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It seems that NFPA 70E is not quite matured yet with regard to PPE selection in Table 130.7(C)(9)(a).
It states:
The assumed short-circuit current capacities and fault clearing times for various tasks are listed in the text and notes to Table 130.7(C)(9)(a). For tasks not listed, or for power systems with greater than the assumed short-circuit current capacity or with longer than the assumed fault clearing times, a flash hazard analysis shall be required in accordance with 130.3.
FPN No. 1: Both larger and smaller available short-circuit currents could result in higher available arc-flash energies. If the available short-circuit current increases without a decrease in the opening time of the overcurrent protective device, the arc-flash energy will increase. If the available short-circuit current decreases, resulting in a longer opening time for the overcurrent protective device, arc-flash energies could also increase.
It essentially says that if your fault current or clearing times vary from the tested condition, the table isn't reliable. I would maintain that with variable fault currents available from the utility and OCPD's, you can't use the table.
Ron
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Thanks Ron, I was aware that you were also interested in 70E and appreciate your comments. Here's a bit from the front matter in 70E that I would like opinions on too: The seventh edition, published in 2004, reflects several significant changes to the document. The major changes emphasize safe work practices. Clarity and usability of the document were also enhanced. The name of the document was changed to NFPA 70E, Standard for Electrical Safety in the Workplace. The entire document was reformatted to comply with the NEC Style Manual, providing a unique designation for each requirement. The existing parts were renamed as chapters and were reorganized with the safety-related work practices relocated to the front of the document to highlight the emphasis, followed by safety-related maintenance requirements, safety requirements for special equipment, and safety-related installation requirements. The chapter on safety-related work practices also was reorganized to emphasize working on live parts as the last alternative work practice. An energized electrical work permit and related requirements were incorporated into the document. Several definitions were modified or added to enhance usability of the document, and Chapter 4 was updated to correlate with the 2002 NEC.
Essential to the proper use of Chapter 4 of this standard is the understanding that it is not intended to be applied as a design, installation, modification, or construction standard for an electrical installation or system. Its content has been intentionally limited in comparison to the content of the NEC in order to apply to an electrical installation or system as part of an
employee’s workplace. This standard is compatible with corresponding provisions of the NEC, but is not intended to, nor can it, be used in lieu of the NEC. It can be debated that all of the requirements of the NEC, when traced through a chain of events, may relate to an electrical hazard, but, for practical purposes, inclusion has not been made of those provisions that, in general, are not directly associated with employee safety. In determining what provisions should be included in Chapter 4, the following guidelines were used:
(1) Its provisions should give protection to the employee from electrical hazards.
(2) Its provisions should be excerpted from the NEC in a manner that maintains their intent as they apply to employee safety. In some cases it has been judged essential to the meaning of the excerpted passages to retain some material not applying to employee safety.
(3) The provisions should be selected in a manner that will reduce the need for frequent
revision, yet avoid technical obsolescence.
(4) Compliance with the provisions should be determined by means of an inspection during
the normal state of employee occupancy without removal of parts requiring shutdown of the electrical installation or by damaging the building structure or finish.
(5) The provisions should not be encumbered with unnecessary details.
(6) The provisions should be written to enhance their understanding by the employer and employee.
(7) The provisions must not add any requirements not found in the NEC, nor must the intent of the NEC be changed if the wording is changed.
Chapter 4 of NFPA 70E was therefore intended to serve a very specific need of OSHA
and is in no way intended to be used as a substitute for the NEC. Omission of any requirements presently in the NEC does not in any way affect the NEC, nor should these omitted requirements be considered as unimportant. They are essential to the NEC and its intended application, that is, its use by those who design, install, and inspect electrical installations.
NFPA 70E, on the other hand, is intended for use by employers, employees, and OSHA. Please note that Chapter 4 was updated to correlate with the 2002 NEC. Look here for some useful information concerning this subject: http://www.ieisa.com/Arcblast.html [This message has been edited by Joe Tedesco (edited 06-28-2005).]
Joe Tedesco, NEC Consultant
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Around here OSHA is virtually all employer driven. There is not an office within 150 miles and the go teams only respond to serious accidents. When something does happen the fines get piled on and the teams may look next door while they are there but there is no real OSHA inspection program. It will be the employer's risk analysis department and the insurance carrier that drives the day to day monitoring of compliance. This will become a job responsibility (and "condition of employment") for the field supervisor in any company I would want to work for. Safety starts in the field and has to include the dilligence of EVERYONE on the job site. Employees should use the appropriate PPE and challenge anyone who is not using it.
These days, with lawyers driving "risk", it could literally be YOUR job at stake when some 3d party gets hurt and your company gets sued out of existance.
Greg Fretwell
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Mike, However, with working with this sort of stuff all the time, I carry a "Fault-Loop Impedance meter", from getting that value from the meter, you can work out through basic Ohms Law, what the actual Fault current value will be. I have never seen or worked with that type of equipment. Can you tell more about how it is used and what it does? Thanks Don
Don(resqcapt19)
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Don, I have never seen or worked with that type of equipment. Can you tell more about how it is used and what it does? There is a thread here that we had about Fault loop testing. If you have any more questions regarding this sort of testing, by all means, ask away. ![[Linked Image]](https://www.electrical-contractor.net/ubb/wink.gif) Mike. ![[Linked Image]](https://www.electrical-contractor.net/mt/EP.gif) [This message has been edited by Trumpy (edited 06-29-2005).]
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Mike,
I looked at the tester information and I don't see how they give me a real world fault current. It appears to me that they use the impedance and the voltage to do a calculation. I don't see how that takes into account the strength of the supply system. It appears that this is more of an infinite bus type of fault current calculation and would not be useful for PPE selection.
Don
Don(resqcapt19)
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Don, I don't see how that takes into account the strength of the supply system Just what do you mean by that Don?. Exactly what is an infinite bus?, I'm sure I can help. ![[Linked Image]](https://www.electrical-contractor.net/ubb/smile.gif)
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Mike,
Systems can only supply so much fault current. It looks like the testers calculate an available fault current based on circuit impedance, voltage and the assumption that the supply system can supply unlimited current (infinite bus).
The calculations for selection the correct PPE require an actual available fault current and not a theoretical maximum. The incident energy produce by the arc often goes up with a lower available fault current. This is because a lower current will often result in a longer time before the OCPD operates and this results in a higher incident energy and will require a higher level of PPE protection.
Don
Don(resqcapt19)
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