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How the study is calculated
Draw the system on the one-line, from the utility or battery to each load. The tool then works through a study in this order:
- Fault current at every point.
- Interrupting duty of each device.
- Coordination of the devices on each protection path.
- Incident energy and arc flash boundary at each equipment bus.
- One label for each location.
Each result can be traced to the equation and table below.
Examples
EV charging hub with battery storage, 480Y/277 V
The main has a maintenance switch, which shows how 240.87 lowers the bus incident energy. The utility fuse governs the line side.
Office building, 208Y/120 V
The reduced arcing current at 208 V falls below the main's short-time pickup.
Battery energy storage, 1500 V dc
Six fused racks each feed their share of a combiner arc.
Water well pump station, 480 V
Taken from a 2026 well station study. The Bay-O-Net fuse sets the switchboard line-side energy (14 cal/cm²); downstream buses stay below 1.2.
Booster pump station, 480 V
Taken from a 2026 booster station study: four 75 hp soft-start pumps. The 15 kVA lighting panel reaches 4 cal/cm² because its feeder takes 2 s to clear.
Substation battery, 125 V dc
The arc resistance method gives a lower arcing current, which does not reach the breaker's instantaneous pickup.
AC fault current
The method is ohmic per-unit on a 1000 kVA base, following IEEE 551. A bus impedance solution gives the fault current at every point and the share that passes through each device.
Sources and impedances
- Utility: Z1 = VLN/I3φ at the given X/R. Z0 = 3VLN/ILG − 2Z1.
- Transformer: Z = %Z/100 · V²/kVA, split by X/R. A delta–grounded-wye unit blocks zero sequence, so a secondary line-to-ground fault sees the transformer Z0 = Z1.
- Cables: R and X from NEC Chapter 9 Table 9 (75 °C, by conduit type), divided by the number of parallel sets. Choosing 25 °C instead scales R by (Tk+25)/(Tk+75). Zero sequence is Z1 + 3Zreturn, taking a return conductor the same size as the phase.
- Induction motors: X″ = 0.17 pu on the motor's kVA, about 6 times full-load current. Motors on a drive contribute nothing.
- Inverters (battery PCS, PV, bidirectional chargers): a constant current of k times rated, 1.2 by default. It is added in magnitude along the inverter's path to the fault.
Results at each point
Single-phase 120/240 V: the 240 V fault uses the full-winding impedance plus both line conductors. The 120 V line-to-neutral fault uses the half winding, 1.5R + j1.2X on the 120 V base, plus the line and neutral conductors. The primary loop impedance is (2Z1+Z0)/3 for a line-to-neutral connection and 2Z1 for line-to-line, reflected by the turns ratio.
Interrupting duty
Each device is checked against the fault at its line-side bus. When the fault X/R exceeds the device's test X/R, the duty is multiplied by the factor below. Molded- and insulated-case breakers use the peak basis; power breakers and fuses use the rms basis.
| Device | Test X/R | Basis |
|---|---|---|
| UL 489 breaker, rating 10 kA or less | 1.73 | Peak |
| UL 489 breaker, more than 10 kA up to 20 kA | 3.18 | Peak |
| UL 489 breaker, above 20 kA | 4.9 | Peak |
| LV power circuit breaker, unfused (IEEE C37.13) | 6.6 | rms |
| LV fuse | 4.9 | rms |
DC fault current
The method is nodal analysis of the resistive network, with each conductor counted twice for the loop. Cable resistance comes from NEC Chapter 9 Table 8.
- Battery: a voltage source behind its internal resistance. The resistance comes from the manufacturer's short-circuit current, R = V/Isc, or is entered directly (IEEE 946). Parallel strings divide it.
- PV array: 1.25 × Isc per string, from NEC 690.8(A)(1).
- Rectifier or charger: its current limit.
- Time constant: L/R comes from the path inductance (Table 9 reactance at 60 Hz) and the battery inductance. It is reported for information; the peak is taken as the steady fault current.
Protective devices and coordination
Each curve is a band. The lower edge is the minimum trip or melt time; the upper edge is the total clearing time. The curves are generic for each device family; final settings need the manufacturer's published curves.
- Thermal-magnetic MCCB: the thermal band is anchored at 200 % of rating by size. Instantaneous is ±20 %, clearing in 1.5 cycles (0.025 s).
- Electronic trip (LSIG):
- Long time: pickup 1.05 to 1.2 × Ir, I²t through tr at 6 × Ir.
- Short time: ±10 % pickup, flat or I²t delay.
- Instantaneous: ±10 %. Clearing is 1.5 cycles for molded-case frames up to 800 A, and 3 cycles for 1200 A molded-case frames, insulated-case and power breakers.
- Insulated-case frames keep a fixed instantaneous override when instantaneous is off.
- The NEC 240.87 maintenance switch adds an instantaneous element while it is on.
- Motor circuit protector: instantaneous only, ±20 %. The overload relay (Class 10, 20 or 30) is shown on the plot, but it does not interrupt faults.
- LV fuses (UL 248 Classes J, RK1, RK5, L, CC and T): representative melting curves. Classes J, RK1, RK5 and L time-delay hold at least 10 s at 500 %; Class CC time-delay holds 12 s at 200 %. The curve becomes constant I²t at high current.
- Bay-O-Net C12 (current sensing): melting and total clearing curves digitized from the ETAP library plots in two 2026 water station studies. The two plots agree within 0.5 %.
- Fuse links, K and T: the ANSI C37.43 melting currents at 300 s (600 s above 100 A), 10 s and 0.1 s, plus one cycle of arcing.
- E-rated power fuses: melt at 200–240 % of the E rating in 300 s (220–264 % in 600 s above 100E), per ANSI C37.46.
- Damage curves:
- Transformer: ANSI/IEEE C57.109, with the thermal points 2×/1800 s to 25×/2 s and I²t = 1250. Category II adds the frequent-fault curve from 0.7/Z. The curve can be shifted to 58 % for line-to-ground faults on delta–wye units.
- Inrush is plotted at 12 × for 0.1 s.
- Conductor: ICEA P-32-382, (I/A)²t = 0.0297 log[(T2+234)/(T1+234)] for copper.
Selectivity between adjacent devices is checked up to the fault available at the downstream device. NEC 700.32, 701.32 and 708.54 require selective coordination for emergency, legally required standby and critical operations power systems.
AC arc flash: IEEE 1584-2018
The low-voltage model covers 208 V to 600 V, bolted fault current 0.5 to 106 kA, gaps 6.35 to 76.2 mm and working distances of at least 305 mm. Ibf is in kA, G and D in mm, T in ms.
Enclosure correction (Eqs. 11 to 14, Table 7): the equivalent enclosure size EES is the mean of the adjusted height and width. CF = b1·EES² + b2·EES + b3 for a typical enclosure, or its reciprocal for a shallow one (below 600 V, under 508 mm high and wide, and 203.2 mm deep or less).
Clearing time: the study is run twice, at the full and the reduced arcing current, and the larger energy governs. For each case, every device between the source and the fault is checked at its own share of the arcing current (the bolted-fault current division), and the fastest clears. Arcs lasting more than 2 s are limited to 2 s (IEEE 1584-2018 B.1.2); the limit can be turned off.
Where the bus has a main, two locations are reported:
- the bus, cleared by the main;
- the line side of the main, cleared by the next device upstream, often the transformer primary fuse.
Single-phase systems are not covered by IEEE 1584. The three-phase model is applied to them as a conservative estimate.
| Equipment | Gap | Working distance | Enclosure (h × w × d) |
|---|---|---|---|
| Switchgear and switchboards | 32 mm | 24 in | 20 × 20 × 20 in |
| Panelboards and MCCs | 25 mm | 18 in | 14 × 12 × 10 in |
| Disconnects (shallow) | 25 mm | 18 in | 14 × 12 × 8 in |
| Cable junction boxes | 13 mm | 18 in | 14 × 12 × 10 in |
DC arc flash: NFPA 70E-2024 Annex D.5
Maximum power method (Doan)
Arc power is greatest when the arc voltage is half the system voltage. This gives an upper bound.
Arc resistance method (Stokes and Oppenlander)
Rsys is the battery network's Thevenin resistance. Current-limited sources (rectifiers, PV) are added as a Norton current; with only current-limited sources, the arcing current equals their current.
Enclosures: a factor of 3 on the open-air value, or the Wilkins model IE = k·Earc/(a² + d²). Wilkins uses a = 100, 400 or 950 mm and k = 0.127, 0.312 or 0.416 for panelboard, switchgear and large enclosures.
Several sources: each source's share of the arc lasts until a device on its own path to the fault clears it. A fused battery rack, for example, is cleared by its own fuse. The arc duration used is the share-weighted mean. A PV array keeps supplying current while it is lit, and a rectifier stops only after the shutdown time entered for it.
Labels and shock boundaries
Label contents follow NFPA 70E-2024 130.5(H):
- nominal voltage;
- arc flash boundary;
- incident energy with its working distance;
- minimum arc rating of clothing.
NEC 2026 110.16 adds the date the assessment was completed, and covers service and feeder equipment of any rating in other than dwelling units. The header follows the ANSI Z535.4 format. Following common industry practice, it reads DANGER above 40 cal/cm² and WARNING otherwise. Glove classes follow ASTM D120 maximum use voltages: Class 00 to 500 V ac or 750 V dc; Class 0 to 1000 V ac or 1500 V dc.
| Nominal voltage | Limited approach (fixed part) | Restricted approach |
|---|---|---|
| AC 50–150 V | 3 ft 6 in | Avoid contact |
| AC 151–750 V | 3 ft 6 in | 1 ft 0 in |
| DC 50–300 V | 3 ft 6 in | Avoid contact |
| DC 301 V–1 kV | 3 ft 6 in | 1 ft 0 in |
| DC 1.1–5 kV | 5 ft 0 in | 1 ft 5 in |
NFPA 70E-2024 Tables 130.4(E)(a) and (b). The limited approach boundary for exposed movable conductors is 10 ft 0 in.
Verification
- IEEE 1584-2018 Annex D.2 (480 V, VCB, 45 kA): the arcing currents, EES, CF, incident energies and boundaries all agree to the published figures.
- IEEE 1584 spreadsheet: 51,840 low-voltage cases agree to within 0.0003 %.
- Fault-current hand calculations: the transformer terminals, line-to-ground through a delta–wye unit, parallel cables and motor contribution all agree exactly.
- Water station studies (ETAP, 2026): the well and booster station examples are copied from the study models.
- Bolted three-phase fault currents agree with ETAP within 1 % at the switchboards and 3 % at the lighting panels.
- Incident energy agrees within 2 % wherever the clearing device and time are the same, for example the switchboard line sides cleared by the Bay-O-Net fuse.
- Motor starter buses read higher here because ETAP's H-frame curve clears in 1 cycle, against the generic 1.5 cycles used here.
- Line-to-ground currents differ by up to 13 %. ETAP takes the cable zero-sequence impedance from its cable library; this tool assumes a return conductor the same size as the phase.
- DC arc resistance: the published 256 V battery example (IAEI, 2012) gives 11.43 kA and 4.9 cal/cm².
References
Arc flash and safety
- IEEE 1584-2018, Guide for Performing Arc-Flash Hazard Calculations
- NFPA 70E-2024, Standard for Electrical Safety in the Workplace: 130.4, 130.5(H), Annex D.5
- ANSI Z535.4, Product Safety Signs and Labels; ASTM D120
- D. R. Doan, Arc flash calculations for exposures to DC systems, IEEE Trans. Ind. Appl., 2010
- A. D. Stokes and D. K. Oppenlander, Electric arcs in open air, J. Phys. D, 1991; R. F. Ammerman et al., DC arc models and incident energy calculations, 2010
- R. Wilkins, Simple improved equations for arc flash hazard analysis, 2004
Short circuit, protection and code
- IEEE 551 (Violet Book), IEEE 242 (Buff Book), IEEE 946 (DC auxiliary power)
- UL 489, UL 248, IEEE C37.13; ANSI C37.43 and C37.46 (fuse links and power fuses)
- IEEE C57.12.00 and C57.109 (transformer through-fault capability); ICEA P-32-382
- NFPA 70 (NEC) 2026: 110.9, 110.10, 110.16, 230.95, 240.67, 240.85, 240.87, 430.52, 450.3, 625.41, 690.8, 700.32