Mitigating arc flash hazards in medium-voltage switchgear

Engineers should be aware of design alternatives that can reduce arc flash hazards in medium-voltage systems.

12/09/2013


Figure 1: This diagram shows a line-to-line arc in a 3-phase system. Courtesy: CDM SmithThe term “arc,” which literally means part of a circle, is attributed to Humphrey Davis, an English scientist. In 1802, Davis demonstrated that electric current can flow between two carbon rods separated in air by a short distance in the form of a band of ionized air that looks like an upward bow. In fact, electrical science started with the study of the electric arc. Soon, a number of inventions came forth, such as arc lamps, arc furnaces, spark plugs, arc welders, and others. Today, the electric arc is again a subject of great interest and study because of the hazards it creates in electrical distribution systems due to its intense heat, which can destroy equipment and cause severe or fatal injuries to unprotected personnel who are unfortunate to be in close proximity to it. 

In all electrical equipment, a serious hazard exists for operating personnel due to possible arcing between energized parts and between energized parts and grounded metal enclosures. Hazardous arcing can take place in electrical equipment because of one or more of the following:

  • Accidentally dropping metal tools in energized parts
  • Incorrect alignment of contacts in draw-out circuit breakers
  • Loose connections can cause overheating and minor arcing, which can escalate to an arcing fault
  • Rodents and vermin in switchgear enclosures
  • Defective cable and bus insulation. 

The arc behaves like a flexible conductor and consists of ionized air at very high temperature, in the order of 35,000 F—more than three times hotter than the surface of the sun. It can burn holes in copper bus bars. It can vaporize copper, which when condensed on other parts can cause secondary faults. It can cause pressure buildup and/or an explosion in enclosed equipment. It can cause severe burns and can ignite clothing. 

OSHA and the National Fire Protection Association (NFPA) have adopted specific requirements with regard to the arc flash hazard. OSHA requires that all equipment be marked with a label that indicates the arc flash boundary, the incident energy in the arc, the safe working distance, and the category of clothing and other protective equipment to be used by personnel. Article 110.16, which states that the equipment be clearly and visibly labeled to warn personnel of the potential arc-flash hazard, was introduced in NFPA 70: National Electrical Code in 2002. In 2004, NFPA 70E: Standard for Electrical Safety in the Workplace required that shock and arc-flash hazard analyses be completed to determine the level of personal protective equipment required in each location. 

Incident energy, working distance, and hazard risk category

Incident energy is the measure of the severity of the hazard to workers. This quantity is defined as the energy density in calories/cm2 or Joules/cm2 to which the worker’s face or body is exposed in an arc flash event at the working distance. The working distance is the typical distance between a potential source of the arc in the equipment and the face or body of the person performing the work on the equipment. The value of the incident energy determines the type of mandatory protective clothing to be worn by the worker. Typical working distances defined by IEEE Std. 1584 include:

  • 15kV switchgear: 36 in.
  • 5kV switchgear: 36 in.
  • Low-voltage switchgear: 24 in.
  • Low-voltage motor control centers and panel boards: 18 in.
  • Cables: 18 in. 

Arc flash hazard is quantified by a number called the hazard risk category (HRC). According to NFPA 70E, the relationship between the HRC, the available incident energy, and the type of protective equipment is listed in Table 1. 

Source: NFPA 70E

Arc flash equations, solution

In 1982, Ralph H. Lee published a paper in the “IEEE Transactions on Industry Applications” on calculation of the incident energy in open air arcs, such as in outdoor substations. This paper triggered renewed interest in the arc flash phenomenon. In 2002, the IEEE Industry Applications Society published IEEE Standard 1584: IEEE Guide for Performing Arc Flash Hazard Calculations and issued subsequent amendments in 2004 and 2011 as 1584a and 1584b. The equations in this standard are empirically derived using statistical analyses and curve-fitting algorithms on a huge collection of experimental data (see “Calculating arcing faults”). The equations can be used for systems from 208 V to 15 kV, 50 to 60 Hz, available short-circuit current from 700 A to 106,000 A, and for arcing distances from 0.5 in. to 6.0 in. 

For any electrical equipment, there are two significant parameters that determine the incident energy and, therefore, the type of protective clothing to be used. These parameters are the arcing fault current “Ia” and the duration of the arc “t”. The arcing fault current Ia is less than the bolted fault current (Ibf) because of the voltage drop across the arc or because of the arc resistance. For a given arc length, the arc voltage drop is almost constant for a wide range of current. Consequently, the arc exhibits negative incremental resistance. The term “bolted” signifies a fault through zero resistance, as when the 3-phase wires are stripped, lugged, and bolted together. 

Figure 1 simplifies the relationship between the arcing fault current and the arc voltage drop. The drawing shows why the arcing fault current Ia is considerably less than the bolted fault current Ibf in low-voltage equipment, while it is about 90% of Ibf in medium-voltage and high-voltage equipment. This is because the arc voltage drop, which is approximately 200 V for a 2-in. arc, is a significant part of the circuit voltage in 480 V equipment, while it is less than 10% of the circuit voltage in 4.16 kV and 13.8kV equipment. 

The arc voltage drop depends on several factors including the clearances in different classes of equipment. The relation between Ia and Ibf and the relation between the incident energy E and Ia and t are given in Section 5 of IEEE 1584. These equations are programmed into the arc flash evaluation programs of most distribution system analysis software. These programs require that a short-circuit study be performed first to determine Ibf at the equipment in question. 


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