Over 40 cal/cm^2 equipment entry

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
Yes. The cal/cm^2. Not a problem with that. But the units are interesting.
But the units are interesting. You and other engineers are using cgs units. The centigram, gram, seconds. which is different.
The British and the rest of the world use SI.
:)
Interesting; my curiosity was piqued. I've never seen (and it wasn't easy to even find) J/m^2 in any literature. What safety standards in RoW use that SI measurement? Is there a consensus or regulatory standard?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
You need to perform a risk analysis for each one of your tasks.
For example, what is the risk of creating an arc flash when unbolting covers on equipment that is probably deenergized because the POCO has opened their feed. Then what is the risk of using a non contact sensing device on a hot stick? Finally what risk is involved with making a direct contact voltage measurement after the previous two tasks have been performed?
 
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The Dude

Member
Location
Santa Clarita, CA
Occupation
Electrician
NFPA 70E suggests working on just about anything live is too risky. Table 130.5(G) suggests the cut-off is >12-cal/cm^2. The 40-cal/cm^2 thing is an outdated and unsubstantiated limit that is no longer valid.
12 or 40 or 150, it's still 168 so it doesn't matter. There is no inspection window on the 12KV so we can't get a visual on the blades.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
12 or 40 or 150, it's still 168 so it doesn't matter. There is no inspection window on the 12KV so we can't get a visual on the blades.
Then isolate further upstream where the incident energy is lower or call the serving utility to kill the power.
 

The Dude

Member
Location
Santa Clarita, CA
Occupation
Electrician
You need to perform a risk analysis for each one of your tasks.
For example, what is the risk of creating an arc flash when unbolting covers on equipment that is probably deenergized because the POCO has opened their feed. Then what is the risk of using a non contact sensing device on a hot stick? Finally what risk is involved with making a direct contact voltage measurement after the previous two tasks have been performed?
This is probably our best bet. I didn't know if anyone had encountered this issue before and had come up with another idea.
Thanks for the input.
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
Interesting; my curiosity was piqued. I've never seen (and it wasn't easy to even find) J/m^2 in any literature. What safety standards in RoW use that SI measurement? Is there a consensus or regulatory standard?
There probably are and I may have encountered the them. I generally worked with maximum faults at the 11kV level.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
You never work it hot, but you treat it as such until you establish an ESWC.

They do make arc flash gear rated over 40-cal/cm^2.

The system might benefit from arc energy reduction (NEC sections 240.67 or 240.87) or other arc-flash hazard mitigation solutions.

The arc flash study could also be over conservative if default/generic bus gap, enclosure dimensions, electrode configuration, and working distance were used in the calculation. Field verification of study inputs is always recommended.

Note 40 cal/cm^2 is an arbitrary limit. There really is no basis for this cut-off as you can get seriously injured even far below this incident energy exposure.
Actually there is a basis for that, the percussion wave of the arc energy above that is considered lethal, so they don’t bother going any higher.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
That’s what I was told in a 70E class taught by the NFPA.

What you heard is something many claim, but there isn’t sufficient data to support it. That’s why the reference to a 40-cal/cm^2 cutoff was removed from 70E.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Actually there is a basis for that, the percussion wave of the arc energy above that is considered lethal, so they don’t bother going any higher.
That is an old wives tale told by instructors, from the scare tactics training style.
There is no industry accepted standard method for calculation arc blast pressure.

Arc blast pressure occurs really fast, while incident energy is usually accumulated over a relatively long time. I would not be surprised to find the majority of ≥40cal/cm^2 locations have arcing times measured in seconds, while those less than ≤4cal/cm^2 are measured in milliseconds.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
That is an old wives tale told by instructors, from the scare tactics training style.
There is no industry accepted standard method for calculation arc blast pressure.

Arc blast pressure occurs really fast, while incident energy is usually accumulated over a relatively long time. I would not be surprised to find the majority of ≥40cal/cm^2 locations have arcing times measured in seconds, while those less than ≤4cal/cm^2 are measured in milliseconds.
I often wonder if arc flash that occurs over longer periods of time is as dangerous as the same amount of energy being released in a much shorter period of time. It seems to me that the safety standards I to recognize the difference between releasing a lot of energy over a relatively long period of time versus over a relatively short period of time.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
It seems to me that the safety standards I to recognize the difference between releasing a lot of energy over a relatively long period of time versus over a relatively short period of time.
I would think that should be part of your risk analysis, as you do with the 2 sec rule for an employee to exit the flash zone.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I often wonder if arc flash that occurs over longer periods of time is as dangerous as the same amount of energy being released in a much shorter period of time. It seems to me that the safety standards I to recognize the difference between releasing a lot of energy over a relatively long period of time versus over a relatively short period of time.
Thermal (burn) injury is going to depend far more on the total energy, independent of rate, as long as the temperature is above some minimum value. The ability of PPE to reflect, insulate, and absorb that energy will be only weakly dependent on elapsed time.
Mechanical injury, which conventional PPE is not designed to prevent, is strongly dependent on timing.
Whether there should be standards for mechanical injury protection too is an open question AFAIK.
 
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