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A critical element that is often neglected in industrial manufacturing plants, including processing facilities, is updating electrical system documentation. This includes things like maintaining an accurate electrical single-line diagram or commissioning a short circuit study, coordination study or arc-flash hazard analysis.
As a facility’s electrical system is modified over time to meet the requirements of production and product changes, this documentation must be updated to reflect any modification of the electrical system. A common practice in many industrial facilities is to put off updating the system documentation when facility changes occur, typically due to budgetary constraints, but this practice ultimately leads to unsafe conditions and a higher likelihood of a significant and damaging process-interrupting failure within the system.
This article will provide background on the various components of electrical system documentation, but more importantly, why it’s important to keep them updated.
When facilities are newly constructed, most of the system documentation will be complete and accurate – these documents are usually provided by the equipment manufacturer as part of the construction contract specification requirements. The exception has been the arc-flash hazard analysis, which is now required in all facilities – see NFPA 70E® 2004 Article 110.8(B) (1) (b) (Standard for Electrical Safety in the Workplace) for details. It will typically be necessary to complete this study on the existing equipment.
Following is background on the crucial components of electrical distribution system documentation that should be kept up to date, particularly in existing facilities.
The electrical single-line diagram, or one-line diagram, shows each piece of equipment and how they are interconnected within the system. The single-line diagram begins at the point of the utility service connection and shows each piece of equipment and its overcurrent protective devices, cable sizes and numbers per phase, conduit types and sizes, fuses, transformer sizes and impedances, circuit breakers and their trip units and other details. An accurate single-line diagram is crucial for worker safety any time work is performed on energized electrical equipment or during preparations for a lockout/tagout procedure.
A short circuit study, also called a short circuit analysis, is an engineering study that identifies the amount of fault current available at each piece of equipment in the system. The highest available fault current will always be found at the point of the facility’s utility service connection. Each piece of equipment (and cable) in the system presents impedance, which has the effect of lowering the fault current at that point in the system. It’s necessary to determine the available fault currents so that equipment can be designed (i.e. braced mechanically and electrically) to withstand the maximum flow of current during any potential occurrence of a bolted fault condition. This information is also used in the calculation of arcing fault currents which is used in constructing an arc-flash hazard analysis.
The protective device coordination study is usually performed in conjunction with the short circuit study. It determines the selection and settings of each overcurrent protective device in the system. Proper settings of the device trip units are necessary to selectively isolate faults to the nearest protective device, and to help avoid nuisance operations that may be due to transformer inrush currents or motor starting operations. Circuit breaker trip unit settings are also useful in reducing arcing fault incident energy, and the coordination study is also a critical part of the required arc-flash hazard analysis.
An arc-flash hazard analysis is generally performed for each piece of equipment in the system. It calculates the potential amount of heat energy (called incident energy) that would be released during the occurrence of an arcing fault within the equipment. It is necessary to establish, among other things, the flash protection boundary distance, and also defines the class of personal protective equipment (PPE) that workers must wear to protect themselves while working on the equipment while it is energized.
This study depends heavily on the other three system documentation elements described above. When the facility documentation is not up to date, the arc-flash hazard analysis process will require that the other studies be updated. The cost of updating the electrical single-line diagram, short circuit study and coordination study in this case will usually be included in the cost of the arc-flash hazard analysis. The final cost of the arc flash hazard analysis will depend heavily on the accuracy of the other electrical system documentation. But the quality of the arc-flash hazard analysis will be determined by the accuracy of the documentation, as well as on the skill and ability of the supplier that is selected to perform this work.
Growing focus on employee protection
In the past, the primary focus of electrical system designers has been designing a system that will protect itself in the event of a significant electrical fault while still allowing uninterrupted service to the loads, along with protection of both the system and the loads from overcurrent conditions. Until fairly recently, the focus on worker safety has not been a primary design consideration. However, with the publication of the NFPA 70E standard and its enforcement by OSHA, designers are more aware of the hazards to employees when they work on energized electrical equipment. As this awareness increases, electrical designers are likely to take equipment design strategies into consideration that will adequately protect the system, as well as maximizing the protection of workers from injury.
With the focus now on electrical workplace safety, and the requirement in the standards for an arc-flash hazard analysis to be completed in most facilities, the need for accurate system documentation becomes even more compelling. Once the documentation is updated, it should be revised and maintained whenever electrical system changes occur. These documents should be maintained to depict actual existing conditions in the plant because they are crucial components of the employer’s electrical safety program.