Incident Energy: The amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. One of the units used to measure incident energy is calories per centimeter squared (cal/cm2).[1. National Fire Protection Association. 2008. NFPA 70E-2009: standard for electrical safety requirements for employee workplaces. (Quincy, MA): NFPA.]
When you hear arc flash and arc flash studies, what most people are really interested in is the incident energy portion of the study. This study helps employees, employers and all other interested parties determine the energy level, and the associated PPE required when working on a specific piece of equipment. Even if your facility or company doesn't do any live electrical work, these studies are important to have to verify that the energy has truly been isolated. Until the equipment has been verified to be de-energized, it must be assumed to be energized, and therefore a danger of an arc flash is possible.
An incident energy study is only one part of a complete power system analysis. The other portions of a complete power system analysis include:
An incident energy (IE) study is something that has to be coordinated by the operator of the facility and the engineer developing the report. The operating conditions of the system can have very dramatic affects to the available incident energy at the location of the fault. The following factors must be considered when setting up the power system model when arc flash and their resultant incident energies are the intended outcome:
Available Short Circuit
Cable length, type and configuration
Protection Settings
And connected motors.
Cable Configuration and Length
Like mentioned above, impedance plays a large part in how much fault current is available. When designing a power system, the short circuit current may be calculated with either very large or no cables, using the transformer as the only impedance. When the short circuit study is completed with this data, the bolted fault and therefore the arcing fault will be much less.
Short Circuit
The available short circuit at a particular bus is determined by the short circuit study. The two factors that determine the available energy are, the amount of current and the amount of time that it is there.
The short circuit current is determined using calculations assuming a bolted fault, like a bus bar has been bolted to the bus and the power turned on. Since an arcing fault has an impedance the maximum power in an arcing fault is Parc = Pbolted x 0.707^2, or about half.
Protection Settings
Like with the short circuit, protection settings may be initial designed assuming bolted faults with little to no cable information. When the cable information is added, the available fault is much less, and therefore the protection settings may need to be tightened. Parc is just one variable in the incident energy equation, the other is time. If the protection is electrically far away from the actual arcing fault, it won't be tripped for a longer period, increasing the incident energy without added reliability.
Connected Motors
When motors are disconnected while they are running, they become generators and the voltage is increased over the gap. If there is an arc, this lessens the impedance (over free air) and current will be supplied into the arc from the motors for a short period of time. This time is determined by the inertial load on the motor, and the size of the motor. For example a larger motor will supply more energy into the fault, and have a larger inertia than a smaller one.
We like to include any motors that are above 25hp in the model and then lump the other connected motor according the load calculator in the Canadian Electrical Code. This allows us to get an accurate picture of what the motors will contribute to the fault.
Operation Data
We mentioned above that it is important to have operations involved from the start, this is because the configuration of the system is very important when determining the incident energy in the case of an arc fault. While getting the worse case short circuit is relatively easy, determining the worse case with regards to incident energy is much harder. The load on the system, how loaded large motors are, what transformers are connected, etc have large impacts on the overall system impedance.
To get the correct data in the report, various operational "norms" need to be compared, and these normal system configurations will be provided by the operations team at the facility. If there is a special configuration during live work, this must be taken into consideration before work begins, and the engineer must be informed so they can determine if this will be a new worse case. A large system may have too many possible configurations to account for at the time of the study, therefore only the regular operating configurations may be looked at.
Conclusion
Incident Energy Studies are an important tool in operating a safe and reliable electrical power system. In the coming weeks we will explain the other power system analysis components mentioned here, Short Circuit and Protection, and add on more Load Flow.
