Minimum quantities for an arc flash

[mikecj]

Everyone,

Are there minimum thresholds for an arc flash to occur? For example, if I have a 5 volt system with 5 mA I can be pretty sure that nothing bad will happen. I'm much less sure if I have a 120 volt, 20 A system.

I'm trying to develop a relatively easy table or graph that would show the HRC levels for any voltage, amperage combination. Has anyone developed such a table?

I know I'm oversimplifying the calculations but I need this for non-electricians.

Thanks,

MikeCJ

 

[ron]

50V is generally the threshold for electrical danger.

Look for the tables in NFPA 70E Section 130.7(C)(15)(a) .
Even those have notes regarding fault current and clearing time that make them not completely easy to reference.

This is someone's summary http://www.witc.edu/continuing-educ...ries for Alternating Current (ac) Systems.pdf

 

[jtinge]

Also take a look at work done by Los Alamos National Labs. It extends analysis of electrical hazards to cover low voltage-high current, and high voltage-low current like might be associated with battery banks and capacitors.

http://www.nsf.gov/bfa/lfo/seminars/pub/LGORDON_NSF_ElectricalSafety.pdf

 

[petersonra]

50V is generally the threshold for electrical danger.

Look for the tables in NFPA 70E Section 130.7(C)(15)(a) .
Even those have notes regarding fault current and clearing time that make them not completely easy to reference.

This is someone's summary http://www.witc.edu/continuing-educ...ries for Alternating Current (ac) Systems.pdf

generally the 50V threshold is for electrocution hazard, not arc flash, although it is hard to see how an arc blast could happen at such a low voltage.

 

[junkhound]

FWIW, the lowest electrocution I have ever seen referenced in the literature was 18Vac circa 1950s? - never have been able to find the details, the only reference was from a San Francisco insurance company, Indemnity Insurance. I'm assuming it was some hospital accident where there was a needle or similar contact.

I've put pits in my glasses from 30Vac, 280 A arc flash. Sunburned my eyelids from 40Vac, 50A.

Blew a steel garage door out and moved a 40 foot by 20 ft tall 2x6 stud wall 6 inches evenly with a 4megamp capacitor bank arc flash discharge at 10 kV. That event BROKE 250MCM cables due to magnetic forces.

 

[steve66]

There was also a transformer size limit in the IEEE recommendations (or somewhere - Maybe NFPA 70E?). I believe it basically said 208V transformers under 112 KVA were unlikely to create a sustainable arc flash, so equipment powered by transformers smaller than that didn't require arc flash labels.

I can't remember if the transformer size has been reduced, or if this recommendation has been eliminated?

Note that this would not exempt the need for a risk assessment before working on live equipment powered by a smaller panel. As always, its better if the power can be shut off before servicing any equipment. And PPE would still be required to work on a live panel (safety glasses, insulated gloves, etc.).

 

[jumper]

There was also a transformer size limit in the IEEE recommendations (or somewhere - Maybe NFPA 70E?). I believe it basically said 208V transformers under 112 KVA were unlikely to create a sustainable arc flash, so equipment powered by transformers smaller than that didn't require arc flash labels.

I can't remember if the transformer size has been reduced, or if this recommendation has been eliminated?

IIRC, it was 240V and it was removed in 2012.

 

[wbdvt]

IIRC, it was 240V and it was removed in 2012.

It was only removed in NFPA 70E and I believe replaced with a note to say refer to IEEE 1584. The current version of IEEE 1584 contains the statement that basically states that if the equipment is supplied by a transformer 125kVA or less and below 240V, then the equipment does not need to be analyzed for incident energy. Does that mean there is no arc flash hazard? Certainly not and most engineers will default to a less than or equal to value of 1.2 cal/cm2. This insures that some PPE should be worn.

There is rumors that in the next upcoming IEEE 1584 that exemption will be lowered and may be based on bolted fault current not transformer size. The testing for the IEEE 1584-2002 version was only able to sustain an arc at 208V for one specimen.

 

[Saturn_Europa]

There is a story there!