Introduction
An arc flash is a sudden and dangerous release of electrical energy that poses a serious risk of injury or even death to individuals exposed to it. This event occurs when insulation breaks down between two conductive surfaces of different electrical potential, often within a high-voltage or low-voltage electrical system. This breakdown creates a short circuit through the air, separating phases or phase and earth.
Once initiated, the arc flash rapidly transforms into a plasma cloud. The high temperatures along the initial arc path cause air molecules to break down into a plasma of positively and negatively charged ions, which are conductive. The electrical fault current flows through this conductive path, releasing a significant amount of energy. This energy is discharged as a sudden explosion of light, heat, hot gases, and molten metal.
The resulting swift and destructive expansion of air and vaporized metal often leads to structural failure of switchboards and the forceful expulsion of molten metal, displaced panels, doors, equipment components, and other debris.
Potential Effects of an Arc Flash
Personnel exposed to an arc flash may suffer severe burns from the arc plasma, radiation, and the ignition of flammable clothing. They may also be burned by metal spray and material combustion. In addition, they may be electrocuted by the projected arc plasma, or suffer trauma to their lungs, eyes, and ears from the pressure wave. Projectiles and shrapnel from the arc flash can also cause secondary injuries. Finally, personnel may be poisoned by toxic gases.
Arc flash incidents can also cause significant financial losses and equipment damage. The initial explosive damage from the arc flash can be severe, and high temperatures can melt metal and cause fire damage. Carbon deposits can reduce the insulation quality of wiring, and the arc flash can also damage adjacent panels and auxiliary plant.
Standards and outcomes of an Arcflash assessment
The United States has played an important role in advancing arc flash safety, primarily due to its specific legal requirements for assessing arc flash hazards and providing appropriate Arc Flash Personal Protective Equipment (AR PPE).
In Australia, arc flash requirements have filtered through mining corporations owned by American companies and have become an important requirement in electricity safety.
The Australian wiring rules AS/NZS3000 mention arc flash in passing, and at this stage, there is no dedicated Australian standard for arc flash. The most common methods for evaluating arc flash hazards and specifying the necessary arcflash rated PPE are NFPA 70E and IEEE Std 1584. The outcomes of an arc flash assessment include:
- Incident Energy at the Working Distance
- Arc Flash Boundary (Closest Approach Distance)
- Hazard Rating Category
Goals and Expected Outcome of Arcflash Assessment
The goals of an arc flash hazard assessment are to:
- Evaluate the potential severity (consequences) of a switchboard arc flash incident. This is done by calculating the prospective energy produced by an arcing fault.
- Evaluate the safe working distance from energized electrical switchboards on the basis of the potential arc fault energy. This is done to ensure that workers are not exposed to dangerous levels of heat, pressure, and radiation in the event of an arc flash incident.
- Propose and develop options for reducing arc flash hazards. This may include measures such as installing arc flash mitigation equipment, improving electrical maintenance practices, and providing training to workers on arc flash safety.
- Determine Arc Rated PPE requirements for employees working on or near electrical switchboards. This is done to ensure that workers are properly protected from the hazards of arc flash incidents.
- Provide a reasonably practicable safe work environment. This means taking all reasonably practicable steps to reduce the risk of arc flash incidents occurring, and to mitigate the consequences of any incidents that do occur.
An IEEE Std 1584 arc flash assessment provides important information about the potential hazards and risks associated with electrical arc flash incidents . Here are some of the key outcomes expected from such an assessment:
Incident Energy at working Levels
The assessment will determine the incident energy levels at various points within the electrical system. The incident energy at the working distance is the amount of energy a surface (or person) exposed to an arc flash will experience at a set distance from an arc. The incident energy reduces exponentially as the distance between the person and the arc source increases. This information is crucial for selecting appropriate personal protective equipment (PPE) for workers who may be exposed to arc flash hazards.
Arc Flash Boundary:
It establishes the distance from an arc source at which the incident energy is equal to or greater than 1.2 cal/cm². This boundary is vital for defining the safe working distance for personnel near electrical equipment.
Hazard Rating Category
Once the incident energy has been assessed a Hazard Rating Category can be assigned, we will discuss this later on in this article.
PPE Requirements
The assessment will provide specific recommendations for the type and level of personal protective equipment that workers should use when operating or working near the evaluated electrical equipment.
Equipment Labeling
Based on the assessment, appropriate warning labels can be affixed to electrical equipment, indicating the potential arc flash hazard and providing necessary safety instructions.
Safety Procedures and Training
It will guide the development of safe work procedures for tasks involving electrical equipment with potential arc flash hazards. Additionally, it will highlight the importance of proper training for workers.
Mitigation Recommendations
The assessment may suggest engineering controls or modifications to reduce the arc flash hazard, such as adjusting the settings of protective devices, installing barriers, or implementing arc-resistant equipment.
Risk Assessment and Management:
The assessment provides a comprehensive understanding of the risks associated with the electrical system, allowing for informed decision-making and risk mitigation strategies.
Arcflash Assessment steps
The flow chart below shows the recommended steps and processes to be followed when carrying out an arc flash assessment. We will discuss some of these steps in the following sections.
Possible situations
There is a balance between protection and reducing arc flash risks that we will discuss later in this article. To achieve this balance, it’s important to conduct arc flash hazard assessments alongside studies on how protective devices work together. The diagram below shows the three potential spots on a switchboard where an arc flash might happen.
- Location 1 – Incomer terminals on the line side of the incomer protection.
- Location 2 – Terminals, main busbars and droppers located between the main incoming protection and the outgoing feeder protection.
- Location 3 – Feeder terminals on the load side of the outgoing feeder protection.
The energy released during an arc flash varies in these three spots because different protective devices (that work at different speeds) kick in to stop the arc. In the usual situation where the protections in different parts of the system are coordinated, the level of danger from the energy is like this:
- The upstream feeder protection acts for a fault at Location 1. The incident energy release at Location 1 is the most severe relative to Location 2 and 3 because the upstream feeder protection is slower than both the switchboard’s incoming and outgoing feeder protection.
- The switchboard’s incoming protection acts for a fault at Location 2. The incident energy release at Location 2 is less severe than Location 1 because the incomer protection is faster than the upstream feeder protection.
- The switchboard’s outgoing feeder protection acts for a fault at Location 3. The incident energy release at Location 3 is generally less severe than both Location 1 and Location 2 because outgoing feeder protection is faster than both the incoming and the upstream protection.
Arc flash calculations should be done at both Location 1 and Location 2. Calculations are not required at Location 3 to avoid the necessity of undertaking a large number of calculations to cover all of the outgoing circuits on a switchboard.
Data Collection
As shown in the flow chart above, the first step in conducting an arc flash assessment is collecting system data. We typically begin at the top or from the power supply. This involves obtaining utility or generation data.
Utility Data
The following data needs to be collected from the utiity;
- Maximum three phase initial symmetrical fault level.
- Minimum three phase initial symmetrical fault level.
- X/R ratio of the supply network impedance, for both the maximum and minimum fault level cases
Backup Generators
The following data from generators is required;
- KVA rating
- Rated power
- Stator resistance
Site Transformers
The following data is required for site transformers;
- Primary and secondary voltage ratings.
- Primary and secondary winding connections.
- kVA rating.
- Tap position
- Positive sequence transformer impedance (%z).
- Positive sequence X/R ratio.
Power Cables
To get a clear picture of the electrical system, we need to create accurate models for specific cables. These include the ones connecting the utility’s power supply point to the switchboard we’re examining.
We’ll also model cables that provide backup power sources like bus-ties and back-feeds. If the downstream board powers induction motors, we’ll consider the cables supplying sub-distribution boards. And we won’t forget to include the cables powering the largest induction motor connected to the assessed board.
This modeling will cover important details like how well the conductors conduct electricity, how many conductors run in parallel, and how long they are. This detailed approach ensures we understand the electrical system’s features and performance accurately
Protective Devices
To get a good understanding on how the electrical system operates, we’re going to create models for different protective devices. These include the main, backup, and the largest outgoing feeder protection devices.
When we’re doing these models, we’ll consider a few things. This includes the specific settings for each protection device, and also the range of times it takes for them to kick in. We’re looking at how fast they can spot a problem and how quickly they can shut down if needed.
Switching Points
All switching points in the electrical system which could affect the fault current levels should be identified by investigating the switchboard configuration and single line drawings. This includes HV and LV switching points such as:
- Secondary selective changeover arrangements
- Ring main switches
- Contingency and backup supply arrangements
Loads
We will need to create a detailed model that encompasses all loads on the site contributing to the overall fault levels. This includes various scenarios involving induction motors, such as those connected direct on-line, connected via starters like Star-Delta or autotransformer, and those linked by regenerative-type variable speed drives.
It’s worth noting that we won’t need to model static loads or motors supplied from non-regenerating variable speed drives. For the motors included, we’ll base our modeling on their worst-case running scenario. For instance, if there are three pumps with one designated for standby, only the two duty pumps will be included.
The largest induction motor supplied from the switchboard under assessment will be individually modeled. The rest of the induction motors can be grouped together, with their combined kW rating representing the lumped motors. Motors on other switchboards within the same facility will be treated as lumped loads connected to their respective boards.
When modeling motors, we’ll consider their kW rating, efficiency, power factor, and locked rotor current.
Arcflash Modeling
A model of the electrical system will then be developed using power system simulation software such as Powerfactory, ETAP, SKM PTW, DIgSILENT, EasyPower, PowerCAD, etc. The model will, among other things:
- Allow the maximum and minimum initial symmetrical three-phase fault levels to be determined at the bus of the switchboard under assessment.
- Enable the evaluation of the protection clearance time under maximum and minimum arcing fault conditions.
The terminal points for modelling should include the components of the electrical distribution system from the utility point of supply up to the busbars of the switchboards under assessment, as well as any other sources of fault current including generators and induction motors that would contribute fault current in the event of an arcing fault. This includes induction motors downstream of the board under assessment.
If you don’t have access to one of the pricey power system simulation tools, you can manually calculate arc flash hazard parameters. Manual calculation involves computing the bolted fault currents and trip times from the protection device time-current curves. The IEEE Std 1584 empirical equations (for switchboards ≤ 15kV) or the Lee Method theoretical equations (for switchboards > 15kV) can then be applied to calculate arc flash parameters using the bolted fault currents and trip times as inputs.
System modes of Operation
The operating modes of the switchboard can significantly impact the results of arc flash calculations. For radial single-feed switchboards, only one switching scenario needs to be considered. However, for more complex supply arrangements, several operating modes may be possible, including:
- Sites with more than one electrical utility supply.
- Secondary selective switchboards (main-tie-main arrangements).
- Embedded generation that can be operated independently (islanded) from the electrical utility.
- Switchboards with supplementary/emergency generation.
- Ring main distribution networks with an open point at one of the multiple possible ring main switching locations.
Three phase bolted current
Calculations for both the minimum and maximum fault current conditions are then undertaken in order to identify the worst-case incident energy. This is because most protective relays and breakers follow an IDMT-type curve that may result in high incident energy for lower fault currents. This can be easily illustrated by the following figures.
In the figure above, for an LV fuse, the first fault is only 800A, but the incident energy is 10 cal/cm², CAT 3, because it takes 2 seconds to clear the fault. The second fault has a higher fault current of 1600A but an incident energy of 5 cal/cm², CAT 2, because it only takes 0.2 seconds to clear the fault.
This is all explained by the IDMT nature of the tripping curve. A similar explanation can be made for the LV tripping curve below.
Arc fault contributing branches
Calculations of branch currents contributing to total faults are necessary to determine the arc currents of different branches, which then inform the trip times of protective devices on those branches.
A protective device upstream of a fault can only register the current passing through it, not necessarily the total short circuit current, which might be higher. Therefore, the overall short circuit current should not be used to determine trip times unless the currents from other branches are significantly smaller.
For parallel lines, it’s crucial to compute each line’s contributing currents to establish accurate trip times. When dealing with branch currents through transformers, special care is needed to adjust the branch current by the transformation ratio, as this is a common source of errors.
In the event of a fault on the low voltage side of a power transformer, the protective device on the high voltage side will record a reduced current due to the transformer’s turns ratio.
Regenerative loads
Large loads greater than 37kW behave as regenerative loads. The combined contribution from the induction motors is treated as a separate contributing branch, with the fault current contribution remaining constant for 5 cycles (100 ms) and then decaying rapidly (approximated as an instantaneous step down to zero current after 5 cycles).
Determine the arcing duration
The duration of the arcing fault is determined by assessing the clearance time of protection devices under arcing fault conditions. The ‘fastest acting’ protection device, which would clear the arcing fault, should be identified for each calculation location, and this depends on the location of the fault.
Referring to the diagram we have used before, for a fault occurring at Location 2 (busbars), the ‘fastest acting’ protection device is usually the incoming device, while for a fault at Location 1 (line side), it is usually the upstream feeder protection.
For an overcurrent protection device, the trip time is determined using the time-current trip characteristic of the protection device (where trip time is a function of the arc fault current and the device’s time-current trip characteristic).
If the time-current trip characteristic includes an upper and lower tripping tolerance, the worst-case trip time should be used.
Incident energy and arc flash boundary
The initial step in estimating incident energy involves establishing the working distance, a concept defined by IEEE 1584. This distance refers to the gap between the nearest possible point of an arc and the worker’s body.
In situations where a physical inspection is not feasible, IEEE 1584 offers generalized working distances for different voltage classes, which are essential for assessments.
Following the identification of the working distance for the specific switchboard being examined, one should apply empirical equations from IEEE 1584 to calculate incident energy levels.
Key inputs for this calculation include the equipment’s operating voltage, the working distance relevant to this voltage, the calculated arcing current (both maximum and minimum), and the duration of the arc, which is determined by the reaction time of the associated protective device.
Furthermore, IEEE 1584 provides formulas to ascertain the safe distance from the switchboard where the incident energy poses a lesser risk to unprotected personnel. This safe zone is known as the ‘Arc Flash Boundary.’
According to IEEE 1584, the incident energy at this boundary is set at 1.2 Cal/cm², a level associated with the onset of second-degree burns.
Arc flash classification
When the worst-case arc flash incident energy has been determined, each piece of equipment can be given the appropriate arc flash classification.
Doors open or closed
‘Doors Open’ refers to situations where there is no door or panel separating a person from a switchboard’s live conductors or terminals.
An example of a ‘Doors Open’ situation is an open cubicle door on a motor control center or an open escutcheon on a distribution board. ‘Doors Closed’ refers to situations where a door or panel is present between a person and a switchboard’s live conductors or terminals.
It should be noted that the arc flash classification levels listed in the table above, along with the associated PPE requirements, assume that working personnel will be directly exposed to an arcing fault, i.e., switchboard panel doors are open at the time of the fault, are incorrectly fastened, or are forced open by the internal pressure developed by the arc.
Improving protection settings
As discussed earlier in this blog, arc flash is all about incident energy, which is also related to the time required to clear the fault. The incident energy from an arc fault can be greatly reduced by selecting appropriate protection devices and setting their protection settings optimally.
This optimization is achieved by reducing the protection settings as much as possible, while still maintaining time and current discrimination between protective devices in the electrical system.
Arcflash PPE
References
The following reference material was used in the production of this article.
[1] The Other Electrical Hazard: Electric Arc Blast Burns, Lee, R. IEEE Transactions on Industry Applications, 1982.
[2] Predicting Incident Energy to Better Manage the Electric Arc Hazard on 600-V Power Distribution Systems, IEEE Transactions on Industry Applications, 2000.
[3] ENA NENS 09-2014, National Guideline for the Selection, Use and Maintenance of Personal Protection Equipment for Electrical Arc Hazards.
[4] IEEE Std. 1584-2018, IEEE Guide for Performing Arc-Flash Hazard Calculations.
[5] NFPA 70E-2021, Standard for Electrical Safety in the Workplace.
[6] Electrical Arc Flash Hazard Management Guideline-2019.