The Need for Equipment Data
A good single-line drawing contains basic electrical equipment data on transformers, cables, switches, filters, and generators. It also indicates the normal operating network configuration, revision number, date, and the editor’s information.
Sometimes, a second set of drawings may be included. This is called a cable schedule. The information contained in a cable schedule include the following:
- Number of cables in parallel
- Cable distances
- Bus duct lengths
- Cable types
- Cable sizes
- Cable ratings
Another separate drawing would contain protection settings, current transformer information, settings group used and overcurrent protective device model numbers.
The Need for A Good Equipment Naming Program
A good naming program for a worksite’s electrical equipment is a precursor to producing a good single-line drawing. The site should label equipment using wording that helps electrical workers locate or identify the equipment.
After all labels are affixed on equipment, then someone should perform a point-to-point check. This action will establish the single-line drawing.
NFPA 70E-2021 provides a good example of a single-line drawing in Informative Annex D (Figure D.2.2).
The Need for an Accurate Single-Line Drawing
IEEE 1584-2018, IEEE Guide for Performing Arc-Flash Hazard Calculations, requires that if single-line drawings are not available, these should be provided by the arc flash study. However, certain limitations of the arc flash study model drawings may surface when compared to single-line drawings. Three reasons are explained here.
First, certain exemptions may produce an electrical work condition that is not as safe as it should be. The IEEE 1584 exempts the modeling of certain low power loads. The standard also allows modeling of typical systems to represent a group of systems. Most reputable consultants model the repetitive scenarios, but there is no guarantee they will do so. In other words, the single-line drawing may not be comprehensive enough for energy control purposes.
Another aspect to consider is that most consultants trace conduit visually. This method introduces human error. Several conditions may lend themselves to human error: conduits going behind walls, references to incorrectly documented heights, glare from nearby lights, or conduits entering an overpopulated wall junction box.
Finally, if a third-party consultant is hired, he or she is often left to gather data alone. This possibility allows error because of the consultant’s unfamiliarity of the electrical lay-out. A simple fix would be to require an on-site worker to accompany the consultant all the time data is being collected.
End-Goal: Creating A Safer Work Condition
Consider these and other points as your site plans to create and/or maintain single-line drawings. The end goal is to provide a safe working environment for employees. This starts with knowing the equipment thoroughly.
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