A single line diagram, also referred to as a one-line diagram, is usually a single page document that represents a facilities electrical distribution infrastructure.
It will have one single line shown for bus (or cable) to represent all three phases. It will also have symbols that represent breakers, meters, relays, and any other control items that you may have present. It may also include the ANSI protective functions that exist in your equipment.
Many times, before you start working on your gear you need to consult the one-line diagram to be able to ensure you are shutting off the correct upstream breaker. But what if you don’t fully understand how to read a one-line diagram? There are several lines and a lot of symbols all over the page. There usually should be a legend that will help you make sense of the diagram, but one is not always provided. This document will help with some of the confusion and give you confidence that you are correctly reading your one-line diagram.
As mentioned above there is usually a legend provided in either the upper or lower corner that is just like a map legend. It will tell you what each symbol means by using universally accepted symbols. A sample is provided below. It will have the relays, meters, breakers, transformer, and any other types of devices that you might normally find one a one line.
Once you have located and familiarized yourself with the legend, you can start reading your one line. A one line typically starts at the top of the page and works its way down. It will start with the utility or other means of incoming power and its disconnecting device. It will then flow down to the distribution equipment like a switchboard or MCC and then finally it will end with the loads, like a motor or panelboard. A one line may have different voltages present on the page and should have transformers shown to help you identify when you are changing from one voltage to another.
You can see in our below example that the utility is feeding our one line represented by a breaker. It then flows into a transformer and into the main switch gear for the facility, in our case MVSWGR, Medium Voltage Switchgear. This main switchgear then distributes the power to various parts of a plant. You can see on the left side there is a transformer that then feeds down to SWGR1. SWGR1 will typically be 480V in our example. The MVSWGR also feeds a Medium Voltage MCC with motors attached. If we continue down the left had side we see that SWGR1 feeds two items, MCC and MSB 2. The MCC has motors attached and the MSB 2 has several distribution breakers that will go on to feed smaller loads.
Let’s look at the above example. We want to work on MSB 2, so we need to identify where MSB 2 is on the one-line, what upstream equipment is feeding MSB 2, and what disconnect we need to lock out tag out so that we can safely work on MSB 2. To do this, we first need to identify where MSB 2 is on the one-line diagram. We then need to follow the line from MSB 2 upstream to see what is feeding MSB 2. Remember the lines are representative of cables or bus in real life.
We start at the top of the diagram and follow the equipment down. We see that the power flows from Utility to XFMR 1 to MVSWGR to XFMR 2 down to SWGR 1 and finally to MSB 2. So, to be able to work on MSB 2, we need to lock out tag out the breaker in SWGR 1 that feeds MSB 2. This will allow us to ensure that MSB 2 is in fact de-energized, which will allow us to safely work on MSB 2.
Every piece of electrical distribution equipment you purchase should come with a one-line diagram.
You should also have an overall one line of your site that shows every piece of equipment. It is important to keep these one lines up to date as new gear is added, gear is removed, or changes are made. One-line diagrams are the easiest way to tell how a piece of equipment is being fed and from where. With an updated one line, you can ensure that you are safely disconnecting the gear before beginning to work on it.
Review your equipment’s’ one-line and ensure that they are accurate and up to date. Try reading a one-line that you know is accurate and see if you can properly identify a piece of equipment and what types of disconnects it includes as well as what it is feeding.
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