operation of disconnects, switches, and grounding switches because of lack of interlocks12 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).
1.13 ARC FLASH HAZARD CALCULATION STEPS
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 5 and 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.
5 Calculate the incident energy, arc flash boundary, and the PPE level.
1.13.1 NFPA Table 130.7(C)(15)(a)
NFPA table 130.7(C)(9)(a) [17] relates the hazard risk category and PPE with respect to maintenance tasks to be carried out on energized equipment within arc flash boundary For example, if a worker is inserting or removing a starter “bucket” from MCC, the risk category is 3, but if he is just reading a panel meter while operating a meter switch, the risk category is zero. This table is applicable for specific three-phase short-circuit currents for specific durations as specified in the many foot-notes to the table. The IEEE equations are based upon an arc in an enclosure, with one side open, and it seems logical to relate the hazard level with respect to task activity; the following observations apply:
All commercially available computer programs calculate the arc flash hazard for a bus fault. It is opined that for safety and conservatism, HRC for a bus fault should be considered. For example, the HRC for a fault downstream of a low voltage or medium voltage MCC (a fault on the load side of the motor starter) can be zero, while for a fault on the bus, cleared by an upstream device, it can be 3 or 4. MCC as a whole is labeled for the bus fault—a fault on the bus can shatter the isolation partition in the MCC on arc blast pressure, unless the equipment is arc resistant.
It will be practically difficult to provide multiple arc flash hazard labels on the same equipment, and lay down procedures for various personal protective outfits based upon the maintenance task to be performed.
A rigorous calculation procedure for task-based HRC is not available, though guidelines in this table can be applied.
Multiple tasks are normally performed in one single instance.
The recommendations of the table can be misapplied (see Section 3.13).
The table is judgmental; there are no mathematical calculations.
The calculations, examples, and study cases in the book consider a bus fault, which gives maximum HRC on an equipment and ignore the nature of the task activity.
1.14 EXAMPLES OF CALCULATIONS
Example 1.1
This example is for calculations as per 2002 Guide. It is included here for reference. The results are compared with 2018 methodology in Chapter 3.
Calculate the incident energy using IEEE equations for a 30-kA three-phase bolted fault current in a 13.8-kV metal-clad switchgear, resistance grounded. The arcing time is 30 cycles for the arc fault current through the protective device.
Using IEEE Equation (1.11), calculate arcing current:
