In a fraction of a second, an electrical incident can claim lives and cause permanently disabling injuries. In fact, hundreds of deaths and thousands of burn injuries occur each year due to shock, electrocution, arc flash, and arc blast — and most could be prevented through compliance with NFPA 70E: Standard for Electrical Safety in the Workplace®. Originally developed at OSHA’s request, NFPA 70E responds to the latest information about the effects of arc flash, arc blast, and direct current (dc) hazards, and recent developments in electrical design and Personal Protective Equipment (PPE).
The 2015 NFPA 70E helps you assess electrical risks on the job, making users more aware of the potential for devastating loss.
The 2015 edition of NFPA 70E introduces a major change in how stakeholders evaluate electrical risk — so that owners, managers, and employees can work together to ensure an electrically safe working area and comply with OSHA 1910 Subpart S and OSHA 1926 Subpart K.
- Key changes throughout the Standard replace the phrase “hazard analysis” with “risk assessment” to enable a shift in awareness about the potential for failure.
- Change in naming from “Hazard Risk Category” to “Arc Flash PPE Category.”
- Elimination of Hazard Risk Category 0.
- Requirement added for proper maintenance of electrical equipment for both energized and de-energized maintenance.
- Updated tables add clarity to requirements, such as the restricted approach boundary dimensions in Table 130.4 (D)(a).
- New requirement 320.3 (A)(1) covers risk assessment associated with battery work.
- New subsection in 130.2 (A)(4) provides requirements where normal operation of electric equipment is permitted.
- Informative Annex E has updated text to correlate with the redefined terminology associated with hazard and risk. This annex provides clarity and consistency about definitions as well as risk management principles vital to electrical safety.
What is Emissivity?
The material emissivity (written as “?” or “e”) is the relative power of its surface to emit heat by radiation. Materials are assigned an emissivity value between 0 and 1.0. Emissivity is a measure of a material’s ability to emit infrared energy. The emissivity of a surface is the ratio of the energy radiated from it to that from a blackbody at the same temperature, the same wavelength and under the same viewing conditions.
The Importance of Emissivity in Electrical Thermography
Manufacturing facilities are full of equipment requiring periodic infrared (IR) inspection. The goal is to obtain an accurate assessment of equipment health. The most critical part of the assessment is properly compensating for the various emissivity values of all the components measured. Slight errors in emissivity compensation can lead to significant errors in temperature and ?T (difference in temperature) calculations. Electrical cabinets are a good example, as they may contain materials with emissivity values ranging from 0.07 to 0.95.
The graph shows how calculated temperatures can be adversely affected when the imager’s emissivity value is set too high. In this example, the emissivity of the target is 0.50; the graph shows the apparent temperature when the imager’s emissivity setting is stepped down from 1.0 to 0.50. When emissivity is properly compensated for, the actual temperature is shown to be 12.2° higher.
Magnitude of Error
One of the most misunderstood concepts in thermography is the degree to which errors in emissivity settings (and errors in window transmissivity compensation) will affect temperature and ?T (difference in temperature) accuracy. As demonstrated by the Stefan-Boltzmann Law, the radiated infrared energy emitted by a target surface is exponentially related to the absolute temperature of that surface.
Therefore, as the temperature increases, radiant energy increases proportionally by the absolute temperature to the 4th power. Incorrect camera settings such as emissivity and infrared window transmission rates will result in incorrect temperature values. Furthermore, because the relationship is exponential, this error will worsen as the component increases in temperature. Consider the effect on ?T comparisons, which are by their nature a comparison between different temperatures. The resulting calculations are apt to be radically understated, which could easily lead thermographers to misdiagnose the severity of a fault.
For some components, it can be difficult to determine the correct emissivity value. In the case of a highly polished component like a bus bar, the actual emissivity may be so low as to make temperature measurement impractical. It is strongly recommended that thermographers understand the surface of the primary targets. Once identified, those surfaces should be treated with a high-emissivity covering so that all targets have a standardized emissivity.
Thermographers can apply electrical tape, high-temperature paint (such as grill paint), or high-emissivity labels. When all targets have a standard emissivity, refection issues are minimized and measurement errors from reflected ambient energy are greatly reduced. High-emissivity targets of varying shapes can also provide a useful point-of-reference both for the thermographer and the technician making repairs.