PPE, or the maximum HRC for that type of equipment. Also, arc flash boundary needs to be specified.
1.11 MAXIMUM DURATION OF AN ARC FLASH EVENT AND ARC FLASH BOUNDARY
A maximum duration of 2 seconds for the total fault clearance time of an arc flash event is considered, though, in some cases, the fault clearance time can be higher. In IEEE 1584 Guide Annexure B, it is stated that: “if the time is longer than 2 seconds, consider how long a person is likely to remain in the location of the arc-flash. It is likely that a person exposed to arc flash will move away quickly, if it is physically possible and 2 seconds is a reasonable maximum time for calculations. A person in a bucket truck or a person who has crawled into equipment may need more time to move away.”
Tables 1.10 and 1.11 show the arc flash boundary calculations according to IEEE 1584 Guide equations for a bolted three-phase fault current of 30 kA, in 13.8-kV switchgear, 13.8-kV system resistance grounded, and also in a 480-V MCC, and 480-V system high resistance grounded.
For a 30-kA, 2-second fault in 13.8-kV switchgear, the incident energy boundary is 3539 in equal to 295 ft. For 1-second fault duration, it is 144.6 ft. The arc flash boundary at which a worker can be exposed to 1.2 cal/cm2 of incident energy and sustain threshold of second degree burns seems to be very large.
The working space, in an electrical room switchgear installation in front of electrical equipment, is limited and may be as low as 5–6 ft, and must meet NEC requirements. Many companies, as a policy, keep the electrical rooms locked, and a worker must have the required PPE outfit before entering the electrical rooms.
TABLE 1.11. Arc Flash Boundary and Incident Energy Release for 30 kA of Bolted Fault Current (Arc Flash Current = 16.76 kA rms) in 480-V MCC, 480-V System High Resistance Grounded, Working Distance = 18″, Gap = 25 mm
| Arc Duration in Seconds | Arc Flash Boundary in Inches | Incident Energy, cal/cm |
| 0.050 | 36 | 3.8 |
| 0.5 | 147 | 38 |
| 1.0 | 225 | 75 |
| 1.5 | 288 | 113 |
| 2 | 343 | 151 |
TABLE 1.12. Statistics of Arc Flash Incidents
| Accident Occurrence | Percentage |
| When the operator or worker is working with equipment doors open | 65 |
| When the operator happens to be in front of a closed door and the equipment is not arc resistant | 10 |
| When the operator is not present at all, and the equipment is not arc resistant | 25 |
1.11.1 Arc Flash Hazard with Equipment Doors Closed
There is some controversy in interpreting the intent of NFPA 70E, whether the arc flash hazard exists at all times, with the equipment door closed, or it exists only when the doors of an energized equipment are opened for maintenance? Let us first consider most common reasons for arc flash accidents:
human error
mechanical faults
failed connections, loose connections, and terminals
Adverse ambient conditions and pollution. This should consider pollution specific to plant operation, that is, corrosive gases and vapors may be present.
rodents.
Table 1.12 shows statistical data of arc flash incidents, when these can occur. This shows that some arc flash hazard exists, even when someone is walking around the closed-door electrical energized equipment, though this probability is relatively small. The IEEE Guide equations are based upon the hazard calculations with the door open. With the door closed, the hazard level will be less. It is not so easy to calculate it. The manufacturers are reinforcing the latching mechanisms and strengthening the doors, yet the withstand capability of the doors in closed position under an arc flash event is a question mark. Only when the equipment is arc resistant is the incident energy level outside the equipment zero, so long as no doors and panels that are not intended to be opened are not opened (see Chapter 13).
To resolve this conflict, NFPA 70E 2012 adds:
It is the collective experience of the Technical Committee on Electrical Safety in the Workplaces that normal operation of the enclosed electrical equipment, operating at 600V or less, that has been properly installed and maintained by qualified persons is not likely to expose the employee to an electrical hazard.
It is also the opinion of the committee that there is little risk in performing normal operations of electrical equipment and devices, such as opening and closing circuit breakers, motor control centers (MCCs), or starters. When the committee states “interacting with equipment in a manner that could cause an arc flash hazard,” it refers to operations, such as racking circuit breakers or installing and removing MCC buckets.
1.12 REASONS FOR INTERNAL ARCING FAULTS
The internal arcing fault can be caused by various factors, such as:
1 Insulation defects due to quality deterioration of components
2 Overvoltages of atmospheric origin or generated within the equipment due to switching transients
3 Incorrect operation due to not adhering to the procedures
4 Inadequate training of the persons involved with installation, maintenance, and operation
5 Breakage or tempering of safety interlocks
6 Highly polluted atmospheric conditions, high humidity, temperatures, corrosive gases, and salt-laden winds
7 Overheating of the contact area due to presence of corrosive agents, loose connections.
8 Faulty assemblies
9 Lack of proper protection and inadequate relaying. Though protective relaying by itself will not prevent an arcing fault, it will limit the arc fault energy and consequently equipment damage.
10 Insufficient maintenance and testing procedures, like insulation resistance records, infrared testing, and partial discharge measurements which can indicate a deteriorating insulation situation.
