Overview of Arc Flash
Arc flash is a hazardous condition related with the release of energy caused by an electric arc due to short circuit of electricity.
An electrical arcing is current passing through vapor of the arc terminal conductive metal or carbon material.
The extremely high temperatures of these arcs can cause fatal burns at up to about 1.5 meter and major burns at up to about 3 meter distance from the arc.
Additionally, electrical arcs expel droplets of molten terminal material that shower the instant area, similar to, but more extensive than that from electrical arc welding.
Immediate effects of an arcing fault:
- Extreme Heat, Pressure Waves, and Sound Waves
- Intense Light
- Molten Metal, Shrapnel and Vapor
- Power System failure
- Electrical Component Damage
- Injury or Death of Operator
Major Cause of Arc Flash
Basically, two type faults is associated with an arc flash incident
- Arcing short Circuit
- Bolted Short Circuit.
Arc-Flash Hazard Analysis
Purpose of Arc-Flash analysis can investigate an operator potential exposure to arc flash energy, which may be required for the purpose of injury prevention and determination of the minimum Personal Protective Equipment (PPE) ATPV rating.
The incident energy and flash boundaries are determined based on the following standards for Arc Flash Analysis:
- National Fire Protection Agency (NFPA) 70E 2018.
- IEEE Standards 1584-2002
- CSA Z462-2015
31.5kV Gas Insulated Switchgear (GIS)
In this article we will discuss on IEEE Standards 1584, which is a guide for Performing Arc-Flash Hazard Calculations.
IEEE Standards 1584 presents methods for the calculation of arc-flash incident energy and arc-flash boundaries in three-phase ac systems to which electrical operators may be exposed.
It also define the analysis process from data collection to final results, formula which is needed to find incident energy and the flash-protection boundary and discusses about available alternative software solution.
An arc-flash hazard analysis is performed in association with or as a continuation of the short-circuit study and protective-device coordination study.
Result of short-circuit study and protective-device coordination study is used to determine the following:
- The electrical equipments momentary duty of fault current, short-circuit (withstand) rating and interrupting rating.
- Time requirement for electrical circuit protective devices to isolate overload or short-circuit conditions.
Above result is very much important to perform an arc flash hazard analysis, and results of the arc-flash hazard analysis are used to identify the flash-protection boundary and the incident energy at assigned working distances throughout any position or level in the overall electrical generation, transmission, distribution, or utilization system.
Step 1: Gather the system and installation data
Field data collection is the major task in an arc-flash hazard study. Even for a plant with nominally updated single-line diagrams, time-current curves, and short-circuit study on a computer, the field part of the study will take about half of the effort.
All low-voltage distribution and control equipment plus its feeders and large branch circuits data also required for this study.
Before start arc-flash study, It is very important to review of the single-line diagrams and electrical equipment layout arrangement with people who are familiar with the actual site. To show the current system configuration and orientation, the diagrams may have to be updated.
All alternate feeds must be included in single-line diagrams.
Step 2: Identify the system modes of operation
For a simple radial distribution system there is only one mode of operation, but a more complex system can have many modes of operation like more utility feeders in service, Generators running in parallel etc.
It is important to determine the maximum and the minimum available short-circuit currents for different modes of operation.
Step 3: Calculate the bolted fault currents
Obtain the symmetrical root-mean-square (RMS) bolted fault current and X/R ratio at each point of concern where people could be working. It can be done with the help of Commercially available programs like ETAP, SKM etc. which can run thousands of buses and allow easy switching between modes.
Step 4: Calculate the arc fault currents
The predicted three-phase arcing current must be found to determine the operating time for protective devices.
Arc fault currents can be determined by computer software like ETAP, SKM. But also, by the following equations
Calculation of Arcing Current (Eq. 1 of IEEE 1584 Page 10)
For system voltage under 1000 V
Calculation of Arcing Current (Eq. 2 of IEEE 1584 Page 10)
For system voltage of and above 1000V
Step 5: Find characteristics of the protective device and the duration of the arcs
Up-to-date system time-current curves (TCC) may have been found in the field survey. If not, it is best to create them, commercially available software like ETAP, SKM etc. makes this task easy. Alternatively, for a very simple study, it is possible to use manufacturer’s data for protective device characteristics.
For fuses, time-current curves may include both melting and clearing time in the manufacturer catalogue. If so, we can use the clearing time.
If Manufacturer show only the average melt time, then add 15% to that time up to 30 milliseconds, and 10% above 30 milliseconds to determine total clearing time.
If the arcing fault current is above the total clearing time at the bottom of the curve (10 milliseconds), then use 10 milliseconds for the time.
For circuit breakers with integral trip units, time-current curves include both tripping time and clearing time in the manufacturer catalogue.
Circuit breakers operated with relay , In the time-delay region, the relay curves show only the relay operating time .
For relays operating in their instantaneous region, allow 0.016 seconds on 60 Hz systems for operation.
The opening time of circuit breaker must be added.
Particular circuit breakers opening times can be verified by consulting with manufacturer.
Below Table from IEEE 1584 shows recommended circuit breaker operating times.
Step 6: Document the classes of equipment and system voltages
Following Table from IEEE 1584 is define the system voltage and the class of equipment.
Step 7: working distances Selection
Arc-flash protection is always based on the incident energy level on the operator’s face and body at the working distance, not the incident energy on the hands or arms. The degree of injury in a burn depends on the percentage of an operator’s skin that is burned.
Following Table from IEEE 1584 is define the Typical working distances.
Step 8: Calculate the incident energy for all equipment
Calculation Of Normalized Incident Energy (Eq. 4 of IEEE 1584 Page 11)
Calculation Of Actual Incident Energy (Eq. 6 of IEEE 1584 Page 11)
We can also calculate the incident energy with the help of Computer software like ETAP, SKM etc.
Step 9: Calculate the flash-protection boundary for all equipment
Calculation Of Flash Protection Boundary (Eq. 8 of IEEE 1584 Page 12)