I manage a group of electricians and technicians that install, test, and maintain fire alarm panels. In our company's efforts to apply 70E and other safety guidance, our energized equipment definition includes >50V per 70E, but also includes >10joules. If you do the math, the 10J criteria makes it nearly impossible to work near any exposed connection in a fire alarm panel (e.g. to simply plug in a RJ45 cable to download a program) without donning PPE as if we were working on live terminals in a 120V panelboard. The hazards related to the 10J criteria are typically discussed in the context of the stored energy of a capacitor - and I acknowledge that it is generally accepted that this energy level can be fatal. However, I can't help but wonder if we are mis-applying the 10J criteria when evaluating a 24VDC notification appliance circuit or 25V speaker circuit. While these circuits can deliver 10J of energy, the math doesn't seem to support the delivery of this energy into the "load" of a human being. Does anyone know of some formal guidance or standards that would provide more specific information for <50V systems?
NFPA 70E - less than 50V
- Thread starter brianz
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- New York, 40.7514,-73.9925
Re: NFPA 70E - less than 50V
Where does the 10 joule number come from? Most references to PPE relate to cal/cm^2
Where does the 10 joule number come from? Most references to PPE relate to cal/cm^2
Re: NFPA 70E - less than 50V
The 10J criteria can be found in the electrical safety programs for university and DOE research laboratories. As far as where it came from or its basis...I don't know. However, if you Google on "10 Joules" or "10 Joule Hazard" you'll find sites that discuss the fact that 10J can be fatal. The hazard is discussed almost exclusively in the context of capacitors and inductors.
The 10J criteria can be found in the electrical safety programs for university and DOE research laboratories. As far as where it came from or its basis...I don't know. However, if you Google on "10 Joules" or "10 Joule Hazard" you'll find sites that discuss the fact that 10J can be fatal. The hazard is discussed almost exclusively in the context of capacitors and inductors.
zog
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- Charlotte, NC
Re: NFPA 70E - less than 50V
Actually the 70E is changing all calorie/cm2 references to Joules/cm2 (So it is all metric). 1 Calorie=4.1868 Joules so 10 Joules = 2.39 Calories. 1.2 cal/cm2 is the onset of a 2nd degree burn so I would agree that your 10J system has a sufficent arc flash hazard.
Additionally you need to review the 50V rule. The 70E states "Energized parts that operate at less than 50V to ground shall not be required to be denergized if there will be no increased exposure to electrical burns or to explosion due to electric arcs."
That being said, where are you getting 10 Joules from? From your description I doubt there really is a 10J/cm2 hazard at 18" in this panel.
Actually the 70E is changing all calorie/cm2 references to Joules/cm2 (So it is all metric). 1 Calorie=4.1868 Joules so 10 Joules = 2.39 Calories. 1.2 cal/cm2 is the onset of a 2nd degree burn so I would agree that your 10J system has a sufficent arc flash hazard.
Additionally you need to review the 50V rule. The 70E states "Energized parts that operate at less than 50V to ground shall not be required to be denergized if there will be no increased exposure to electrical burns or to explosion due to electric arcs."
That being said, where are you getting 10 Joules from? From your description I doubt there really is a 10J/cm2 hazard at 18" in this panel.
Re: NFPA 70E - less than 50V
The 10J concerns come from the exposed terminals of a circuit in the panel that is rated, for example, 24VDC, 3A. The present thinking is that since 1 Joule = 1W-sec, then the connection represents an energy availability of 72J. Perhaps this is an incorrect way to evaluate the hazard?
The 10J concerns come from the exposed terminals of a circuit in the panel that is rated, for example, 24VDC, 3A. The present thinking is that since 1 Joule = 1W-sec, then the connection represents an energy availability of 72J. Perhaps this is an incorrect way to evaluate the hazard?
petersonra
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- Northern illinois
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- engineer
Re: NFPA 70E - less than 50V
I think you need to rethink a little.
IIRC, the energy release that triggers the different PPE requirements is not total energy released but the energy released over a specified cross sectional area until the fault is cleared.
You need to think about whether the capacitor can in fact discharge the 10 J of stored energy fast enough to trigger the standard. I do not believe this is very likely.
I think you need to rethink a little.
IIRC, the energy release that triggers the different PPE requirements is not total energy released but the energy released over a specified cross sectional area until the fault is cleared.
You need to think about whether the capacitor can in fact discharge the 10 J of stored energy fast enough to trigger the standard. I do not believe this is very likely.
Re: NFPA 70E - less than 50V
Thanks to all for the responses.
I'm assuming the standard that is "triggered" as was stated...is NFPA 70E (true?). As to "rethinking"...I hope that is the result of my post. Guidance in our electrical safety documentation includes the following statements:
"Conditions for a serious (potentially lethal) shock across a critical path, such as the heart, are:
1.More than 30-V root mean square (rms), 42.4-V peak, or 60 V dc at a total impedance of less than 5000 ohms.
2.10 to 75 mA.
3.More than 10 J."
As you can see, 10J is simply stated without qualification as to the type of source, the rate of release, or the energy/cross sectional area. So, what is the explanation that the simple example of a pair of terminals exposing a 24VDC/3A control circuit is not the hazard indicated by #3 above?...Or, is it?
Thanks to all for the responses.
I'm assuming the standard that is "triggered" as was stated...is NFPA 70E (true?). As to "rethinking"...I hope that is the result of my post. Guidance in our electrical safety documentation includes the following statements:
"Conditions for a serious (potentially lethal) shock across a critical path, such as the heart, are:
1.More than 30-V root mean square (rms), 42.4-V peak, or 60 V dc at a total impedance of less than 5000 ohms.
2.10 to 75 mA.
3.More than 10 J."
As you can see, 10J is simply stated without qualification as to the type of source, the rate of release, or the energy/cross sectional area. So, what is the explanation that the simple example of a pair of terminals exposing a 24VDC/3A control circuit is not the hazard indicated by #3 above?...Or, is it?
petersonra
Senior Member
- Location
- Northern illinois
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- engineer
Re: NFPA 70E - less than 50V
you are confusing two seperate issues.
arc flash is a totally different event from electrocution.
arc flash issues requires PPE to be donned that is dependant on what level of energy is liekly to be released in the event of a fault.
electrocution prevention requires only insulation from the voltage source.
you are confusing two seperate issues.
arc flash is a totally different event from electrocution.
arc flash issues requires PPE to be donned that is dependant on what level of energy is liekly to be released in the event of a fault.
electrocution prevention requires only insulation from the voltage source.
Re: NFPA 70E - less than 50V
You're right, thanks for making the distinction. So, it would also appear that the guidance from the safety folks in my company may be confusing the two issues or at least combining them into a single set of criteria - perhaps for simplicity of the guidance. Right or wrong, the hazard guidance I must follow, as quoted in my previous post, is clearly framed from a shock hazard perspective. Although the prime concern regarding the 10J criteria may be flash hazard, I believe it is also true that paralysis of chest muscles and ventricular fibrillation occurs in a range from 2.4W to 12W, which unfortunately looks a lot like 10J at the high end. Given that shock hazard would be the main concern of our safety folks in my simple scenario (24VDC/3A), maybe a better question for me to ask is what does "Not Specified" mean in NFPA 70E Table 130.2 for the Prohibited Approach Boundary for <50V systems? Does it mean that any approach distance >0.03" (from Annex C, C.2.1) is acceptable? "Avoid Contact" as used for the 50-300V range seems more clear.
You're right, thanks for making the distinction. So, it would also appear that the guidance from the safety folks in my company may be confusing the two issues or at least combining them into a single set of criteria - perhaps for simplicity of the guidance. Right or wrong, the hazard guidance I must follow, as quoted in my previous post, is clearly framed from a shock hazard perspective. Although the prime concern regarding the 10J criteria may be flash hazard, I believe it is also true that paralysis of chest muscles and ventricular fibrillation occurs in a range from 2.4W to 12W, which unfortunately looks a lot like 10J at the high end. Given that shock hazard would be the main concern of our safety folks in my simple scenario (24VDC/3A), maybe a better question for me to ask is what does "Not Specified" mean in NFPA 70E Table 130.2 for the Prohibited Approach Boundary for <50V systems? Does it mean that any approach distance >0.03" (from Annex C, C.2.1) is acceptable? "Avoid Contact" as used for the 50-300V range seems more clear.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: NFPA 70E - less than 50V
you do not need any seperation distance to prevent electrocution, only insulation, and that is not always needed.
arc flash is a totally seperate issue, having nothing to do with electrocution at all. it is a violent burst of energy, VERY hot, that can cause very serious burns if enough energy is released. the only safe way to deal with arc flash is to stay away from an area there is an arc flash hazard. If you are forced to enter such an area, you must have the proper training and PPE suitable to protect you from the flash if it were to occur.
A 9V battery is quite capable of causing a spark if shorted out, but the arc generated is not a serious hazard to you. It may well cause a small burn, but its not likely to be serious (although a 9V NiCAD battery can release enough current thata short circuit can get very warm).
A 480V panel with a large amount of short circuit current available can generate a huge arc flash releasing an enormous amount of energy in a very short period of time right on top of you. If you are the recipient of such an event, you will be seriously injured or die unless you have taken proper precautions.
you do not need any seperation distance to prevent electrocution, only insulation, and that is not always needed.
arc flash is a totally seperate issue, having nothing to do with electrocution at all. it is a violent burst of energy, VERY hot, that can cause very serious burns if enough energy is released. the only safe way to deal with arc flash is to stay away from an area there is an arc flash hazard. If you are forced to enter such an area, you must have the proper training and PPE suitable to protect you from the flash if it were to occur.
A 9V battery is quite capable of causing a spark if shorted out, but the arc generated is not a serious hazard to you. It may well cause a small burn, but its not likely to be serious (although a 9V NiCAD battery can release enough current thata short circuit can get very warm).
A 480V panel with a large amount of short circuit current available can generate a huge arc flash releasing an enormous amount of energy in a very short period of time right on top of you. If you are the recipient of such an event, you will be seriously injured or die unless you have taken proper precautions.
zog
Senior Member
- Location
- Charlotte, NC
Re: NFPA 70E - less than 50V
"you do not need any seperation distance to prevent electrocution, only insulation, and that is not always needed"
Whoa! So I can just stay 1" away from a 69kV bus and not get shocked??? Both the NFPA 70E and OSHA 1910.269 have minimum approach distances for electrical shock. I hope you were refering to 120V systems with that statement.
"you do not need any seperation distance to prevent electrocution, only insulation, and that is not always needed"
Whoa! So I can just stay 1" away from a 69kV bus and not get shocked??? Both the NFPA 70E and OSHA 1910.269 have minimum approach distances for electrical shock. I hope you were refering to 120V systems with that statement.
Re: NFPA 70E - less than 50V
Thanks for all the contributions to this post. I have calculated the flash protection boundary per 70E, 130.3(A) for various transformer ratings. It is unlikely we have a calculated flash protection boundary that is >12".
Thanks for all the contributions to this post. I have calculated the flash protection boundary per 70E, 130.3(A) for various transformer ratings. It is unlikely we have a calculated flash protection boundary that is >12".
Is there something special about 18"?
zog
Senior Member
- Location
- Charlotte, NC
Re: NFPA 70E - less than 50V
18" is the distance most arc flash calculations are figured. You have to pick a distance to do a cal (or joule)/cm2 calculation, 18" is the average distance from a worlers outstreched hand to thier chest.
When you calculate a flash protection boundary you are actually calculating at what dostance there is 1.2 cal/cm2 of heat energy, to determine what PPE is necessary you need to find the amount of heat energy at the recicing surface which is usually 18".
18" is the distance most arc flash calculations are figured. You have to pick a distance to do a cal (or joule)/cm2 calculation, 18" is the average distance from a worlers outstreched hand to thier chest.
When you calculate a flash protection boundary you are actually calculating at what dostance there is 1.2 cal/cm2 of heat energy, to determine what PPE is necessary you need to find the amount of heat energy at the recicing surface which is usually 18".
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: NFPA 70E - less than 50V
I stand corrected. For normally encountered voltages insulation will suffice.
zog
Senior Member
- Location
- Charlotte, NC
Re: NFPA 70E - less than 50V
"Can anyone comment on the meaning of the 70E statement as it relates to a <50 VDC circuit?"
All voltages in the 70E are AC RMS or DC , it is the same thing.
"Can anyone comment on the meaning of the 70E statement as it relates to a <50 VDC circuit?"
All voltages in the 70E are AC RMS or DC , it is the same thing.
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