The Unseen Plant Danger
The big question is, “Where do I start and what needs to be done?” To initiate a program, it will be necessary to develop a data collection program to identify all the electrical equipment in the plant. This data collection process may be done by qualified employees within your company or by outside contractors. This data gathering program will need to be done only once, and if all the required data is documented the first time, it will simply require updating if there are any changes. The study will require several steps to complete. The following is a list of possible steps and a description of each.
Step 1: Data collection
Before an electrical study can be initiated, it will be necessary to document all of the electrical equipment in the plant. This will include the power line information, switches, fuses, transformers, capacitors, switchgear, motor control centers, cables, and motors. The nameplate information on each device should be recorded.
During the data collection process, it would be a good time to check, test, and clean the individual components. This would include the circuit breakers, disconnect switches, fuses, overloads, and contactors. Ensure components are of the correct value and record the settings on adjustable devices. Check for loose wires and corroded connections. Each motor control center (MCC) bucket should be removed to evaluate the condition of connections that are not normally visible on the back, and the internal MCC bus work.
All data collection should be scheduled during a time when the power will be OFF. This work can be done with qualified personnel within your organization, or it may be done using outside help. This will be the most time consuming part of the study.
Step 2: Develop a single-line/one-line diagram
Using the electrical data collected in step 1, develop a single-line drawing, to be maintained, that shows the power flow to the plant and the plant devices. Each power company switch or company-owned disconnect switch should be indicated on the single-line diagram. If there is a number the power company assigned to a pole for a switch, this number should be documented and placed on the single line diagram. If the disconnect is owned by the operating company, a unique number should be created and placed on the pole supporting the disconnect device, and the number indicated on the single line. This will help in identifying the switch should there be a requirement to have the switch operated and locked out. A unique number will help in identifying the switch, avoiding any confusion.
Step 3: Model the electrical system
Several companies offer software programs that may be used to perform the required calculations for an arc flash study. These may be purchased and personnel trained to use the software, or an outside electrical engineering firm may be contracted to provide the study. In order to do a study, the data collected is used to create an electrical model of the plant with the software. Once the data has been put into the program, it becomes a matter of selecting the required study.
Step 4: Complete the required studies
Each of the studies is built into the software program and requires the click of a mouse to call up and display. The following list describes the studies required.
- A system fault study. This study is required to calculate the amount of fault current that will be available at each system location.
- A system coordination study. This study will determine if the electrical protective devices are coordinated with the upstream device in order to provide adequate protection. By analyzing the coordination of the protective devices, it may be possible to make modifications to the settings and reduce the arc flash intensity.
- An arc flash study. This is the study that all of this work leads up to. The arc flash study provides the following data:
- The incident energy at the equipment location. This is the amount of arc flash energy that an individual would be subjected to in a fault condition. This value is a function of current and time the current is available. The amount of incident energy determines the class of PPE that would be required if working on energized equipment.
- The working distances calculated for the incident energy calculated.
- The required information should be printed on a warning label that is placed on the door of the equipment. The 2009 edition of NFPA-70E 130.3 (C) states “Equipment Labeling. Equipment shall be field marked with a label containing the available incident energy or required level of PPE.” The software will provide labels that list both. An example of a label providing both is shown in Figure 1.
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