Source: Reference [23].
| Material | Resistance in MΩ |
| Rubber gloves or soles | >20 0 |
| Dry concrete above grade | 1 0–5.0 |
| Dry concrete on grade | 0.2–1 0 |
| Leather sole, dry, including foot | 0.1–0.5 |
| Leather sole, damp, including foot | 0.05–0.2 |
| Wet concrete | 0.01–0.05 |
Figure 1.8. Shock hazard categories according to IEC.
Source: Reference [23].
See also Section 2.4.
1.7 FIRE HAZARD
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].
| Zone | Boundaries | Physiological Effects |
| AC-1 | Up to 0.5 mA, curve a | Perception possible but usually no “startled” reaction |
| AC-2 | 0.5 mA up to curve b | Perception and involuntary muscular contractions likely but usually no harmful physiological effects |
| AC-3 | Curve b and above | Strong involuntary muscular contractions, difficulty in breathing. Reversible disturbances of heart function. Immobilization may occur. Effects increasing with current magnitude. Usually no organic damage to be expected. |
| AC-4a | Above curve c1 | Pathophysiological effects may occur, such as cardiac arrest, burns, or other cellular damage. Probability of ventricular fibrillation increasing with current magnitude and time |
| c1–c2 | AC-4.1: Probability of ventricular fibrillation increasing up to about 5% | |
| c2–c3 | AC-4.2: Probability of ventricular fibrillation increasing up to about 50% | |
| Beyond curve c3 | AC-4.3: Probability of ventricular fibrillation increasing above 50% |
a For 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.
1.8 ARC FLASH HAZARD ANALYSIS
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/cm2 exists, 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.