Application of 2007 NESC Arc-Flash Requirements
Application of 2007 NESC Arc-Flash Requirements
(OP)
I'm curious how others are dealing with the 2007 NESC arc-flash protection requirements and the apparent contradictions with existing NFPA 70E?
In particular, I'm wondering about the provisions for dealing with systems less than 1000 V. The NESC says that in lieu of doing an actual arc-flash study, a hazard of 4 cal/cm2 can be assumed under 1000 V. For 480 V systems, this can be absurdly low when compared with the requirements of NFPA 70E and IEEE 1584. This is especially true when dealing with the 480V portion of a pad-mounted transformer that probably has minimal primary protection.
Does anyone know the basis or origin of the 4 cal/cm2??
Also, for substations using metal-clad switchgear, the IEEE 1584 calculations would seem to be much more relevant than the NESC tables when dealing with switchgear. Anyone made any decisions on this?
In particular, I'm wondering about the provisions for dealing with systems less than 1000 V. The NESC says that in lieu of doing an actual arc-flash study, a hazard of 4 cal/cm2 can be assumed under 1000 V. For 480 V systems, this can be absurdly low when compared with the requirements of NFPA 70E and IEEE 1584. This is especially true when dealing with the 480V portion of a pad-mounted transformer that probably has minimal primary protection.
Does anyone know the basis or origin of the 4 cal/cm2??
Also, for substations using metal-clad switchgear, the IEEE 1584 calculations would seem to be much more relevant than the NESC tables when dealing with switchgear. Anyone made any decisions on this?






RE: Application of 2007 NESC Arc-Flash Requirements
Have you seen the change proposals? Here is what the working group said:
"The working group rejected the original CPs and agreed to develop a revised CP that would address worker
safety associated with electric arc exposure, while at the same time acknowledging industry needs and unique
working conditions. It was recognized by the Working Group that rule changes in Section 41 for employers,
and Section 42 for employees, were necessary to accomplish this task. This CP focuses on those issues and
will provide work rules that are appropriate for performing work associated with electric supply and communication
systems."
I think we are seeing the compromise between those who are looking at the potential hazard and those who are looking at the actual safety record of the industry. Some of the dissenting comments are interesting too.
I agree that the arc in a box method of IEEE is more relevant for metal clad than the open air single phase fault that ArcPro (and NESC tables) use.
RE: Application of 2007 NESC Arc-Flash Requirements
Thanks - are the Change Proposals accessible on-line? I am interested in figuring out how this came about. I suspect you're right - as usual a compromise.
Dave
RE: Application of 2007 NESC Arc-Flash Requirements
RE: Application of 2007 NESC Arc-Flash Requirements
So far I've structured my approach using the NESC tables for live-line work, ArcPro for work in the area but not on energized equipment (sub inspections and the like) and use PTW w/ either NFPA 70E or IEEE 1584 methods for the < 1000 volt work. You can't use the tables < 1000 volts and the 4 cal/cm2 alternate is absurd if you look at calculated results. Switchgear configuration for padmounts and panel for metering sockets? Trying to catch work performed by meter technicians, locators, tree-trimmers, etc. with the appropriate working exposures.
I agree with you on using IEEE 1584 for metal-clad gear and MV faults although I don't believe the commercially available software is applicable for MV systems? I've also started conversations with Cooper Power Systems on their standard bayonet fusing charts and the resulting exposures for padmounted applications looking for a different fuse or different sizing.
When using the NESC table for live-line work are you just considering the total clearing time of the primary protective device or the backup device per IEEE 1584? How do you handle intermediate fault values, especially between 0 and 5kA?
I have some utility customers encountering conflicts with installations where utility equipment is located in customer switchgear - which code applies? The Utility says today they only need FR or to prove cotton won't combust - the facitlity owner says the utility must meet the Owner's rated FR standard under NFPA 70E. The utility does not have FR rated to the 70E calculated values so we countered with requiring a plant outage - not well received.
I think this new code needs a lot of discussion - have you considered requesting any interpretations from the code committee?
Dave
RE: Application of 2007 NESC Arc-Flash Requirements
Thanks for the input. I think we're both seeing this in basically the same way.
Just a couple of points:
I thought the tables in NESC were computed using ArcPro, so for similar conditions and assumptions, ArcPro calcs should match the NESC table?
IEEE-1584 has methods for calculation medium-voltage arc-flash. Above 15 kV, I believe they simply use the Lee equations, but below 15 kV, there are specific equations in 1584. We use EasyPower and it calculates medium-voltage (and above) arc-flash levels using IEEE-1584. This still seems appropriate to me for metal-clad switchgear, even in utility substation.
We don't consider backup protection (breaker failure) per se in doing arc-flash calculations per NFPA 70E and IEEE 1584. For main breakers you do have to use the next device upstream to determine fault duration.
I can (sort of) see some logic to the 4 cal/cm2 value for 120/240V and 208V systems, but at least for 480V and above, we will use the IEEE 1584 calculations.
RE: Application of 2007 NESC Arc-Flash Requirements
"For main breakers you do have to use the next device upstream to determine fault duration."
But that would be only if you need the AFH on the line side of the breaker, say between the low side of the transformer to the line side of the main breaker, Correct?
Also are you using the CLF feature in EasyPower?
RE: Application of 2007 NESC Arc-Flash Requirements
Right - for any work on the main breaker compartment or section, you would have to assume the fault could be on the line side of the breaker, so the fault would have to be cleared by the next upstream device.
I generally don't bother with the CLF option in EasyPower. Just based on time of clearing, the arc-flash becomes so low that the difference isn't worth worrying about. The big problem with fuses is that often the fault current is not high enough to get into the current-limiting range of the fuse. This is especially true when the fuse is on the primary of a transformer and fault is on the secondary side.
RE: Application of 2007 NESC Arc-Flash Requirements
RE: Application of 2007 NESC Arc-Flash Requirements
RE: Application of 2007 NESC Arc-Flash Requirements
They did an accompanying article on our project that is linked in the first paragraph or so under (see box).
RE: Application of 2007 NESC Arc-Flash Requirements
RE: Application of 2007 NESC Arc-Flash Requirements
RE: Application of 2007 NESC Arc-Flash Requirements
If your logic is that we don't need to worry about PPE because we have a very reliable system and we know what we are doing, you aren't going to get very far in discussions with OSHA, etc.
You can separate it into two categories: Hazard and Risk.
For arc-flash, the **hazard** is determined by the worst case arc energy that can be released at a particular point in the system, based on the physical location, the fault current and the clearing time. The hazard does not change unless you change the system. The **risk** is a function of what is being done. Someone just opening a cubicle door is at a much lower risk than someone who is racking a breaker into an energized bus.
But if there is a fault, the energy released would be basically the same. So does it make sense to allow someone just opening a door to wear a lower level of PPE than someone racking in a breaker for the same HAZARD level. That's the question. It's a valid question and a legitimate position. I don't agree with it, but others support it.
NFPA 70E 2004 does not provide a mechanism for this, but I'm sure it under consideration in the upcoming revision. Stay tuned.
RE: Application of 2007 NESC Arc-Flash Requirements
RE: Application of 2007 NESC Arc-Flash Requirements
Just to add to your point:
The current arc flash analysis standards and recommendations for PPE are ONLY to minimize "burn" induries due to arc-flash. All other common sense precautions to safe guard against flying debris and shrapnels are still to be implemented and are not and nor can they be covered by code books.
Any class of PPE in cases like the one described by you are of no help.
RE: Application of 2007 NESC Arc-Flash Requirements
RE: Application of 2007 NESC Arc-Flash Requirements
I see like a lot of good discussion, but I think I missed the answer to the original problem...
Is it a good approx to assume 4 cal/cm^2 for <1000V systems??
The calculations for the arc energy are very dependent on the arc duration...
Has anyone seen test data where they've actually been able to get a LV arc to sustain long enough to get the calculated energy exposure?
RE: Application of 2007 NESC Arc-Flash Requirements
4 cal/cm2 is much too low for a 480 V system. It might be a reasonable starting point for a residential 120/240 V system, but I don't know of anyone who think it makes sense for 480 V. I think the NESC committee just punted on this one.
480 V and 277 V arcs can be sustained for a long, long time. At 240 V it is much harder to sustain an arc, and even more difficult (but not impossible) at 208 V and 120 V.
IEEE 1584 has a method for calculating approximate arc-flash energies at voltages to 15 kV based on actual test data. 480 V has tremendous arc-flash potential, but arcing currents will be significantly reduced from bolted fault currents.
RE: Application of 2007 NESC Arc-Flash Requirements
The current arc flash standards and guidlines (IEEE 1584 and NFPA 70E) are only intened to safeguard person workig live on it, so the maximum time to be considered is 2 seconds. The thinking is that by that time either the person is removed from the fault or is dead or is entangled beyond protection. So it does not matter how long the arc persists. The arc flash energy calculated in the total accumulated for the entire event but only long enough as it pertains to safety of the personnel NOT the equipment.
RE: Application of 2007 NESC Arc-Flash Requirements
RE: Application of 2007 NESC Arc-Flash Requirements
Per the calcs I agree with Breakerboy, 4 cal/cm^2 is certainly not a good number to use for a rule of thumb...
that is if the standards have a correct approximation method...
would be interested to see any test data of the LV side of life...