Using arc flash studies to enhance safety
- Review the steps for arc flash incident calculations.
- Explore the uncertainties in utility calculations.
The science of arc flash studies is geared to find the worst-case scenario and protect workers from these conditions. This is not an exact science, but a series of best-case/worst-case approximations. To understand the approximations, it is important to look at the major factors-and steps-in the arc flash incident-energy calculation.
Steps in arc flash calculations
The following are steps typically taken in arc flash incident energy calculations.
- Data is collected. This data includes utility short-circuit contribution, site transformer, circuit breaker, fuse, relay, and cable data.
- Data is input to a computer model, usually using a computer program. Different scenarios are often built to mimic different system operating conditions within the facility, such as switchgear with closed tie breakers, motors and pumps thatmay be running, and alternative power sources like onsite generation or uninterruptible power supply (UPS) systems.
- A short-circuit study is run to determine the fault levels at various locations in the system.
- A coordination study is often run next. This plots the relays, fuses, and breakers in the system to determine trip levels and see which device will operate first in a fault.
- The arc flash study is run. This study is run in each scenario, using the fault currents previous calculated and the trip times for the breakers and fuses as loaded in the coordination study. This energy is calculated at the working distance for the equipment type specified.
Uncertainties in utility calculations
The collected data is the first step of uncertainty in the calculations. This data consists of data from transformers, circuit breakers, relays, fuses, cables, and the utility. Much of the data necessary for an arc flash study is the same data necessary for a short-circuit study. The utility is asked to provide short-circuit data at the point of connection to the facility being studied. This short-circuit data is from a calculation from other data, much of which is dependent upon other conditions. These include conditions such as online generation and transmission-line configuration. The amount of power being fed into the power grid affects the amount of power available on the lines.
The type of generation can also make a difference. A fault affects a large turbine generator differently than it does a solar cell. A wind turbine thatis online part of the time may not be contributing to the grid at the time of the downstream fault. Many of these conditions are minimized by the transmission system itself, however. The transmission system components that affect and contribute to a fault include transformers and transmission lines. Fault current can flow through a transformer in proportion to the transformer’s impedance. This value is largely unaffected by outside conditions. Transmission-line impendence, however, is calculated based on conditions such as line length, conductor size, and conductor configuration. This means the impedance can be affected by such things as construction techniques, splicing, and even wind and weather. These effects are small, but they serve to demonstrate the uncertainty in the data at the utility level.The utility often will supply a range of values to account for these possible variations. This range may be accounted for in the computer model by using different scenarios, usually a maximum and minimum value. (Figure 2)
The next, and possibly greatest, level of uncertainty in the short-circuit calculation is the site’s cable data. Cable data is often estimated. This is not due to any lack of competency of the study engineer or the data collector. This is simply due to the near impossibility in obtaining exact cable lengths. Cables may have loops or pull slack in them, most likely made during installation bya thoughtful electrician who wants to create a better-looking installation or allow for future work. Simply measuring the conduit or cable tray may not account for these loops or pull points in the cabling. The length is estimated as accuratelyas possible, but these differences can cause variations in the calculations. The other problems with cable data include installation methods. Whether the cable is installed in aluminum conduit, electrical metallic tubing, or cable tray can affect the impedance of the installation, therefore, the short-circuit current through the system. Unless all cable changes including splices, wiring sizes, conduit and tray transitions, etc.are accounted for, the exact impedance cannot be known. The result is an estimation thatis very close to the real conditions. It is yet another level of uncertainty, though.
Different scenarios are often created in the computer model to calculate short-circuit and arc flash energy under different site conditions. These scenarios may include differing utility data, as previously mentioned. They should also include conditions such as tie-breaker operations, onsite generation or UPS operation, as well as motor loads. Motors, especially large synchronous motors, can contribute current back toward a fault as their magnetic field collapses. The motors chosen to be represented in the different scenarios create changes in the short-circuit current at differing points in the system. Often, two scenarios are created-one with no running motors and one with all motors. This creates a case of maximum and minimum fault-current contribution from motor loads.
The breaker/relay-coordination study provides trip and operating times for overcurrent protective devices. Trip curves are looked at to determine the operating order for various points in the system. In some cases, changes may be recommended. Breaker and fuse trip curves are provided by the manufacturer. These have uncertainty included in the form of shaded areas on the curve. These “tolerance bands” represent the fact that their operating speed may be dependent on ambient temperature, previous current levels, installation, and other factors. This is largely due to the fact that these devices are thermallyoperated and depend on the heat generated by the current flow through the device. Electronic devices are often more reliable and have smaller tolerance bands then thermal-magnetic breakers.(Figure 3) The choice of arc flash study method can be one of the final means of uncertainty. There are several calculations available to the engineer.
The choice of calculation can affect the resulting incident energy as well as the arc flash boundary. Ralph Lee presented one of the first ways to find the incident energy, and this still persists today. The arc flash boundary for this method is generally closer to the source than other methods because it assumes a quicker decay in energy. The current IEEE1584: IEEE Guide for Performing Arc Flash Hazard method is the most common method, but it is not recommended for all situations. Differing methods exist for transmission voltages, and there are other methods for distribution voltages as well. NFPA 70E: Standard for Electrical Safety in the Workplace stipulates an arc flash hazard study but does not endorse any method.
IEEE arc flash study method
Within the IEEE method, the arc flash incident energy is calculated based on equipment type and working distance. These can vary depending on the outlook of the study engineer. Is a large, low-voltage switchboard a panelboard or switchgear? Should an 18-in. boundary be used for low-voltage switchgear, or a slightly more distant 24 in.? These choices affect the resultsand the protective measures taken, in various degrees. The
IEEE 1584 equations represent data from numerous tests performed. The data from these tests have been analyzed and a computerized curve fit was applied to arrive at the set of equations. This means that, although the data may describe the conditions of the tests very well, the tests may not reflect the conditions of the site being studied. This will always be the case of curve-fit-type equations. It is one of many reasons why a better, physics-based model is needed.
One of the final unknown variables in the arc flash hazard calculation is the equipment setup. Calculation methods are based on open or closed box setups. The equipment inside the box is not accounted for. The additional equipment can absorb or redirect the energy from the blast. Where this will go is difficult, if not impossible, to predict. It is commonly assumed that an arc flash incident will be initiated in the back of the cubicle or gear, with the energy radiating out toward the worker. If material is between the worker and the point of initiation, the results can be unpredictable. The material may either shield the worker or become additional projectiles hurling out of the cubicle.
All these factors combine to form a science that is far from exact. In the best case, these effects combine to cancel each other out. In the worst case, they combine to skew the results from reality. This is one reason why most equipment is posted with the results of the worst-case calculation. Arc flash hazard studies are used to make workers aware of the extent of the hazard.
While exact results for the location, operating conditions, time of day, and weather may not be available, worker protection in the form of knowledge of the hazard is extremely important. With that information, people can take appropriate, common-sense actions to protect themselves.