The Unseen Plant Danger
Ensure electrical safety at your operation by understanding and implementing safety standards.
by Edward J. Muglach
It’s early in the morning. Your maintenance crew has been working all night repairing an electrical problem on a crusher motor. The repairs are complete and your electrician has gone to remove his lock and hold tag to re-energize the electrical system. You wait patiently for the crusher to start when, BAM! There is a bright flash of light and a loud noise from the switchgear building. All power in the plant is lost. “What happened?” you yell to the other employees as they are running toward the switchgear building.
You follow everyone to the switchgear building. There is a lot of smoke in the area, and you expect the worse. Just as you get to the building, your electrician walks out the door. An arc flash had been generated when he closed the disconnect switch, but he was wearing his Personnel Protection Equipment (PPE), that protected him from the arc flash. There is equipment damage, and you are going to have to be down a little longer, but there was no serious injury or loss of life.
What is an arc flash? An arc flash is the sudden release of electrical energy through the air when there is a breakdown between two or more current-carrying conductors or one conductor to ground. An arc flash gives off intense heat, light, and pressure.
The intense heat, up to 35,000 degrees Fahrenheit, can melt metal conductors allowing metal droplets to fly in all directions. The temperature increase can ignite clothing. The flash of light can blind employees, and the pressure from the rapidly heated air can place extreme forces on workers, damaging their hearing and causing other pressure related injuries. All of this can happen in a fraction of a second. Treatment of a worker that survives an arc flash incident can require years and may cost in excess of $1,000,000.
In an effort to prevent or significantly reduce electrical accidents, the Occupational Health and Safety Administration (OSHA) contracted the National Fire Prevention Association (NFPA) to develop a standard for working around electrical equipment. NFPA-70E, “Standard for Electrical Safety in the Workplace,” was developed to introduce standards and to develop guidelines for a workplace electrical safety program. NFPA-70E has been around since 1976, but was not well recognized until 2000 when tables were introduced to allow workers to choose PPE. In 2002, the equipment labeling requirement was introduced. OSHA revised its electrical standard, Subpart S of 29 CFR part 1910, to rely heavily on the information in NFPA-70E.
In the future, there is going to be a stronger emphasis on the condition, maintenance procedures, and troubleshooting techniques for electrical systems in a plant. New methods and procedures for doing work around electrical equipment will have to be implemented. Extra training will be required of workers doing electrical maintenance. The training would be to ensure the employees understand the hazards of working on energized equipment. The safest way to work on electrical equipment is with it de-energized, but that is not always possible.
If an arc flash program has been initiated by your company, you are ahead of the game. If not, there are several things that will be required to initiate a program. The first step would be to understand NFPA-70E and the procedures it addresses.
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.
All of the work described above, along with an accurate available fault current from the power company at the feed point to the plant, will be required to accurately provide an arc flash study.
Article 110.3 of NFPA-70E states, “Responsibility. The safety-related work practices contained in Chapter 1 shall be implemented by employees. The employer shall provide the safety-related work practices and shall train the employee who shall implement them.” There are also guidelines for training an employee to be a qualified person that can work on energized electrical equipment.
If a program of this type has already been initiated within your company, you are ahead of the game. If there is not an active program in your company, arrangements should be made to initiate one. It will take time to collect data and to complete the studies. The sooner it is initiated, the sooner you will be able to meet the “Standard for Electrical Safety in the Workplace” in your plant. Taking steps to implement the practices discussed in this article will help to protect your employees and keep your electrical equipment functioning as intended.
To come: Edward J. Muglach is the senior principal engineer for Vulcan Materials Co.