We have had customers ask about the blast pressure above 40 cal/cm^2. We've heard that being in an arc flash of this size will cause instant death from the blast pressure or an arc flash of this size will result in a closed casket instead of an open casket. We have also seen marketing information still available on the internet making similar claims
First of all, greater than 40 cal/cm^2 arc flash hazard has no direct relationship to blast pressure, this is a myth associated with arc flash hazard calculations. Test data supports that this is a myth and it has been debunked for some time now. Industry is taking a while to be re-educated because many electrical safety instructors incorrectly taught this myth for years.
This is a very good topic so there’s a lot of information here.
While some of the following are opinions of Rozel, we reference applicable source material when possible. We base our opinions on the latest revisions of applicable standards, and annually attend the IEEE electrical safety workshop where the latest updates and papers are presented and discussed. As licensed engineers we are required to stay educated and we work tirelessly to stay up to date and ahead of changes to our industry.
Worker Incident Energy Cutoff - Codes and standards are meant to represent a minimum level of compliance, and it is certainly acceptable to develop a more stringent criteria, we see many companies take this stand. As an example, it is not uncommon for us to see a company create an incident energy (arc flash measurement value) cutoff of 40 cal/cm^2 for their workers. The cutoff is not based on a standard but rather at the value of arc flash measurement that the company defines their risk level. More often we see an 8 cal/cm^2 cutoff. Any work above such a cutoff this would require an outside contractor. 8 cal/cm^2 is not typically considered as a lethal value but this is actually quite dangerous, yet we do have PPE to protect workers above this level, just as PPE is available above 40 cal/cm^2.
“Table Method” aka “Category Method” for determining arc flash hazards
There are two methods for determining PPE (“table” or “calculation” method). This is a one or the other approach. They are not to be confused with each other as they have different parameters, requirements, etc. We have another post which compares some of the calculations between the two methods, that can be found here (article on Category vs Incident Energy calculations). People will frequently confuse the two, or mention PPE by category when referring to the ATPV (this is arc thermal protective value which is measured in cal/cm^2) of the garment. Some garment manufacturers even include text on the garment that mentions Category X along with ATPV (this is another discussion).
- If someone mentions “category” this refers to a “table method”. The table method (NFPA 70E 2021 - Table 130.7 (C)(15)(a)) lists specific types of equipment with specific parameters of fault current and clearing time. As long as you meet the required parameters, then you select the appropriate category of arc flash PPE. Note, this method does not mention anything about the incident energy value. The highest PPE category, 4, requires a minimum arc rating of 40 cal/cm^2. This is the only top end cutoff which mentions 40 cal/cm^2. The parameters in this table are actually rather conservative, and would yield values under 40 cal/cm^2 if calculations were performed. There is one important "gotcha" with the table- you must meet the required parameters to use the table. This means that if you don’t know the both fault current AND the clearing time, then the table cannot be used. In order to know the fault current at each point in your system so that you can utilize the tables, you must model and calculate at each point. Although some try, it is almost impossible to do this without engineering software which is what we use for the actual arc flash hazard evaluation. It is possible to begin calculations with maximum fault current, also known as infinite bus method., more information about infinite bus is discussed here: What is Infinite Bus Method (for transformer fault current)? However, using the same value everywhere can also be dangerous; transformers downstream in a facility might give an even higher fault current value, see more details here: (Arc Flash Size). In other words, with many systems, you must conduct an analysis to use the table. Check out NFPA 70E's free viewer to find this information in the standard.
- CSA Z462 2021 (Canada's equivalent to NFPA 70E) Clause 126.96.36.199.15 Arc Flash PPE category method is actually going to have a new arc flash PPE PPE category 5, 75 cal/cm^2 for 600V class switchgear. Members from CSA Z462 have been instrumental in advancing NFPA 70E over the last several editions and this should be an indication of things to come for NFPA 70E. If nothing else it proves that 40 ca/cm^2 is not the upper limit for proper protection (please don't be misconstrued that 75 cal/cm^2 is the upper limit either). Read more about their changes here.
- Incident Energy Calculation Method for determining arc flash hazards. The other option for determining the PPE is the calculation method. This is all you should use with complex systems as described above, otherwise you risk errors. There is no 40 cal/cm^2 top end cut off for this method. Calculations are performed using an engineering software (EasyPower, SKM, ETAP are a few examples). After calculating the incident energy, appropriate PPE is determined using NFPA 70E 2021 - Table 130.5(G) pg 29. As you can see in this table, there are two "buckets", above and below 12 calories. 20, 40, 60, 80 calories, they’re all treated the same, as long as you have appropriate PPE.
Blast Pressure and Incident Energy
There has been a concern about “blast pressure” and its correlation to 40 cal/cm^2, i.e. “open casket versus a closed casket if a pressure blast occurs above 40 cal”.
As mentioned above, the correlation between incident energy and blast pressure is a myth. In fact, blast pressure is associated with high amounts of fault current. Interestingly, high arc flash values may come from low fault current or high fault current. The concept of high fault current causing a large arc flash is intuitive. However, low fault current can cause a breaker to take longer to trip, this increases the time available for an arc flash to grow resulting in a higher incident energy rating. We see this scenario a lot with generator feeds, the highest arc flash values are often when fed from a generator with low available fault current. Data does not currently tell us when high fault current causes high blast pressures, we do not know the cutoff when this becomes a concern. More discussion regarding fault current and arc flash size is covered in the same post as was mentioned above (Arc Flash Size).
What we do know though is the pressure wave of the blast itself this has generally been accepted as not being a concern with most equipment (capacitors are an exception). There was an IEEE paper presented and published in 2019 (ARC FLASH PRESSURE DOOR EJECTION MEASUREMENT Hugh Hoagland Et al.) that discusses and explains hazards associated with “arc blasts”. In summary, the potential hazard due to expansion of the arc flash is of secondary effects (door ejection, falling off ladder), rather than the concussive effect. To quote the presentation:
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.
Door ejection is a real concern. However, in many cases this can occur as low as 2-4 cal/cm^2. This is a great example of why standards provide a minimum threshold. According to NFPA 70E, “operation of a [circuit breaker]” in normal operation mode does not require PPE (at any value). Our policy is to recommend wearing PPE for incident energy values greater than 4 cal/cm^2. This is because in a fault, doors are very likely to come off of equipment. (see NFPA 70E 2021 - table 130.5(C) pg 27)
None of this should take away from the effort to mitigate risk. This could include adding/replacing circuit breakers, adding IR windows to equipment, or creating/ensuring dead fronts exist behind doors where maintenance needs to be conducted on live equipment.
Another article on Hugh Hoagland’s website written by him in 2012 supports the same, it can be found here: http://archive.constantcontact.com/fs005/1101637591588/archive/1110049668373.html#LETTER.BLOCK51
Hugh Hoagland is the founder of ArcWear, which does 90% of the world’s arc flash testing of protective apparel. Hugh serves on many international standards committees including NFPA, ASTM, IEEE, IEC, and has helped develop electrical and flash fire safety legislation and standards in the U.S., Europe and internationally.
Here is a similar article written by Jim Phillips in 2017:
Jim Phillips is a Technical Committee alternate for NFPA 70E, the committee is who writes and finalizes the standard. He is also the co-chairman of IEEE-1584 (Guide for Performing Arc-Flash Hazard Calculations), this is the standard that details our equations for arc flash calculations.
NFPA 70E has taken a very direct path towards prioritizing risk before work begins. It is now relatively straightforward for a trained individual to calculate arc flash hazards utilizing engineering software. However, ultimately if we can avoid the hazard then we are better off, this is identified in a risk analysis. Consider a large enclosed pad mount transformer, operating normally, doors closed, in good condition. An individual may walk by this very large arc flash hazard with no risk of ever being hurt. Additionally, a trained worker may carefully swing open the door on this transformer for visual or IR inspection with no risk either as long as proper risk mitigation has been put into place.
One good example of where great risk analysis and mitigation is practiced is in the nuclear environment. A work place with extremely high hazards yet they have one of the best records of avoiding injury or incident. You can ask Brian Hall about this, before Rozel he worked at a nuclear facility creating, auditing and maintaining SOPs including infrared inspection on high arc flash hazard areas.
We want to provide you with the most current data to support your electrical safety decisions, that is our obligation.