A single line diagram as the name implies, is a one-line diagram. In Electrical Terms, it is used to show how electrical power is distributed within an installation be it a factory, shop office, mall, or even a hotel. Most non-domestic installations have on display in their Utility or Electrical Rooms, this Single Line Diagram on display. It is a type of block diagram that is usually framed, and typical sizes range from A3, to A1 or A0. AO is usually the largest size on display. It is framed and hung in a location that is clearly visible to anyone working in that Electrical Room.

The Line Diagram can show the electrical power coming in from the Source i.e., the Utility Company such as TNB in Malaysia. It can also illustrate the distribution of power from one room to another such as from the Main Medium Voltage (11kV or 33kV in Malaysia) Room, to the Power Step Down Transformer Room, to the Main Switch Board Room operating at Low Voltage and even to the standby Generator Room if applicable.

You can also identify the symbols used in the Single Line Diagram to represent the different types of Components, such as Circuit Breakers, Power Transformers, Switchgears, Bus-Bars, Capacitors and even Conductors. Such Components are shown by standard symbols on the Line Diagram, and they are usually explained in the Legend on the same Diagram. The use of Symbols to denote Electrical Components is usually standard across many countries.

On the diagram, it is also clearly stated the type, size and rating of the components used, such as the types of cable, size of cable, type of Switches used be it either an Air Circuit Breaker, or the Moulded Case Circuit Breaker or MCCB. There are many choices of equipment that can be used in a single electrical panel, with differing make and models available. As far as possible, all the relevant information must be on display on the Single Line Diagram so that it makes for complete and easy reference for anyone reading it.

Single line diagrams used to be drawn by hand when I started my Electrical Journey in a Consultants office in 1992. Nowadays software such as AutoCad is used, and this makes it much easier to edit and make necessary changes as are often required. Having a digital copy also aids in easier sharing and distribution of these drawings to different parties involved in the same project.

It is often the case that a physical installation such as a building will undergo some renovation or upgrading work during its lifetime, and electrical items are often modified in such cases.

 Sometimes new cables are laid to new Electrical Panels or certain equipment is replaced due to wear and tear. Some components must be replaced during an upgrading process and replaced with equipment with a higher rating. In such a case, the Single Line Diagram must be updated as well.

The line diagram is usually designed by a Project Electrical Consultant.  The Electrical Consultant has to use their professional skills to design an installation to be safe and free of any electrical hazards, as it is within their Professional Duty of Care to do. This is to avoid any untoward incident to members of the public for instance, arising from the faulty design of the electrical infrastructure under their supervision.

It is referred to during the many stages of construction, by the installer or Electrical Contractor. Any omission or deviation from the specification can be seen upon project completion as even the make and model of the electrical components are specified on the Single Line Diagram.

Unfortunately, as is the case, many existing buildings do not have on display any Single Line Diagram. A Contractor who wishes to work upon the electrical installation at such a site, has to physically trace the location, route and size of the existing cables, and other components, to be able to work safely and to get a better understanding of the power distribution within that site. This can be tedious work that is also time consuming.

Safety is another important consideration with Single Line Diagrams as some circuits need to be isolated (turned off) before work can begin safely on site.  If the Diagram is not correct or accurate, the wrong switch could be operated and turned on, and the actual area of work could still be energised and have live electrical power. This could lead to an unsuspecting worker being electrocuted.

This is why most countries have Regulations that state that every panel must have a Line Diagram on display, if possible, on the nearest wall, for easy reference. The Regulations go one step further, to stipulate that, besides the need for accurate and visible Single Line Diagrams, they must be endorsed by a licensed and Professional Electrical Engineer, usually a Chartered Engineer.

The person who signs on the drawing is certifying that the installation represented on the diagram is accurate. It denotes that he or she has checked to ensure the correct selection of cable and component sizes has been used in the installation of that Electrical Installation. Such a certification is usually for life and there have been instances, whereby the Professional Engineer has been hauled up to court, or even put in jail!

This has occurred when there was an electrical mishap that resulted in death. Such an event would have been caused by the negligence of the Professional Engineer who certified that the design as represented on the drawing, was done in accordance with the prevailing regulations and Standards.

It should be replaced when it becomes faded, and this does happen over a few years of the original drawing being framed on display. If it is to be replaced, it should be re-certified by the original professional engineer who is responsible for the original design.

So, the Single Line Diagram is an important tool that is often referred to by different people during the lifetime of an installation. It is often mandatory and the failure to display a clear, accurate and certified Line Diagram can bring with it a significant fine.