Jerry, plant safety manager at the Cornerstone auto plant, smiled as he hung the latest safety commendation plaque on the wall, which he’d just received for another accident-free month. As he stood back to admire how it blended in with the other safety awards, Walter, the plant electrician, charged into his office waving a copy of the 2009 edition of NFPA 70E. “Jerry,” he yelled. “We have a problem.”
Sensing the urgency in Walter’s tone, Jerry promptly asked for an explanation. “The 2009 edition of NFPA 70E changes a lot of things in the 2004 edition that we based our safety program on,” Walter blurted out. “We’re going to have to change our equipment PPE labels, our safety procedures, and our PPE requirements in a number of areas!”
In the end, Walter was correct. This example demonstrates how changes in NFPA 70E can affect electrical safety in all types of workplaces.
Arc flash protection boundary
It’s important to note that the 4-ft rule [130.3(A)(1)] has been greatly changed. In the 2004 edition of the standard, the arc flash protection boundary (AFPB) for systems 600V or less was 4 ft — based on a fault clearing time of six cycles and a bolted 3-phase fault current of 50kA. In the 2009 edition, the fault clearing time was reduced to two cycles for the same fault current, representing a threefold reduction in incident energy. NFPA 70E-2009 requires calculation of the AFPB when the product of the fault current (in kiloamperes) and the clearing time in cycles exceeds 100kA cycles.
In the 2004 edition, there was an expression for calculating the AFPB for cases that exceeded the parameters for the 4-ft criterion. That expression has been removed from the body of the standard and placed in Annex D, which is labeled, “This annex is not part of the requirements of this NFPA document but is included for informational purposes only.”
The 2009 edition of NFPA 70E still requires calculation of the AFPB for conditions that exceed the 4-ft criterion parameters in the 2009 edition. However, no guidance is given on how to do that — no reference to Annex D. If the product of the current and time exceeds 100kA cycles and is, let’s say, 150kA cycles — and the expression in Annex D of the 2009 edition (same as in the 2004 edition) [D.2(d)] is used to calculate the AFPB — the result is about 2.5 ft, which is significantly less than 4 ft.
Figure 1 (click here to see Figure 1) shows the NFPA 70E defined shock protection boundaries and a generic AFPB. The locations of the shock boundaries are clearly indicated in Table 130.2(C) of NFPA 70E.
The 2009 edition also introduced changes in training requirements. Employee training must now be documented, and the electrical safety program must be audited to ensure that the principles and procedures of the electrical safety program are being followed. The use of an outside auditing company is advisable because of possible conflicts of interests in having an internal team perform this function.
Even if an independent entity is used to do the audit, the frequency of the audit is left up to the employer. However, this defeats the purpose of an audit unless the plant safety manager is given that authority. A recent IEEE Industry Applications magazine article entitled “Auditing is the Key,” authored by Shahid Jamil, John D. Aeiker, and Daryld Ray Crow, which appeared in the January/February 2010 issue, forms the basis of a good auditing process to ensure an effective electrical safety program.
Overcurrent protection, meetings, and documentation
A number of additions and deletions in the 2009 version of the standard are outlined in the sidebar Requirements Added or Deleted in the 2009 Edition of NFPA 70E. One item that requires the maintenance of overcurrent protection in accordance with manufacturers’ instructions or industry consensus standards is particularly significant. This new requirement is meant to save lives, reduce personal injuries, and minimize or eliminate equipment damage.
Section 110.4(B) of the 2004 edition required that the coordination between the host employer and the contract employer include a meeting and documentation. The 2009 edition does not include this requirement. In the event of an accident, the information provided by the host employer to the contract employer is sometimes disputed. How can this be resolved without some type of mutually agreed upon documentation? Even if NFPA 70E does not require it, documentation of the coordination between the parties is essential and may become a key factor in the event of a lawsuit following an incident in which someone is seriously injured.
What’s not included in the standard?
Two issues of ongoing concern to engineers have not been addressed in the 2009 edition of this standard. First is the lack of guidance on how to calculate the appropriate short circuit values to be used in arc flash analysis; the second is guidance on what type of arc flash analysis method to use.
Typically, when an electrical engineer is called upon to perform a short circuit calculation, he or she is sizing electrical equipment for short circuit current withstand capability or selecting breaker sizes. In these cases, the engineer will attempt to determine the maximum anticipated short circuit current that the equipment may experience, which is the correct approach for these types of calculations. When the same engineer is asked to perform an arc flash analysis, he or she may automatically take the same approach. The assumption is that higher short circuit currents yields higher incident energies. However, this is not necessarily true.
To understand this better, let’s look at the circuit shown in Fig. 2 (click here to see Fig. 2). If an electrician is working on the terminal box of this motor and a short circuit occurs in the box, the arc incident energy released can be higher than at the upstream panel or motor control center (MCC). You may argue that the cable will reduce the short circuit current from the value at the MCC — and you’d be right. That’s the problem.
Place Fig. 2 here.
<<caption>>PPE levels change based on where you’re working along the circuit path.
If the fault current is 1,200A at the panel, the protective device shown in Fig. 3 (click here to see Figure 3) will interrupt it in about 0.012 sec. That results in an I2t value of 17.28kA sec. This is a rough measure of the incident energy to which a worker will be exposed. But if the cable between the panel and the motor terminal box reduces the short circuit current to 1,000A at the motor terminals, the interrupting time is now 50 sec and the I2t value is now 50,000kA sec, which is significantly higher than 17.28kA sec. Ultimately, the worker is exposed to a greater incident energy and thus potentially more severe burns and injury. Therefore, the presence and characteristics of protective devices must be taken into account when selecting the short circuit current for use in the arc flash analysis and consequently the selection of the appropriate personal protective equipment (PPE) for working on the equipment.
Currently, there is no arc flash analysis method in the body of the standard. Annex D to NFPA 70E, which is not part of the standard and is only provided for informational purposes, includes two methods that can yield drastically different results. I addressed this issue in a paper I presented at the 2006 NFPA World Safety Conference in Orlando entitled, “Are You Safe at 4 Feet?”
This same issue was also raised by Benjamin O. Medich, P.E., Ballinger, in a September 2007 EC&M article entitled, “Calculating Arc Flash Energy Levels.” The Medich article offers an approach that requires the engineer to do the analysis using both methods in NFPA 70E Annex D and then selecting the most conservative results. Software packages, which offer arc flash analysis capabilities, generally include both methods. The engineer can execute the program for each method separately and choose the conservative result. This is a reasonable approach and something I hope future editions of this standard might address.
Many industrial owners use the NFPA 70E “Table Approach” for labeling electrical equipment to indicate the required level of worker PPE. Although this is generally an acceptable approach, in cases in which the short circuit conditions at a piece of equipment exceed those for which the table is applicable, NFPA 70E requires an arc flash analysis be performed in accordance with 130.3. However, 130.3 does not provide any guidance as to how it should be conducted.
Line by line comparison
When comparing the 2004 and 2009 editions of NFPA 70E directly, the differences become clear. To review changes and deletions to the 2004 edition of the standard, visit EC&M’s website for additional information. The Table, which was too lengthy to print in its entirety in this article, is available exclusively online. Besides changing some of the 2004 requirements, some new ones have been added to Table 130.7(C)(9). For example, infrared testing done regularly to detect any equipment hot spots will now have to be conducted by workers wearing various levels of PPE ranging from Hazard/Risk Cat. 1 to 3, depending on the voltage level and equipment type.
Another addition to this table is the identification of PPE levels for work on energized conductors and circuit parts fed directly by a branch circuit of a panelboard, switchboard, or MCC. Refer to the above discussion of the short circuit current at the motor terminals versus the panel feeding the motor for an understanding of this change.
The 2004 edition of NFPA 70E required a PPE level of 2 for 600V switchgear if the doors are closed when racking a breaker and a level of 3 when the doors are open. To be conservative, such equipment would have been labeled PPE 3. Now, according to the 2009 edition, it should be labeled PPE 4, whether the doors are open or closed. Many plants will need to review their equipment labeling and re-labeling practices as necessary based on the revised table or on calculations if the table parameters are exceeded.
As can be seen from the Table, there are quite a few pieces of equipment where the 2009 edition requires higher PPE levels than the 2004 edition, and changes to NFPA 70E do affect the safety of electrical workers. Compliance with NFPA 70E is not a one-time set of actions but rather an ongoing activity as the standard evolves — it must become part of an industrial plant’s ongoing audited safety program.
McCauley is a forensic engineer and president of Consolidated Consulting Corp. in River Forest, Ill. He can be reached at [email protected]