It is currently Wed Jan 26, 2022 10:04 pm

Author Message

 Post subject: How good is the MVA method for short circuit calculations?Posted: Fri Sep 06, 2013 11:38 pm

Joined: Mon Oct 08, 2012 2:59 am
Posts: 24
Location: India
I have done a few sample calculations (by hand) of MVA method for short circuit calculations and verified the same with the results with ANSI and IEC 60909 SC calculations in ETAP. I have observed that the MVA method is coming very close to ANSI calculations (error ~ 1% - acceptable error). But the error in IEC calculations is exceeding 1%.
I am interested in IEC method because of local regulations.

The question is - "Is MVA method of short circuit calculations for symmetrical bolted fault current acceptable/good enough for arc flash hazard calculations?"

Top

 Post subject: Posted: Fri Feb 14, 2014 8:04 am
 Sparks Level

Joined: Wed Jun 10, 2009 5:00 pm
Posts: 267
Location: Toronto
The MVA method is a modification of the Ohmic method where the impedance of a circuit equals the sum of the impedances of components constituting the circuit. Using the admittances, it follows that the reciprocal of the system impedance is the sum of the reciprocals of the admittances of the components. By very definition, the circuit component admittance is the maximum current or KVA at unit voltage which would flow through the circuit or component to a short circuit or fault when supplied from a source of infinite capacity. In practice, the MVA method is used by separating the circuit into components and calculating each component with its own infinite bus.

The disparity between calculation results from ANSI, IEC comprehensive method etc. can be explained by different approximations and assumptions adopted by different methodologies. For example, some techniques ignore equipment X/R ration in the analysis and apply different correction factors to equipment ratings. Disregarding the equipment X/R ratio introduces up to 15% uncertainty in a single step of adding two impedances alone while applying different correction factors does not quantifies and does not corrects the error but only amplifies it.

Both Ohmic and MVA methods are reliable and produce accurate short circuit currents as long as the impedance for equipment (source, transformers, cables etc.) within the electrical system is broken down into X and R components, the components are grouped together and impedance between the source and the fault is calculated using the formula Z = (R^2+X^2)^0.5

_________________
Michael Furtak, C.E.T.

Top

 Display posts from previous: All posts1 day7 days2 weeks1 month3 months6 months1 year Sort by AuthorPost timeSubject AscendingDescending
 Page 1 of 1 [ 2 posts ]

 All times are UTC - 7 hours

 You cannot post new topics in this forumYou cannot reply to topics in this forumYou cannot edit your posts in this forumYou cannot delete your posts in this forumYou cannot post attachments in this forum

 Jump to:  Select a forum ------------------ Forum Library / Articles The Lounge    Question of the Week - What Do You Think?    Arcflashforum.com Feedback and Announcements    Off Topic Discussions    News in Electrical Safety Arc Flash and Electrical Safety    General Discussion    Electrical Safety Practices    Equipment to Reduce Arc Flash Dangers    Personal Protective Equipment (PPE) Arc Flash Studies    General Discussion    Arc Flash Labels    Software for Arc Flash Studies    System Modeling and Calculations    NEW! Electrode Configuration Library – 2018 IEEE 1584 Codes and Standards    CSA Z462 Workplace Electrical Safety    EAWR Electricity at Work Regulations, HSE - Europe    OSHA CFR Title 29    IEEE 1584 - Arc-Flash Hazard Calculations    NFPA 70 - National Electrical Code - NEC (R)    NESC - ANSI C2 - National Electrical Safety Code    NFPA 70E - Electrical Safety in the Workplace    2015 NFPA 70E Share It Here    Arc Flash Photos    Your Stories    What's Wrong Here? by Joe Tedesco
© 2022 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883