Arc Flash Study for MCCs and Control Panels
“Arc flash” isn’t just a vague danger to any electrician who has opened a live MCC bucket. It’s real and violent energy that can turn copper wires into plasma in a matter of milliseconds. NFPA 70E and IEEE 1584 have pushed plants to document and control that risk through arc-flash studies over the past 20 years.
Arc-flash energy can be reduced significantly with faster clearing times and properly maintained protection systems. A few milliseconds of improvement can save equipment — and lives.
This guide shows you how to build or maintain MCCs, switchgear, or PLC cabinets. It tells you what to collect, how to model in ETAP or SKM, how to read the results, and how to label and fix things. This book is for real-life engineers and technicians who do this work, not just talk about it in books.
What is an Arc Flash Study and Why Do You Need One?
An arc-flash study tries to figure out how much energy (cal/cm²) a worker might be exposed to when an arc happens. That number tells you what level of PPE to wear and what the label on the door says.
According to NFPA 70E, businesses must check for electrical hazards and put up signs on equipment that could hurt anyone. IEEE 1584 2018 gives the maths for AC systems that range from 208 V to 15 kV. Plants in the U.S. usually do the analysis again every three years or after big changes to the electrical equipment.
Core Standards: Purpose of the Standard and Key Use
NFPA 70E (2024) Safety for employees who work with electricity, IEEE 1584 (2018) sets limits, PPE, training, and labelling. The model for calculation gives equations for arc-flash and incident-energy boundaries.
NEC 240.67 / 240.87 Regulation OSHA 1910 Subpart S says that circuit breakers and fuses must use methods that reduce energy for breakers that are more than 1200 A. It requires employers to assess the risk of arc-flash and offer protection.
IEC 61482-1-1/2 and EN 50110 are important for European readers, but NFPA 70E and IEEE 1584 are more important for people who work in manufacturing in the U.S.
Checklist for Collecting Data
Field data, not software, is the best place to start a study. Before you begin modelling, get the following:
- X/R ratio and utility short circuit current at the service entrance
- One-line diagram that is up to date
- Connections, voltage, impedance, and kVA of the transformer
- The sizes, materials, and lengths of conductors
- Types and sizes of enclosures (switchgear, MCC, panelboard)
- Details about breakers and fuses, such as ratings, settings, and trip units
- MCC bucket data: type of feeder, size of starter, and overloads
- Big motors, VFDs, soft starters, and loads that are connected
- Any UPS, BESS, or PV/PCS systems that are linked to the same bus
Put everything into a spreadsheet before putting it into ETAP or SKM. Each row should have a tag number or panel ID that it goes back to.
Modelling in ETAP, SKM, or EasyPower
- Use the right IEEE 1584 library defaults for setting up electrodes (VCB, VCBB, HOA).
- Input the size of the box, as the size of the box affects the energy calculation.
- Set a reasonable working distance: 18 inches for panels and MCCs and 24 inches for switchgear.
- Include the motor’s contribution and the capacitor’s discharge when it makes sense.
- Be careful with VFD outputs and DC circuits; each tool works with them in its own way.
- Be careful when entering breaker parameters; if you forget to set the trip, the energy number could double.
Short-Circuit and Coordination—First
The amount of incident energy depends a lot on how long it takes to clear a fault, so check your short-circuit and coordination studies first.
Study Main Output: Why It Matters for Arc Flash
| Study Type | Output | Importance |
|---|---|---|
| Short-Circuit | Current fault current | Sets the starting point for calculations of incident energy |
| Coordination of Protection | Times for clearing devices | Less time clearing means less exposure to energy |
Better coordination speeds up the clearing process. A lot of engineers try out Zone-Selective Interlocking (ZSI) or differential schemes to speed up trips without losing coordination.
Calculating Incident Energy in Real Life
The empirical equations in IEEE 1584 2018 are based on more than 2000 test shots. Inputs are:
| Input | Units | Normal Effect |
|---|---|---|
| kA of fault current | kA | More current means more energy |
| The length of the arc | s | Time to clear—most important factor |
| Space between conductors | mm | Less space, more energy |
| Size of the enclosure | mm | Confinement boosts energy |
| Distance to work | mm | Longer distance means less exposure |
If breakers clear in less than five cycles, most MCCs fall between 4 and 8 cal/cm² at 480 V. If protection is slow, old gear can go over 30 cal/cm².
Requirements for Labelling (NFPA 70E)
Every piece of equipment that might need energised work must have a long-lasting label that says:
- Nominal system voltage, arc-flash boundary distance, incident energy (cal/cm²), or PPE category, arc-flash date, and the name of the engineer or company responsible
Make sure labels are up to date. Check again after changing the settings on the device or adding a new source. If you don’t take care of things, the study and your compliance record could be invalid.
PPE and Limits
- Arc-Flash Boundary: the distance at which exposure is 1.2 cal/cm².
- Limited / Restricted Approach: NFPA 70E’s shock-protection distances 130.4 Table
- Workers can get PPE right from Table 130.5(G) if you put the incident energy on the label. Most electricians in the U.S. wear shirts, pants, or coveralls that are rated 8, 20, or 40 cal/cm².
Keep in mind that PPE doesn’t make the work safe; it just lowers the risk of injury when everything else fails.
Ways to Reduce Risk
| Method | Explanation | Regular Reduction % | Comment |
|---|---|---|---|
| Fuses that limit current | Peak fault current should be between 40 and 60% | Easy to add on | |
| ZSI (Zone‑Selective Interlocking) | Communication between upstream and downstream for faster trips | 30%–70% | Needs breakers that work with it |
| Switch for maintenance | Setting for a temporary fast trip | Up to 90% | |
| Arc‑flash relays (optical) | Detect light or current and trip in less than 8 ms | 70–95% | Common on new switchgear |
| Bus differential protection | Finds internal problems right away | 90%+ | Best for MCCs and switchgear |
| Remote operation/racking | Keeps the worker out of the area | 100% | Complete avoidance of exposure |
Cases in Point
- Food Plant in Ohio: We upgraded the MCCs with ZSI breakers, which cut the incident energy from 28 to 7 cal/cm². The PPE level went from Category 3 to 2.
- Paper Mill in Georgia: Two main breakers got maintenance switches added, and clear times went from 0.35 s to 0.07 s.
- Texas Municipal Water Plant used the ETAP arc-flash module and found that the 600 V panel was connected to the wrong protective setting. The cost of repair was $300, which saved a possible injury citation.
- Data Centre in Arizona: An arc-flash relay was put on the UPS tie, which cut energy use at the DC bus by 80%.
- OEM Panel Builder: Added the ability to print labels directly from SKM reports to the cabinet serial database for tracking compliance.
Special Cases and Mistakes
- Upgrading legacy trip units in MCC buckets may help with long delays.
- VFD-fed loads: turn on arc-fault filtering in models; drives can send DC links back.
- UPS and BESS: According to IEEE 943, DC arcs need to be calculated separately.
- For imported panels, check the size of the enclosure and the venting data, or the model may overpredict energy.
- Bad data: Rounding the length of a conductor or using a guessed panel impedance gives you results that don’t make sense. Always measure or figure things out the right way.
Keeping Records and Doing Maintenance
Under change control, keep all of your study files, breaker settings, and label logs. NFPA 70E says to look over:
- Every three years, or sooner if the system changes.
- After adding big loads, transformers, or safety devices.
- When the levels of faults in upstream utilities change.
Teach qualified workers how to read the labels and the plant’s own PPE matrix. In any OSHA investigation, your paperwork is proof that you followed the rules.
Look at the Table to See How Much It Costs Versus How Much It Is Worth
| Count of Equipment | Normal Cost (USD) | Key Benefit of the Review Cycle |
|---|---|---|
| Small Plant (fewer than 20 panels) | $5,000–$8,000 for three years | Meets the bare minimum for compliance |
| Medium Site (20–80) | $10,000–$25,000 for three years | Better safety and lower insurance rates |
| Big Facility (>80) | $25,000–$60,000 for three years | Maintenance and tuning of devices in one place |
Insurers are asking for proof of an up-to-date arc-flash study more and more before they renew policies.
Useful Software and Tools
- ETAP Arc Flash Analysis: Full IEEE 1584 2018 engine and label export
- SKM PowerTools: Commonly used in U.S. businesses; great for customising reports
- EasyPower: Has an easy-to-use interface for quick “what if” studies.
- ETAP Mobile / EasyPower Viewer: Scanning labels and validating data in the field
A lot of engineers keep an internal “calculator” spreadsheet that they use to quickly check how incident energy trends compare to breaker clearing time and arc gaps.
Sources
Learn more in our NFPA 70E training guide.
See how to apply calculations in our IEEE 1584 arc flash tutorial.
25 Common Questions (FAQ)
Q1: How often do you need to update an arc-flash study? Ans: Every three years or after changes to the system, like adding new transformers, MCCs, or breaker settings.
Q2: What has the biggest effect on incident energy? Ans: Time to clear the fault. Faster protection cuts down on energy use by a lot.
Q3: Do VFDs make the risk of arc-flash higher or lower? Ans: They usually lower upstream energy, but if they aren’t blocked, they can feed DC arcs. Be careful when modelling them.
Q4: What should be on an NFPA 70E label? Ans: Voltage, incident energy or PPE category, boundary, and study date.
Q5: What’s the difference between the incident-energy method and the PPE category? Ans: The category method uses standard PPE tables, while the incident-energy method figures out the exact cal/cm².
Q6: Is a “Category 0” still good? A: No, NFPA 70E took it away. The lowest level is Category 1.
Q7: Is it possible to print labels straight from ETAP or SKM? Yes, both can be exported to DYMO and Brother label sizes.
Q8: What is a switch for maintenance mode? Ans: A key switch that temporarily speeds up the trip curve while work is going on.
Q9: How close can I get while testing? Ans: Stay away from the arc-flash boundary shown on the label.
Q10: Do I need to do an arc-flash study for 208 V panels? Yes, if the transformer is bigger than 125 kVA. IEEE 1584 covers three-phase down to 208 V.
Q11: How do I figure out how much energy an arc flash in DC has? Ans: For DC systems, use IEEE 1584-1 or NFPA 70E Annexe D.6.
Q12: What kind of software do utilities like? Most of them accept ETAP, SKM, or EasyPower. Brand isn’t as important as consistency.
Q13: Is it okay to use data from 2010? Ans: No, the 2018 model changed the equations. You need to redo them with the new parameters.
Q14: What are the best arc-flash relays? Answer: The Littelfuse PGR-8800 and Schweitzer AFR series are common in U.S. plants.
Q15: Does humidity change the results? Ans: Not much of an effect; the size of the gap and the enclosure are more important.
Q16: What is the formula for the arc-flash boundary? Ans: From IEEE 1584: Distance = [(Ei/Eb)^(1/x)] × working distance, which software usually fixes on its own.
Q17: What kind of material should labels be made of? Answer: Polyester that is not metal and can withstand UV rays and temperatures up to 176°F.
Q18: Who can do an arc-flash study? Answer: A qualified electrical engineer who knows how to do short-circuit and coordination analysis.
Q19: Why every three years? Answer: NFPA 70E 130.5(G) says that is the time frame for reevaluating the system.
Q20: Do panels with 24V DC need labels? No, but if DC > 50 V, add shock warnings.
Q21: How do you rate arc-flash PPE? Ans: The ASTM F1959 test gives the arc-thermal performance value (ATPV) in cal/cm².
Q22: What does “selective coordination” mean? Ans: Making sure that only the closest protective device trips during faults.
Q23: Does the energy of an incident stay the same throughout? Ans: Usually the same; the model assumes that the conditions are the same on both sides.
Q24: How long does it take to do a full study? Ans: One to three weeks, depending on the size of the facility and the quality of the data.
Q25: Is it possible for arc-flash energy to be zero? Ans: Only by completely shutting off the power and making sure there is no connection—otherwise, there is always a risk.
