Accurate single-line diagram a must for compliance

Electrical system safety and regulatory compliance begins with an accurate single-line diagram

By Joseph Weigel

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An uninterrupted flow of clean power is a critical element in any facility. Without it, production operations are impossible. Components, from the incoming service entrance switchgear to the low-voltage distribution and control devices, support the processes, safety and security, lighting, environmental and other critical applications.

Electrical distribution systems are well engineered and will be safe and reliable provided they receive regular, adequate maintenance and periodic testing. Because of its high reliability, many facilities don’t consider the support it provides until a failure occurs. In most industrial plants, electrical system maintenance is deferred because of budget constraints and system documentation isn’t updated to reflect field changes.

It’s common to find functioning industrial electrical distribution equipment that hasn’t been maintained and tested, is obsolete, or is well beyond its design service life. There’s often no indication of a potential problem until a fault occurs and the overcurrent protective device fails to operate, resulting in injury or damage to equipment and property. These failures often result in indirect costs, such as lost production or scrap, and also may present a risk to employees who are working on, or in the vicinity of, the equipment.

Resource documents such as NFPA 70B (Recommended Practice for Electrical Equipment Maintenance, 2006 Edition), the OEM’s installation and maintenance instructions, and other resources are available for guidance on proper maintenance intervals for electrical distribution and control equipment.

Maintain documentation, too

A critical element often neglected is maintenance of electrical system documentation, including a single-line diagram (sometimes called a “one-line diagram”), short circuit study, coordination study and arc-flash hazard analysis.

In new facilities, most system documentation is complete and accurate because it’s provided by the manufacturer as part of the contract. But the arc-flash hazard analysis is a relatively new requirement that hasn’t commonly been performed for new facilities, and for most it will be necessary to complete this study on the existing equipment.

As a facility’s electrical system is modified over time, this documentation must be updated to reflect the modifications. Many plants neglect to update the documentation when changes occur, which can lead to unsafe conditions and a likelihood of a failure. These documents and their purposes are:

Electrical single-line diagrams (one-line diagrams) show each piece of equipment and how they are interconnected. The single-line diagram begins at the utility service connection and shows equipment overcurrent protective devices, cable size and quantity per phase, conduit type and size, fuses, transformer sizes and impedances, circuit breakers and their trip units, and other details. An accurate single-line diagram is crucial for safety any time work is performed on energized equipment or during preparations for a lockout/tagout procedure.

Short circuit study (short circuit analysis) identifies the amount of fault current available at each piece of equipment. The highest available fault current always will be found at the service entrance. Each piece of equipment (and cable) introduces impedance, which reduces the fault current at that point in the system.

It’s necessary to determine the available fault currents so equipment can be designed to withstand the maximum current during a potential bolted fault condition. This information also is used to calculate arcing fault currents, which are used to construct an arc-flash hazard analysis. Arc-flash hazard analysis 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.

This electrician is wearing Hazard/Risk Category 2* gear per regulations.
Figure 1. This electrician is wearing Hazard/Risk Category 2* gear per regulations. (Oberon Co.)

Protective device coordination studies usually are performed in conjunction with the short-circuit studies to determine selection and settings for overcurrent protective devices. Proper trip settings are necessary to isolate faults to the nearest protective device, and to help avoid nuisance tripping. Circuit breaker trip unit settings are useful in reducing arcing energy and the study is a part of the arc-flash hazard analysis.

Arc-flash hazard analysis is a study performed for each piece of electrical equipment. It quantifies the heat energy (called incident energy) that could be released during an arcing fault. It establishes, among other things, the flash protection boundary distance and defines the personal protective equipment (PPE, Figure 1) that workers must wear when working on energized equipment.

This study uses the documentation described earlier. When documentation isn’t up-to-date, an arc-flash hazard analysis will require that the other studies be updated. The cost of updating the electrical single-line diagram, short-circuit study and coordination study usually are included in the cost of the arc-flash hazard analysis.

The quality of the arc-flash analysis is determined by the accuracy of the documentation, as well as the skill and ability of the analyst.

Protect your employees

In the past, designers focused on systems that could protect themselves during an electrical fault while allowing uninterrupted service to the loads, and on protecting the system and the loads from overcurrent. Until recently, worker safety hasn’t been a primary design consideration. However, with publication of the NFPA 70E standard and its enforcement by OSHA, designers must be more aware of the hazards to employees working on energized electrical equipment.

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