NFPA 70E’s Approach to Considering DC Hazards

NFPA 70E’s Approach to Considering DC Hazards

2012 edition of the standard contains new information  to address the hazards associated with direct current (DC) voltage sources  

Next to the National Electrical Code, NFPA 70E, “Standard for Electrical Safety in the Workplace,” is the largest selling document among more than 300 NFPA codes and standards. Although not a code itself, standards are written to be adopted into law by governmental bodies. NFPA 70E is a recognized consensus standard that was developed at the request of the Occupational Safety and Health Administration (OSHA), a group that can levy penalties for not following best safety practices, such as those outlined in this standard. A companion document is IEEE 1584, “Guide for Performing Arc Flash Calculations, 2002.”

Photovoltaic Array
NFPA 70E has expanded its coverage of DC power systems, which includes PV arrays.

Up until publication of the 2012 edition of NFPA 70E, both of these documents had focused almost exclusively on alternating current (AC). However, this is no longer the case. Additional information has been included in the 2012 edition of NFPA 70E to help protect employees in workplaces where direct current (DC) voltage sources are present, such as large battery systems for telecommunication, data center, photovoltaic, and electric vehicle charging systems — to name just a few. Estimates are that the industry’s understanding today of DC hazards is about where it was 15 to 20 years ago for AC hazards. With the collaboration of the IEEE Stationary Battery Committee’s Code Working Group and the research/development community, however, the NFPA 70E technical committee is adding rules to the standard to close the gap and address these hazards.

Hazard vs. risk

One of the biggest changes in NFPA 70E is to recognize that hazard and risk are two separate concepts. That distinction has not been clear in the past, but will be clarified in the 2015 edition. A hazard (such as high DC voltage in a stored energy system) is a potential source of harm to a worker. Risk is a combination of the likelihood that a harmful incident will occur. While a hazard either does or does not exist, the risk level has varying degrees and may increase or decrease, depending on a number of factors.

The risk level may not only rise as you approach an energized conductor, but it may also increase depending on if and how contact is made. NFPA 70E tells you to first identify the hazard (hazard analysis) and then determine the amount of risk (risk assessment). Minimizing risk starts with good design. For example, the risk associated with a 480V battery system can be reduced if the design allows the battery to be partitioned into low-voltage segments before you work on it. Other mitigation methods, such as separating the positive and negative conductors as far apart as possible and installing insulated covers on battery inter-cell connector bus bars or terminals, are also part of good system design.

Training, of course, is always a prerequisite. Work should always be performed by qualified persons who use safe work practices and the proper tools. Personal protective equipment (PPE) needs to be used judiciously based on the risk, not just on the hazard. In situations where workers believe that excessive PPE limits their dexterity, impairs their vision, or raises their body temperature, additional considerations may be required to reduce the risk and the necessary PPE levels.

Defining the hazard

One of the big discussions within the NFPA 70E technical committee has been “what is a safe voltage?” For most applications, the codes have used 50V rms (which is 70V peak) as the shock threshold. But the peak voltage of 50VDC is still only 50V. However, there is also evidence to suggest that voltage is only one factor; frequency and current must also be considered.

Commentary Table 340.2, published in the 2012 edition of the “NFPA 70E Handbook,” indicates the current and voltage thresholds for physiological effects on a human body are almost twice as high for DC as compared to AC. While some portions of the 2012 standard indicate that 50V is the level where energized conductors are to be considered hazardous, Table 130.7(C)(15)(b) for use of insulating gloves and insulated hand tools and Table 130.4(C)(b) for approach boundaries both identify 100V as the appropriate shock hazard level for DC systems. This disparity may be clarified in the 2015 edition, because public input and comments were submitted requesting harmonization at either the 50VDC or 100VDC level.

The two primary hazards addressed by electrical workers today are shock and arc flash. Because the hazards are different, the techniques, boundaries, and PPE must address the risks associated with both types of hazards.

Following the guidance in Sec. 130.4(A), a simplified approach to shock hazard analysis is:

1) Determine polarity differences of potential;

2) Determine the nearest approach boundary to be crossed based on (new) Table 130.4(C)(b);

3) Apply proper shock protection techniques.

To cross a “limited approach boundary,” you must:

• Be qualified, as per 130.4(D); or

• Be continuously escorted, as per 130.4(D)(2); and

• Work under an energized electrical work permit (EEWP), as per 130.2(B)(1); and

• Use insulated tools and equipment, as per 130.7(D)(1).

For DC shock protection, the limited, restricted, and prohibited approach boundaries are now defined in NFPA 70E-2012 Table 130.4(C)(b). This table identifies four conditions at 11 DC voltage ranges between 100V and 800kV. It is estimated that more than 90% of all DC systems likely to be encountered are in one of the three ranges below 1,000VDC. For example, the prohibited approach boundary ranges from 1 in. when below 300VDC to as much as
16.5 ft when above 500kV. Rubber gloves and insulated tools are required in all cases, as per Table 130.7(C)(15)(b).

The greater challenge has been to quantify the arc flash hazard. Little research has been performed to characterize DC arc flash. The current version of IEEE 1584 has no DC arc flash guidance, nor is it likely that it will have anything about DC arc flash in the next edition either.

NFPA 70E provides some very conservative guidance in Annex D.8 for calculating DC incident energy. The basis for the calculation is that the maximum power possible in a DC arc will occur when the arcing voltage is one-half of the system voltage. What limited testing has been done has shown this approach to be extremely conservative; there are more variables than this simplistic approach provides. However, until further research is conducted, it is the best we have.

NFPA 70E states, “Where selected in lieu of the incident energy analysis of [section] 130.5(B)(1) [then] table… 130.5(C)(15)(b) shall be used to determine the hazard/risk category.” However, it appears that the formula in the Annex and the data in the above referenced table do not always align. The committee is considering a change in the next edition, but, in the meantime, it is strongly recommended to rely on the calculation method provided in Annex D.8. NFPA 70E Handbook Table H.3(b) provides suggestions on selection of PPE when using the calculation method.

One of the most common sources of DC is from a stored electrical energy device such as a capacitor bank, battery, or fuel cell. You might be able to wait for a capacitor to bleed down, and you might be able to turn off a fuel cell, but waiting for the energy to run down on a battery system could take years. Therefore, working on a battery system is always considered energized electrical work — or what used to be called “hot work” (referring to electrical energy, not heat).

Battery Risk Assessment Chart
Fig. 1. When analyzing the risks of working on a battery system, you need to address four key hazard categories.

NFPA 70E-2012 covers batteries and battery rooms in Art. 320. This entire chapter has been modified from the previous edition, which had several installation requirements, such as floor loading, equipment layout, and the like. Such installation requirements are beyond the scope of NFPA 70E, so they were removed. (Note: Many of these removed requirements reappear in Art. 480 of the 2014 edition of the NEC.) Some battery room or enclosure requirements that may sound like installation requirements remain, such as access only by authorized personnel, proper illumination, battery monitoring, and warning signs (e.g., notice of electrical and chemical hazards, requirement for PPE, and limited access).

Batteries are somewhat unique in that they have a chemical hazard as well as an electrical one. Doing electrical work can in some cases expose a worker to the chemical hazard, and vice versa, so both are addressed in Art. 320. Electrolyte (chemical) hazards vary depending on the type of battery, so the risk is product-specific as well as activity-specific. For example, nickel-cadmium (NiCd) and vented lead-acid (VLA) batteries allow access to liquid electrolyte, thereby potentially exposing a worker to a chemical hazard when performing certain tasks. By contrast, valve-regulated lead-acid (VRLA) and certain lithium batteries are designed with solid or immobilized electrolyte so that a worker can only be exposed to electrolyte under failure conditions. NFPA 70E provides PPE guidance for both electrolyte methods.

Thermal Risk Assessment
Fig. 2. Thermal hazards can exist on DC power systems.

A section on DC ground fault detection requires the worker to know the type of DC grounding. An informational note describes four different battery grounding types, some of which normally have ground fault detection — whereas in other types a ground fault detector would be useless or could even create a safety hazard.

Chemical Risk Assessment
Fig. 3. One of the possible dangers of working with batteries is chemical exposure.

A paper presented at the recent 2013 NFPA Conference & Expo in Chicago introduced an example of how a risk assessment might be constructed for a battery system. The same thought process could be applied to other types of systems. It separates the types of hazards: shock, chemical, arc flash, etc., and identifies the likely exposure (risk) depending upon the type of task being performed. A large, complex flow chart from that presentation has been broken down into five smaller flow charts for presentation in this article (see Figures 1 through 5).

(Click here to see Figure 4).

(Click here to see Figure 5).

Note: Three simple messages are embedded in all these figures: (1) The hazard always exists unless it can be engineered out of the design. (2) Risk increases in proportion to one’s exposure to the hazard. The risk is zero if one is not exposed to the hazard. (3) The level of PPE should be proportionate to the degree of risk. The figures shown here are somewhat simplistic and are meant only to be models for evaluating risk.

In summary

NFPA 70E safety requirements are evolving as the knowledge of DC hazards grows. The science of DC arc flash is still not completely understood and will likely remain that way until the industry steps forward with money to fund the necessary research. There is an existing joint research project between NFPA and IEEE to characterize arc flash needs. It is estimated that only about $2 million is needed to complete the DC portion of that particular study. Until the DC hazard is properly quantified, however, the hazard can be determined using the conservative approach found in NFPA 70E in conjunction with a common sense risk assessment similar to the approach shown in the flowchart.

McCluer is the senior manager/external codes & standards, North America for Schneider Electric, working out of the Dallas office. He can be reached at [email protected].

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