The 2020 Edition of the National Electrical Code and the 2021 Edition of NFPA 70E define the term available fault current as: “The largest amount of current capable of being delivered at a point on the system during a short-circuit condition.”
The terms “fault current” and “short circuit current” are interchangeable, and both terms are used depending on which standard is being referenced. The NEC and NFPA 70E use fault current, and calculation standards such as IEEE 3002.3-2018 and IEC 60909 use short circuit current.
The calculations are performed to predict how much current could flow to electrical equipment under evaluation, such as panelboards, motor control centers, switchboards and similar equipment, to determine if the equipment has an adequate interrupting or withstand rating. If it does not have an adequate rating, the equipment is considered to be overdutied. Short circuit calculations are more important than ever because the results are also used as part of an arc flash study.
Although the calculations may seem complicated, they are really just an application of Ohm’s Law—as in I = V/Z, where I is the current in amperes, V is the system voltage and Z is the impedance. I will discuss other details in subsequent columns. Fortunately, power system software makes performing the study much easier—as long as you know what you are doing. Otherwise it will be the proverbial “garbage in, garbage out.”
Begin at the beginning
Short circuit studies can require a staggering amount of data. One of the first steps should be to contact the electric utility company to determine how much short circuit current they have available—typically at the service entrance. A simple way of thinking is this: If the dam breaks, how big is the flood?
Another important part of the study is to create or update the facility’s one-line drawing. This provides the road map of the electrical system. The impedance data is at the heart of the study and determining the impedance of transformers and conductors is very important.
Bolted short circuit current
Section 110.9 of the NEC states: “Interrupting Rating: Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the current that is available at the line terminals of the equipment.”
This requires verifying the equipment can adequately interrupt or withstand the available fault current. The equipment in the study has an upper limit of how much current it can interrupt or withstand that is defined by standards such as UL and ANSI. If the available fault current exceeds the interrupting or withstand rating, the equipment could fail, sometimes catastrophically.
Short circuit studies must be based on an appropriate standard such as “IEEE 3002.3-2018 - IEEE Recommended Practice for Conducting Short-Circuit Studies and Analysis of Industrial and Commercial Power Systems.” The calculations are based on a “bolted” fault, which means the short circuit current has no additional impedance at the contact point. Only impedances such as the source, transformers and conductors are considered.
Arcing short circuit current
Beginning with the 2000 edition of NFPA 70E, Hazard Risk Category tables were introduced for use in selecting arc rated clothing and PPE. The tables contain notes that define the maximum available fault current where the PPE selection is valid. Why is short circuit current so important with PPE selection? Because short circuit current is one of two main variables used in calculating the severity of an arc flash known as incident energy. While there are many other secondary variables, the short circuit current defines the intensity or explosiveness of an arc flash.
The other important variable is the duration of the arc flash, defined by how long it takes an upstream overcurrent device to operate, which is also dependent on the available fault current.
When “IEEE 1584 - Guide for Performing Arc-Flash Hazard Calculations” was first published in 2002, it introduced a method for calculating the arcing fault current during an arc flash. The most recent 2018 edition has improved upon the first edition. When an arc flash occurs, the short circuit current jumps across an air gap between the conductors, which introduces additional impedance. This impedance reduces the current, so the arcing short circuit current will always be lower than the bolted current. This lower arcing current may result in an upstream overcurrent device taking longer to operate and creating a longer arc duration. This means the total incident energy exposure could be greater and more hazardous.
My next column will explore the details of short circuit studies.
About The Author
PHILLIPS, P.E., is founder of brainfiller.com and provides training globally. He is Vice-Chair of IEEE 1584 Arc Flash Working Group, International Chair of IEC TC78 Live Working Standards and Technical Committee Member of NFPA 70E. He can be reached at [email protected].
"Kama ilivyoelezwa katika NFPA 70E, ""mzunguko wa kosa"" ni ""kiasi kikubwa cha sasa kinachoweza kutolewa katika hatua fulani kwenye mfumo wakati wa hali ya mzunguko mfupi"" na ""mzunguko wa kosa"" ni maneno yanayoweza kubadilishwa, na maneno yote mawili hutumiwa kulingana na kiwango kinachohitajika." NEC na NFPA 70E hutumia mkondo wa kosa, na viwango vya hesabu kama vile IEEE 3002.3-2018 na IEC 60909 hutumia mkondo wa mzunguko mfupi. Hesabu hizi ni kutekelezwa kutabiri kiasi cha sasa inaweza mtiririko kwa vifaa vya umeme chini ya tathmini, kama vile paneli, vituo vya kudhibiti motor, switchboards na vifaa kama hivyo, ili kuamua kama vifaa ina kutosha kuvuruga au kuvumilia rating. Kama haina rating ya kutosha, vifaa ni kuchukuliwa kuwa overdutied. Shortscreen ni muhimu zaidi kuliko wakati mwingine wowote kwa sababu matokeo ya shortscreen ni muhimu kwa ajili ya utafiti wa arc flash. Ingawa hesabu inaweza kuonekana ngumu, ni kweli tu maombi ya Sheria ya Ohm kama katika I = VZ, ambapo I ni sasa katika amperes, V ni mfumo voltage na Z ni impedance. Nitazungumzia mambo mengine ya kina katika safu zilizofuata. Kwa bahati nzuri, programu ya mfumo wa umeme hufanya kufanya utafiti rahisi sana - mradi tu unajua unachofanya. Vinginevyo itakuwa msemo <unk> taka ndani, taka nje.<unk> Anza mwanzoni Short mzunguko masomo inaweza kuhitaji kiasi staggering ya data. Moja ya hatua za kwanza inapaswa kuwa kuwasiliana na kampuni ya huduma ya umeme ili kuamua ni kiasi gani cha mzunguko mfupi wa sasa wanaoweza - kwa kawaida kwenye mlango wa huduma. Njia rahisi ya kufikiri ni hii: Ikiwa bwawa huvunjika, mafuriko ni makubwa kadiri gani? Sehemu nyingine muhimu ya utafiti ni kuunda au kusasisha kituo cha moja-line kuchora. Hii hutoa ramani ya barabara ya mfumo wa umeme. Data impedance ni katika moyo wa utafiti na kuamua impedance ya transformers na conductors ni muhimu sana. "Kifungu cha 110.9 cha NEC kinasema: ""Mipango ya kuvunja: Vifaa vilivyopangwa kuvunja sasa katika viwango vya kosa lazima viwe na kiwango cha kuvunja kwa voltage ya mzunguko wa jina angalau sawa na sasa ambayo inapatikana kwenye viunganishi vya mstari wa vifaa.""" Vifaa vya utafiti vina kikomo cha juu cha kiasi cha sasa kinachoweza kuvunja au kuvumilia ambacho hufafanuliwa na viwango kama vile UL na ANSI. Kama sasa kosa inapatikana inazidi kuvunja au kuvumilia rating, vifaa inaweza kushindwa, wakati mwingine catastrophically. "IEEE 3002 - Mazoezi yaliyopendekezwa ya IEEE kwa Uendeshaji wa Mafunzo ya Mzunguko mfupi na Uchambuzi wa Mifumo ya Nguvu ya Viwanda na Biashara - ""Mzunguko mfupi"" ni mtiririko wa umeme wa umeme wa umeme wa umeme wa umeme wa umeme wa umeme wa umeme wa umeme wa umeme." Impedances tu kama vile chanzo, transformers na conductors ni kuchukuliwa. Kuanzia mwaka 2000 toleo la NFPA 70E, meza za Jamii ya Hatari za Hatari zilianzishwa kwa ajili ya matumizi katika kuchagua nguo na PPE zilizo na kiwango cha arc. Meza zina maelezo ambayo kufafanua kiwango cha juu inapatikana kosa sasa ambapo uteuzi PPE ni halali. Kwa nini umeme wa mzunguko mfupi ni muhimu sana katika uteuzi wa PPE? Mzunguko mfupi ni moja ya vigezo viwili vya msingi vinavyotumiwa kuhesabu uzito wa arc flash inayojulikana kama nishati ya tukio. Ingawa kuna vigezo vingine vingi vya pili, mkondo wa mzunguko mfupi hufafanua nguvu au mlipuko wa arc flash. Mabadiliko mengine muhimu ni muda wa arc flash, inayofafanuliwa na muda gani inachukua upstream overcurrent kifaa kufanya kazi, ambayo pia inategemea sasa kosa inapatikana. IEEE 1584 - Mwongozo wa Utendaji wa Arc-Flash Hazard Calculations (IEEE 1584 - Mwongozo wa Utendaji wa Arc-Flash Hazard Calculations) ni mfumo wa kiufundi wa umeme wa umeme wa umeme wa umeme wa umeme wa umeme. Toleo la hivi karibuni la 2018 limeboresha juu ya toleo la kwanza. Wakati arc flash hutokea, mkondo mfupi sasa kuruka katika pengo hewa kati ya conductors, ambayo inatoa impedance ziada. Hii impedance hupunguza sasa, hivyo arcing mkondo mfupi sasa itakuwa daima chini ya sasa bolted. Hii chini arcing sasa inaweza kusababisha upstream overcurrent kifaa kuchukua muda mrefu kufanya kazi na kujenga muda mrefu arc. Hii ina maana jumla ya tukio nishati mfiduo inaweza kuwa kubwa na hatari zaidi. Safu yangu inayofuata itachunguza maelezo ya masomo ya mzunguko mfupi. "Kuhusu mwandishi: PHILLIPS, P.E., ni mwanzilishi wa brainfiller.com na hutoa mafunzo ulimwenguni kote.""" Yeye ni Makamu Mwenyekiti wa IEEE 1584 Arc Flash Working Group, Mwenyekiti wa Kimataifa wa IEC TC78 Live Working Standards na Mwanachama wa Kamati ya Ufundi ya NFPA 70E. Anaweza kupatikana kwenye [email protected].