how to perform Short Circuit Calculations 2

[Roboteck]

I’m trying to learn how to perform the Short Circuit Calculations and how to calculate the Short Circuit current rating so my panels will meet the NEC2005 article 409.
Does anybody know of a book/class or even a self teaching method that I can follow?
Please help.

Thanks

 

[xnuke]

From what I understand, unless the contents of the panel are a listed/labeled single assembly, you basically need to follow UL 508A, Supplement SB, as referred to in the FPN in 409.110(3).

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rbulsara]

For proper learning: IEEE standard 242 or the Buff Book.
and IEEE "Red Book"

Visit

http://www.ieee.org.

Once you know the basics, you can invest in a software program.

For free and half-hearted information, (whicn could confuse you more than help) you can google search the web for the topic.

Remember at some point you need to work with experienced electrical engineer in the subject to understand the nuances of short circuit calculations.

 

[Roboteck]

Dear xnuke,
what does "listed/labeled single assembly" mean?

Thaks

 

[xnuke]

A listed/labeled single assembly is any assembly that comes listed or labeled by a nationally-recognized testing lab (NRTL), e.g., UL, or someone authorized to list or label it under the authority of a NRTL. It may be as simple as a single listed/labeled component, or a group of components that come from the manufacturer pre-assembled and listed/labeled. Either way, the short-circuit current rating (SCCR) must be provided for this component or assembly.

For example, on a retrofit, assume the functionality of a control panel needs to be changed and all of its components are mounted on a backpanel. The backpanel may be removed with all of its components on it. Then, a panel shop can construct what is known as an open industrial control panel by mounting components on a new backpanel. If they are a UL panel shop, they can label the backpanel alone as an open industrial control panel - thus making it a listed/labeled single assembly - and they must provide the short-circuit current rating. It can then be installed in the empty enclosure in the field.

In my opinion, Article 409 will make it a little bit more work to make field modifications to control panels for those unfamiliar with calculating the SCCR, because every time a change is made, the SCCR will have to be reevaluated and the cabinet may have to have its label changed.

Does that answer your question?

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

I don't think that the SCCR for a panel is something that you calculate. I believe that it is a standard that you must design to.
That is, if all the components are rated for an SCC of 25Ka, then the panel under consideration may be safely connected to a system which supplies 25Ka or less. If any component is changed to a component with an SCC rating of 10Ka, then the panel under consideration is safe for use only on systems which cannot supply over 10Ka.

This is a simplified explanation. Components of lower ratings may be used if they are protected so that the current reaching them in the event of a short circuit is less than their SCC rating, and the available SCC may be limited.
An example may be an instance where the available fault current at the transformer is 30 Ka and the panel under consideration is rated for 25Ka.
I would then consider the impedance of the feeders and calculate the available fault current at the service or distribution panel.
Still over 25Ka? Calculate the impedance of the feeder to the panel under consideration and determine the available fault current at the equipment.
Still too high? From the last calculation, estimate how much extra cable will be required to bring the available fault current at the panel under consideration down to 25Ka. Consider running the cable by a longer route.
Is the available fault current at the panel under consideration still too high. Install a dry type transformer with an impedance voltage such that the available fault current is below the rating of the panel under consideration
Alternatively, consider replacing components within the panel under consideration, or installing current limiting fuses.

The panel builders rating is the maximum safe rating.
Feeding the panel safely and calculating the available fault current from the supply system is the responsibility of the owner or installer.
Consider that if you are rating your equipment for 10ka and your competitors are rating their panels for 25Ka, any potential customer who realizes that he can connect one panel directly to his system, but the other panel will need a more expensive installation wil probably go with the higher rated panel.
respectfully

 

[BJC]

A short course on Short Circuit Currents. A pretty good one to.

http://www.geindustrial.com/products/applications/GET-3550F.pdf

 

[mc5w]

The older Bussman and Shawmut catalogs had a back section for calculating short circuit currents. There is utility company power transformer contribution plus motor contribution plus capacitor contribution plus any local generation.

For any transformer that is operating off of 11,500 volts delta or 7,200Y12,470 volts of higher it is reasonable that the primary side of the transformer has infinite available short circuit current and only the impedance of the transformer controls the level of short circuit current. For these primary voltages and higher a secondary circuit is seen by the primary system as just another load. If the primary system is to have good voltage regulation particularly when motors start it needs to have a very stiff impedance. At the terminals of a single phase 3-wire secondary the phase to center tap ( neutral ) is 1.5 times the phase to phase short circuit current. At the terminals of a wye secondary transformer the phase to neutral current is the same as the phase to phase short circuit current. For a 3-wire delta secondary single phase short circuit current and 3 phase short circuit current are a little bit different but not enough that it matters much because you need to add in a safety factor. Oil filled distribution transformers are usually 1.8%, 2.2%, or 5.4% impedance. You would divide the motor full load current by percent impedance to get absolute maximum short circuit current.

You next would add in the impedance of downstream wiring. Since the impedance of a transformer is mostly inductive and the impedance of the wiring is mostly resistive straight addition does not work but there are some formulas ( an equation that you use but do not understand ) that assumes that the resistance of the transformer is around 1%.

Motor contribution is based on the matter that when a short circuit occurs energy stored in the magnetic field of the motor dumps back into the system because of the voltage collapse. This will never exceed motor short circuit current. National Electrical Code 240.86 limits the full load ampereage of motors connected to a single bus to 1% of the short circuit rating of branch circuit breakers and fuses so that series connected ratings for class J fuses ahead of circuit breakers can use a standard design criteria. NEC 240.86 should be applied to panelboards that do not have a series connected rating that is being used.

To some extent, if a short circuit is not that solid induction motors can temporarily act as induction generators and contribute more than 1/2 of a cycle to the fault current.

Similarly, capacitors contain stored energy that dumps back into the system when a short circuit collapses the voltage.

I will separately add a comment next posting.

Mike Cole

 

[mc5w]

Also, National Electrical Code 110.9 does not prohibit equipment from having a greater short circuit rating than the available short circuit current.

Forget about how textbooks state that the first half cycle of a short circuit. Most short circuits do not become solidly bolted until some arc welding or spot welding has occured which means that peak current does not occur until 1.5 to 20 cycles have passed. The asymetrical peak can also theoretically occur in a circuit that is second order ( resistance + inductance + capacitance ) but that would usually be masked by the welding effect.

A 1/2 or 1 cycle pickup and clear circuit breaker such as General Electric or SquareD molded case branch circuit breakers will not allow as much damage as a 2 or 3 cycle clear time circuit breaker such as ITE Seimens.

I have encountered an instance where a circuit breakers rated for 5,000 amps of short circuit current was ruptured by the current from a 15 KVA oil filled transformer that was located 2 overhead spans of wire ( about 150 feet ) from the electrical service equipment. One way for a circuit breakers to rupture at substantially less than its short circuit rating is that some circuit breakers develop arthritis after a few years with the result that the breaker responds rather sluggishly to the sort circuit. Even SquareD, which is the best in the business, says to exercise a circuit ( turning it off and on ) once per year. I have also had the experience of replacing and outdoor outlet quad that was supplied by a Federal Pacific 20 amp breaker and turning it off and back on restored normal circuit breaker function. The customer then finds out that they have 30 or 35 amps of engine block heaters on a single 20 amp circuit. I then have to run a second branch circuit through the same conduit and then get paid for it.

Mike Cole