J. C. Das - Arc Flash Hazard Analysis and Mitigation

Source : Reference [23].

See also Section 2.4.

NFPA and National Fire Incident Reporting Systems (NFIRS) statistics of fire hazard can be viewed on websites. These statistics are based upon:

heat source, that is, arcing

contributing factors like electrical failure or malfunction

equipment involved in electrical distribution, lighting, and power transfer.

TABLE 1.3.Time–Current Zones for 15–100 Hz AC Currents for Hand-to-Feet Pathway

Source : Reference [23].

aFor duration of current flow below 200 ms, ventricular fibrillation is only initiated within the vulnerable period if the relevant thresholds are passed. As regards to ventricular fibrillation, this figure relates to the effects of current which flow in the path from left hand to feet. For other current paths, the heart current factor has to be considered.

In 1999–2003, arcing was the heat source that resulted in 37,700 home fires, 240 deaths, 890 home fire injuries, and $703 million in direct property damage [30, 31].

Fires can develop in electrical equipment due to overloads and loose connections that are not cleared by overcurrent devices. The equipment should be listed by a nationally recognized test laboratory (NRTL), which helps to reduce the fire risk. Some precautionary and design measures are:

Fire detection and suppression equipment should be permanently installed or readily accessible around the electrical equipment. Such equipment could possibly include smoke detectors, sprinkler systems, and portable fire extinguishers.

The workplace should be designed so that escape routes are sufficiently wide, clear of obstructions, well marked and lighted. Normal and emergency lighting and exit signs are important.

Special considerations should be applied to the electrical equipment located in hazardous areas, according to NEC.

All conductors and wiring should be properly sized for protection against overheating (see Article 310 of NEC).

Overcurrent protection should be provided to meet the requirements of NEC.

Motors and generators should be properly protected so that these do not cause a fire hazard.

The transformers should be protected and installed according to NEC, UL, and FM (factory mutual) guidelines. In general, all electrical equipment must be installed, operated, and maintained according to codes and standards (see Chapter 2).

The fire hazards are not further discussed in this book.

As early as December 1970, the Occupational Safety and Health Act required that each employer shall furnish to his employees, employment and place of employment that are free from recognized hazards that are causing or likely to cause death or serious physical harm to his employees. It was not till late 1991 that OSHA added words acknowledging arc flash as an electrical hazard. NFPA published the first edition of NFPA 70E in 1979.

Effective from January 1, 2009, the National Electric Safety Code (NESC) [32] requires that all power generating utilities perform arc flash assessments. The employer shall ensure that assessment is performed to determine potential exposure to an electric arc for employees who work on or near energized parts or equipment. If the assessment determines a potential employee exposure greater than 1.2 cal/cm 2exists, the employer shall require employees to wear clothing or a clothing system that has an effective arc rating not less than the anticipated level of arc energy.

Currently, there are four major guides for arc flash calculations:

1 NFPA 70E, revised in 2018 [17]

2 IEEE 1584 Guide, 2018, which will undergo revisions [9]

3 IEEE 1584a, 2004, amendment 1 [33]

4 IEEE P1584b/D2 Draft 2, unapproved [34].

NFPA 70E 2012, in annex D, table D.1, provides limitations of various calculation methods. This is reproduced in Table 1.4. The standard does not express any preference for which method should be used. Reference [33] recognizes use of knowledge and experience of those who have performed studies as a guide in applying the standard. IEEE 1584 Guide also contains a theoretically derived model applicable for any voltage.

It is recognized that to construct an accurate mathematical model of the arcing phenomena is rather impractical. This is because of the spasmodic nature of the fault caused by arc elongation blowout effects, physical flexing of cables and bus bars under short circuits, possible arc reignition, turbulent flow of plasma, and high temperature gradients (the temperature at the core being of the order of 25,000 K, while at the arc boundary, of the order of 300–2000 K).

IEEE 1584 Guide equations are empirical equations based upon laboratory test results, though the standard includes some of Lee’s equations also.

TABLE 1.4.Limitations of ARC Flash Hazard Calculation Methods

Source : NFPA 70E-2018. © 2018 National Fire Protection Association.

aEquations for higher voltages are included.

If the equipment is maintained under deenergized condition, there is no arc flash hazard. NFPA 70E [17] states that energized electrical conductors and circuit parts that operate at less than 50 V to ground should not be required to be de-energized. Again, it is qualified that the capacity of the source and any overcurrent protection between the source and the worker should be determined and there should be no increased exposure to electrical burns or explosion due to electrical arcs. The IEEE 1584 Guide states that equipment below 240 V need not be analyzed for arc flash unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply. The “low impedance” is not defined. Sometimes, the arc flash hazard can be high even in systems of 240 V. When incident energy exceeds 40 cal/cm 2, the equipment should only be maintained in the de-energized condition. There is no PPE (personal protective equipment) outfits specified for incident energy release >40 cal/cm 2; see Section 1.9for definitions and discussions of PPE.