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

TABLE 1.12.Statistics of Arc Flash Incidents

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 1.12shows 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.

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.

11 Incorrect operation of disconnects, switches, and grounding switches because of lack of interlocks

12 Problems with cable terminations, like inadequate design, faulty installation, and insulation failures

13 Ferroresonance voltages of instrument transformers

14 Inadequate protection for ground faults and improper selection of system grounding

15 Aging under electrical stress

16 Entrance of dust and rodents, corrosion at contact surfaces producing heat, loose contacts creating sparks. Snapping of the connections and wiring due to inadvertent force, human errors, and incorrect operation.

17 Closing on to an existing fault, without prior rectification and analyses of the faulty condition.

18 Sparking produced due to racking of circuit breakers, operation of fuses, and excessive current flows through loose contacts.

19 Human errors, that is, some parts or tools left or dropped inside the equipment during maintenance

This list may not be exhaustive and is indicative only. In the first place, if the electrical installations are inadequately designed and do not meet the requirements of national standards and safety codes, these will be more prone to higher incidents of arcing faults. Design and operational measures can be adopted for enhancing safety (see Chapter 2). The list also implies that improvements in arc flash hazard reduction can be achieved by avoiding the listed items and taking remedial measures. As an example, the incipient breakdowns can be predetermined by proper testing with infra red scans or partial discharge measurements (see Chapter 14).

We can summarize the calculation procedures as follows:

1 Calculate bolted three-phase symmetrical short-circuit currents throughout the system where arc flash calculations are to be carried out. The data collection, single line diagrams, and switching conditions to be studied are all akin to the short-circuit calculations; see Chapters 5and 6.

It will be necessary to consider the various operating conditions, for example:

Normal operation

A tie circuit breaker is closed

Alternate operating situations that is transformers run in parallel, a generator is brought in or taken out of service

Dual feeds.

It will be erroneous to conclude that if the arc flash calculations are based upon the maximum short-circuit currents in any operating conditions, the results will give maximum hazard levels, and other scenarios need not be considered. This is so because at reduced short-circuit currents, the protective relaying times may increase, giving rise to even greater arc flash hazard.

1 The arcing currents can be calculated based upon the system voltage and the bolted three-phase currents and gap length. Calculate second arcing current at 85% Ia.

2 The working distance and gap lengths in all the examples and discussions in this book are according to IEEE standard. There impact on arc flash calculation results is, however, documented in Chapter 3.

3 Input the correct equipment type, switchgear, MCC or panels, or arc in the open air.

4 Conduct a rigorous relay coordination study throughout the distribution system. This is an important step, where alternate protective relay types, relay characteristics, and settings, protection strategies, current limiting devices, equipment selection, and alternate settings during maintenance modes become important. The protection and relaying is important, and for arc flash hazard reduction, it becomes an iterative calculation. With all other parameters welldefined, relaying can be manipulated to reduce HRC.