PPE, you need to be in contact with the serving electric utility to verify the fuse size in the transformer, the amount of available fault current on the primary side of their transformer, and the transformer impedance. Find out if they are even sure which substation the transformer is served from in order to calculate the available primary fault current. Most electric utilities have a dynamic system and will not guarantee that the primary feed will remain static. Also, be aware that if the amount of available fault current is lower than expected, the incident energy could be much higher due to the amount of time it takes for the fuse to let go!

 

[Lxnxjxhx]

-Transient Analysis is difficult, even if you ignore all the small resistances, inductances and parasitic capacitances. It's not the same as steady-state analysis.
-At these power levels, if you ignore the wrong things the results may be spectacular and lethal.
-When a fuse clears, it draws an arc. An arc is "plasma", a so-called fourth state of matter. It's very difficult to model an arc mathematically. I saw one model that was actually a tuned circuit, with an R, an L and a C.
-I'd look for a book or software on this, if you can find them. The power companies do this all the time, so I'd start with them.

 

[pwolpert]

Thanks All !!!! You have been very helpful. The 21,000 AIC is from the power company as per their calculations.

Thanks

 

[jim dungar]

The 21,000 AIC is from the power company as per their calculations.

AIC means Amps Interrupting Capacity it is a term that is only applied to devices that "break" current.

The utility provides available Short Circuit Amps or SCA.

Per 110.9 the AIC of a device must be equal to or greater than the SCA at its line side.

edit: corrected acronym to SCA

 

[charlie]

The 21,000 AIC is from the power company as per their calculations.

Be careful! The 21,000 amperes is probably the worst case for sizing equipment and overcurrent protection. It will not work for PPE!