Arc Flash Standards: IEEE 1584
The IEEE Standard 1584 – IEEE Guide for Performing Arc-Flash Hazard Calculations provides a method for calculating arc flash hazard for electrical distribution systems. The first edition of the standard was released in 2002 using an empirically derived model based on arc flash measurements performed in a laboratory. When further research yielded results differing from the energy predicted by the model, a revised standard was published in 2018. The new edition features more variables such as switchgear design.
The standard provides the following method for calculation:
- Gather data for a short-circuit and protective device coordination study
- Calculate short-circuit current
- Calculate the arcing current
- Determine the trip time for the arcing current
- Calculate the incident energy in cal/cm²
The equations have a limited range of validity for current and voltage although most industrial systems are within these limits.
Data Collection
Details about the electrical distribution system are required to accurately calculate the hazard level. The following lists the information typically required:
- Utility data; fault contribution and protection settings
- One Line Diagrams showing the main electrical equipment
- When one-line diagrams do not show complete distribution, an additional list should be obtained of all three-phase electrical distribution panels. These are the locations that are going to be labeled (and calculated). Loads and single-phase distribution are not applicable.
- Cable List with Sizes & Lengths. If estimated, the lengths that are at least accurate to within about 5m
- Relay and circuit breaker setting schedule
Typically, an onsite data collection effort is required to verify drawings are accurate. This is especially a concern for older installations.
Scenarios
When the electrical distribution system is operated in different modes, scenarios can be created that simulate the entire range of operating parameters. This ensures that the worst-case hazard is found. As an example, the following scenarios may be created:
- Maximum Utility – full contribution of all sources, including motors
- Minimum Utility – minimum contribution of utility and all rotating equipment (such as motors) out of service
- Emergency – with emergency generators supplying part of the system
A calculation needs to be performed for all applicable scenarios and evaluation should be based on the worst-case result. Which scenario yields the worst-case result is determined for each location separately.
Short-circuit Evaluation
The results of the short-circuit evaluation can be used for checking if switchgear and protective devices are adequately rated against short-circuit currents.
During data collection the short-circuit withstand of all switchgear and protective devices is gathered and compared to the maximum calculated short-circuit current. When current-limiting devices are present (such as fuses or molded-case circuit breakers) their limiting effect will be included as far as documentation is available.
The following parameters are evaluated:
- The symmetrical (RMS) withstand current for thermal withstand
- The peak current for mechanical withstand
Any issues with inadequate rating of equipment are immediately reported, as they can create unsafe working conditions.
Protective Device Evaluation
Overcurrent devices that are modelled should be checked for miscoordination for overload and short-circuit currents. When during an overcurrent more than one device trips, these devices are said to not provide full coordination, and this may present operational issues (although not necessarily safety related). In this case alternative protection settings can be explored that do provide improved or full coordination.
A report should include a list of locations where full coordination is not available. Alternative protection settings can be presented with their limits, as this is usually a compromise between operational stability, safety and cost. A setting evaluation will consider the following information:
- Connected loads and system stability
- System coordination
- Arc flash hazard levels
Arc Flash Hazard Calculation
Using the result of the short-circuit evaluation and the protection device coordination studies, the arc flash incident energy and the associated boundary can be calculated according to the equations in IEEE Standard 1584. A report should provide the calculation results, source data and any approximations or analytical variables used.
