Ideally, a single-line, or one-line, drawing (SLD or OLD) would already be in place before an arc flash study is performed.
The SLD is a key input to using the IEEE’s 1584 Guide for Performing Arc Flash Hazard Calculations. The “guide provides mathematical models for designers and facility operators to apply in determining the arc-flash hazard distance and the incident energy to which workers could be exposed…”
The plant personnel are usually in a better position to either update or create an SLD, instead of a contractor/consultant that is seeing the plant for the first time. A robust drawing will serve as a good due-diligence check to ensure the accuracy of the arc flash data collection accuracy and is a good reference for other needed safety functions like identifying electrical sources for lockout/tagout verification (Control of Hazardous Energy).
Who Should Be Involved?
Circuit identification and the establishment of single-line drawings should be performed by electrically knowledgeable and experienced persons, such as qualified electrical workers.
These qualified electrical workers can be electricians or electrical engineers already employed by the plant site. They can also be third-party consultants; however, we do not recommend that consultants/contractors be expected to complete this task by themselves. No matter who the plant site management chooses, the plant site should request a quality assurance program.
What is the Process?
Consider the safety aspects of developing a single-line drawing. It is better for the plant, together with the engineer performing the arc flash study, to establish the drawing during a planned shutdown or outage. During that time, when equipment is maintained in a zero-energy state, the qualified electrical workers can collect all the necessary information on the equipment and create the drawing.
Next, the equipment is labeled. Implementing an organized and clear naming system aids electrical workers in locating and/or identifying electrical panels. Avoid duplicating names, even if across different buildings or processes.
Then, another qualified worker should perform a point-to-point check. Also, terms and conditions for remediating errors and omissions should be set up.
This type of organized system, or a similar one, should govern how single-line drawings are established.
What About Equipment Labels?
The best practice is to NOT combine electrical network information with the arc flash label information. However, this is not specifically prohibited in any standard.
Combining information on the same label can cause errors. In fact, errors caused by human factors have been noted during most of my site visits (more than 60% of the cases). Questions must be asked: Was the arc flash information incorrect because the source was incorrectly modelled? Was it a case of human error? Or had the plant been modified since the last arc flash study?
Refer to NFPA 70E 130.5(H) for all required information to be included on the arc flash label.
