Beyond the 40 Cal/cm² Myth: What You Must Know About High-Energy Arc Flash and Arc Blast Hazards
- Jeff Kershner
- Sep 8
- 5 min read
For years, a persistent myth has circulated in the electrical safety world:
“Anything above 40 cal/cm² will kill you instantly.”
We’ve heard it described in dramatic terms: closed casket funerals, certain death from “blast pressure,” and warnings that no PPE could possibly make a difference.
Here’s the truth: 40 cal/cm² is not a cutoff point for survival.
It’s not a blast pressure threshold, and it’s not a magic number where PPE suddenly stops protecting workers.
Why This Blog?
The common misunderstanding is that higher incident energy (and the heavier PPE required) equals higher blast pressure – as if it’s a one-to-one correlation.
So, if the arc flash label says you need a 40 cal/cm² suit, the assumption is that the arc blast will kill you. That’s not how it works.
The reality is that arc flash and arc blast are different hazards, and they don’t necessarily mirror each other in severity. In most systems, the blast risks are limited to secondary effects* such as a panel door swinging open or a worker losing balance on a ladder. (Large capacitors are one notable exception, but for the majority of equipment, the catastrophic “blast wave” described in old training slides has never been supported by evidence.)
At Guidant Power, our goal is to build trust by providing accurate, up-to-date safety information. When your workforce knows the facts, they are more likely to trust their PPE, follow procedures, and contribute to a stronger safety culture.
Arc Flash vs. Arc Blast: Definitions
First, a basic reminder: while both arc flash and arc blast happen during a an arc fault, they are fundamentally different:
Arc flash is the intense light and heat generated during an electric arc.
Arc blast is the pressure wave that can accompany an arc flash.
Arc flash PPE is designed to protect against arc flash energy, not arc blast pressure. Arc blast, when it occurs, is more often controlled through equipment design, work practices, and safe distances.
Understanding this distinction helps prevent overestimating one hazard while underestimating the other.
Where the 40 cal/cm² Cutoff Came From
The confusion with 40 cal/cm² stems from how NFPA 70E® once structured its “PPE Category Method.” In that method, Category 4 required arc-rated clothing with a minimum arc rating of 40 cal/cm². Many trainers and companies mistakenly took that value as an absolute maximum, when in reality, it was just the highest PPE category defined at the time.
Additionally, for a long time, suits rated higher than 40 cal/cm2 were hard to find. All this led to the impression that 40 cal/cm² represented the final cutoff for safety.
Today, we know better. In fact, Canada’s CSA Z462 standard (their equivalent to NFPA 70E®) already recognizes PPE up to 75 cal/cm² for certain 600V switchgear applications. The key takeaway: protective clothing is available and tested for incident energies well above 40 cal/cm².
Blast Pressure vs. Incident Energy
Let’s address the myth that “high incident energy equals fatal blast pressure.” This is wrong because:
Incident energy (cal/cm²) measures thermal energy at a working distance.
Blast pressure depends on fault current and the dynamics of the fault, not directly on the incident energy value. Large capacitors can also cause arc blast hazards, see NFPA 70E 2024 - R.10
High incident energy can actually come from low fault current if breakers or fuses take longer to trip. That’s why generator-fed equipment often produces high incident energy.
Surprising Blast Pressure Risks With Low Incident Energy
In 2019, Hugh Hoagland and colleagues presented an IEEE paper titled Arc Flash Pressure Door Ejection Measurement. The study found that the real hazard linked to arc blast is not the concussive force that guarantees death, but rather secondary effects such as:
Panel doors being ejected.
Workers falling off ladders or being startled.
The research showed that door ejection can happen at incident energies as low as 2–4 cal/cm²—far below the supposed “40 cal/cm² threshold”.
To quote directly:
“The examples even in IEEE-ESW presentations and in training events have gone so far as to state that a 100 cal/cm² flash suit will ‘just leave a nice corpse.’ This has never been supported by evidence to our knowledge. Over exaggeration leads to lack of trust and could undermine honest efforts at worker protection.”
Equipment doors can and do move in faults, and even a small hazard can become serious without protection. Look at arc blast risk based on its own merits, asking: is a door likely to swing open? Is a worker prone to fall from a ladder should a blast occur?
Bottom line: arc blast and arc flash hazards need to be considered separately.
Protecting From Arc Flash and Blast Risks: Beyond Just Compliance
To account for real-life scenarios that workers encounter in their daily work, it’s important for management to think beyond “what the rules say.” For its part, NFPA 70E® wants companies to focus on risk assessment.
Here are some risks you may not have considered:
While an arc flash label may call for 40 cal/cm2 PPE, the PPE alone isn't necessarily safe enough for working on energized equipment.
If door ejection from arc blast is a risk, then arc flash PPE is not going to protect workers. Instruct your workers to stand where a flying door (or door swinging open) will not hit them when performing the switching action.
When energized work is unavoidable, proper risk analysis, PPE, and clear procedures are a must. De-energizing equipment is still the safest path whenever possible.
Switching on equipment after a fault or repair is considered energized work. A technician may think that a simple task like switching a breaker is “normal operation,” but it’s not if changes have occurred prior to switching. You must wear PPE based on the arc flash hazard label when performing this “non-normal” task. (Learn more about normal operation in this article.)
A 100 cal/cm² arc flash may not guarantee death, but it will destroy equipment and could create cascading hazards.
The Safest Choice: Compliance Plus Training and Procedure
Because electrical safety is much more than checking a box, regular training and detailed procedures are essential in a facility manager’s toolbox.
Nuclear facilities are a great example of a comprehensive approach. They operate with some of the highest arc flash hazards in industry, yet their record for avoiding incidents is excellent—because of rigorous procedures, constant training, and disciplined risk management.
What This Means for You
Don’t buy into old training myths. The “40 cal/cm² = instant death” line was never backed by data.
Don’t confuse PPE categories with incident energy values. They’re separate methods with separate rules (see our related post about Category Method vs. Incident Energy)
Do invest in accurate arc flash analyses. Modern engineering software (like SKM, ETAP, or EasyPower) gives you the data to make the right PPE and work-practice decisions to mitigate both arc flash and arc blast hazards
Do use risk analysis. Ask: Can we de-energize? Can we redesign equipment? Can we reduce risk with IR windows, dead-fronts, or breaker replacements?
The Guidant Power Difference
With over 25,000 arc flash inspections completed and decades of field experience, Guidant Power helps companies separate fact from myth. We understand the nuances of high-energy hazards and guide you in balancing compliance, practicality, and worker protection.
If your facility still operates under the 40 cal/cm² misconception, it’s time for a reset. Let’s build a safety program grounded in today’s standards and real-world data—not outdated training slides.
Contact Guidant Power to schedule an arc flash analysis or review your electrical safety program.
70E®, Standard for Electrical Safety in the Workplace®, NFPA 70®, NEC®, and National Electrical Code® are registered trademarks of the National Fire Protection Association, Quincy, MA. All rights reserved. This informational material is not affiliated with nor has it been reviewed or approved by the NFPA.
Mr. Hoagland is an Author of several IEEE papers and an Associate Editor for the IEEE Electrical Safety Committee.
