LBNL EH&S Division Electrical Safety Engineer
NFPA 70E, Article 110.8(B)(1) requires an Arc Flash Hazard Analysis to be conducted as part of an Electrical Hazard Analysis, whenever conductors are not placed in an electrically safe work condition (de-energized and tested).
An Arc Flash Hazard Analysis is a study investigating a worker’s potential exposure to arc-flash energy, conducted for the purpose of injury prevention and the determination of safe work practices, arc-flash protection boundary, and the appropriate levels of PPE.
II. NFPA 70E guidance on Arc Flash Hazard Analysis
Neither NFPA 70E nor IEEE 1584 (referenced in NFPA 70E) provides a means to calculate or estimate single-phase AC arc energies. The existing formulas are for three-phase AC sources, which have different physical properties.
III. Physical limitations of 120-V single-phase arc evolution
Typically, a three-phase AC arc flash begins when there is sufficient voltage to break down the air gap between conductors. It takes approximately 3 kV/mm to break down air. Therefore, an arc can occur at much lower voltages as long as the gap is sufficiently small. Since most arcs begin as a short circuit or ground fault, the initiation of the arc is not dependent on high voltages. 170 V (peak voltage in a 120-V sine wave) can break down a gap of 0.057 mm (.002 in).
Once the arc has been initiated, it must be sustained in order to be a significant hazard, i.e., one that is capable of causing significant second-degree burns or clothing ignition. Upon initiation of the arc, the air gap becomes ionized. This ionized path (plasma) is an excellent conductor, and will be sustained as long as the system provides and delivers sufficient uninterrupted electrical energy. The plasma grows and expands as it ionizes more air. This expanding plasma is responsible for the arc-flash hazards to nearby workers. It will only be interrupted when an overcurrent device trips or sufficient material burns away from the electrodes to effectively increase the gap. In the case of a three-phase AC circuit, a sustained source of current is available because the three voltage waveforms present are always separated from each other by 120 degrees. Therefore, voltage and current are never at zero. It has been determined empirically that three-phase arcs are very difficult to sustain below 240 V, and the IEEE 1584 Standard has determined that such circuits, if fed via a 125 kVA or smaller transformer, are rarely a significant safety concern.
Unlike three-phase circuits, a single-phase 120-V circuit is not an arc-flash hazard. Although an arc can occur at 120 V, the 60 Hz sinusoidal wave form crosses through zero 120 times each second. This means that the voltage and current both decay to the point where the arc will extinguish within 0.5 cycles, or 0.008 seconds. This prevents the arc from developing enough energy to turn into an expanding, burning plasma.
The result is that the 120-V AC arc can indeed be hazardous, but far less so than three-phase arcs of higher voltages. The 120-V arc presents hazards of localized skin burns, primarily to the hands, as well as minor to moderate injuries resulting from ejected molten material. It will not propagate a plasma capable of producing 1.2 cal/cm2 at 18 inches from the arc source, which is the common definition of an arc-flash burn hazard, or 3 cal/cm2 on clothing, considered to be the threshold for spontaneous clothing ignition. However, it should be noted that 120 V is still a serious shock hazard and that exposures must be reduced and eliminated as possible.
IV. Personal Protective Equipment (PPE)
The following minimum arc-flash PPE shall be required when working within the limited approach boundary (42 inches) of exposed energized 120-V AC circuits. This does not apply for any exposures greater than 120 V AC.
In addition, the following shock-protection PPE shall be worn while measuring 120-V contacts with a handheld test instrument, or when there is a chance that inadvertent contact with exposed conductors may be made:
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