Simplified Short-Circuit Calculations
Thomas Mason, PE
The course contains front-matter which superficially discusses the content, the intended audience and internet locations for follow-up pursuit of the topic.
The core content is made up of two .pdf documents. The first is a complete copy of the look-up tables for determining available short-circuit current on a 480V system. There are 15 tables for 15 common transformers, 8 columns on each table for 8 common distribution feeders and 10 entries in each row, for 10 reasonable feeder lengths. Interpolation is discussed and demonstrated in the second document.
The second .pdf document is a guide for use of the tables. It is a summary of the guides provided with the IEEE spreadsheet program (IEEE.com) and the guides provided with commercial system modeling software and courses in short-circuit calculation.
The course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.
At the conclusion of this course the student will be able to:
This course is intended for practicing design engineers who must select electric power equipment early in the construction project. It is intended for Architects and design supervisors who perform a reasonableness-test on finished designs. It is intended for Construction Managers and Field Inspectors who wish to avoid unpleasant surprises during start-up. It is intended for Contractors who wish to avoid delays by use of intelligent requests-for-information.
Benefit to Attendees
The primary benefit to the audience is that the method presented makes it easy to comply with the National Electrical Code and Occupational Safety and Health Act.
The secondary benefit is that the guidelines included help everyone on the project do their jobs better, with more confidence.
The National Electrical Code requires “field marking to warn qualified persons of potential arc flash hazards.” (2008 NEC 110.16) Note that it does not require the design engineer, with the system one-line diagram and software tools to compute the arc-flash magnitude. It requires the installer, with a screwdriver and pliers to apply a label.
The almost-universal interpretation is for the designer to include a specification for the Contractor to perform and submit the arc-flash calculations, prepare the labels and attach them. When it happens, the Contractor hires an outside engineer to do the work. Usually, it doesn’t happen.
This course offers a simplified first step towards the arc-flash calculations. Several vendors offer no-charge on-line computers for arc-flash, IF THE AVAILABLE SHORT-CIRCUIT CURRENT IS KNOWN. (us.ferrazshawmut.com/arcflash)
There is an apparent ambiguity in that the NEC does not require numerical values. OSHA does. (CFR 1910.132) Regardless of this fine point, the specification to the Contractor is to do the calculations and provide the detailed arc-flash label with numbers.
In this lesson, you are required to download and study the following course contents in PDF format:
Please click on the above underlined hypertext to view, download or print the document for your study. Because of the large file size, we recommend that you first save the file to your computer by right clicking the mouse and choosing "Save Target As ...", and then open the file in Adobe Acrobat Reader. If you still experience any difficulty in downloading or opening this file, you may need to close some applications or reboot your computer to free up some memory.
This course offered a simplified method of calculating available short-circuit current on a commercial or industrial power distribution system. This calculation is required by the National Electric Code (2008 NEC 110.9, Interrupting Rating). It is also required by OSHA in order to prepare a detailed Arc-Flash warning label with j/sq-cm and personal protective equipment level.
The course was a set of look-up tables prepared with commercial electrical system modeling software and accompanied by coaching on how to use the tables correctly.
A multiple-choice quiz was included to provide confidence in the learning achieved.
Related Links and References
For additional technical information related to this subject, please visit the following websites or web pages:
2008 NEC 110.9, Interrupting Rating - http://www.nfpa.com
IEEE 1584 Arc Flash - ieee.com - The spreadsheet used to be available from multiple sources on the internet, but they have disappeared. IEEE offers it for $29. GM has a derived version of the spreadsheet that they give to contractors for free. It includes special GM electrical equipment data. There are ~10 interacting pages of the spreadsheet to produce a single label.
Detailed arc-flash calculator, if you know the SCA - http://www.us.ferrazshawmut.com/arcflash
Publishing software that fought me every step of the way on this course
Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.