How to read a circuit diagram: Part 2

Learning how to understand electrical circuits: practical tips for reading circuit diagrams.

What types of circuit diagram are there?

In addition to the plans listed here in the guide, there are others such as the terminal diagram, which shows the wiring in the electrical system in detail using terminals and terminal strips. This means that a lot of information comes together. For complex circuit diagrams, it is therefore important to first get an overview of the existing documents, the circuit symbols used and the labelling, and only then start reading the diagrams. In principle, a distinction can be made between single-pole and all-pole circuit diagrams.
In the single-pole variant, wires – for example between a power source and a resistor – are represented as a single line, regardless of how many cores the wire consists of. Single-pole variants include installation and overview circuit diagrams, while a circuit diagram is an all-pole circuit diagram. In the case of all-pole circuit diagrams, the individual conductors (including the neutral and protective conductors) are shown in detail so that the path of the current can be precisely traced.

Installation plan

In buildings, the correct placement of lamps, switches, sockets and large appliances such as electric ovens is crucial for a convenient-to-use electrical installation. The installation plan therefore follows the floor plan of the building and places the individual electrical elements in the intended location. Anyone who has the installation plan in front of them and can read it will quickly know where each installation belongs. The plan thus serves as a guide for the electrical installation.

What is an installation plan

The installation plan takes into account the presumed or agreed requirements of future residents of a property with regard to the electrical installation of the building. At the same time, it ensures compliance with legal requirements and regulations, such as the requirement that wires be laid horizontally or vertically, but not at an angle.
If you take, for example, the installation plan for the electrical installation of an entire detached house, the plan describes the main and sub-distribution boards or sub-distribution boards. The main power supply systems with fuse units and main lines are located between the utility company's connection point and the meter or meters.
The installation plan for the sub-distribution board shows, among other things, how many sockets are to be placed in each room and where. Perhaps electric shutters are planned? These also belong in the installation plan, as do the cable connection for the telecommunications network, the doorbell and smart home technology.
You can distinguish between installation plans with or without cable routing. The latter have less information. However, they are clearer when it comes to the placement of the individual components. An installation plan can be very simple, as the following example illustrates:

To understand it, it is good to look at an installation plan of medium complexity. The following installation plan is drawn on a floor plan showing the individual rooms and the position of doors and windows.

As with all wiring diagrams, knowledge of the symbols used is also important for understanding the installation plan. In the plan on the left, for example, you will find symbols for switches, sockets and lamps. These are the same symbols as those used in the example of a very simple installation plan here in the guide. If the symbol for a socket has a number, this indicates the number of sockets on a multiple socket outlet. Symbols for appliances are particularly useful in the kitchen, because it is especially important to think about the placement of large kitchen appliances such as the stove in advance. This installation plan uses colours to make the plan clearer. Switches are green, sockets and connections are blue, lights and wires are red. The only thing missing from this example are labels for the individual equipment (electrical assemblies, components or devices of electrical systems). For example, the letter ‘E’ is used to label lamps.

Colour scheme for the installation plan

Sometimes, different colour codes are used to distinguish the individual areas of an installation plan. Cable trays, for example, are marked in yellow, the lighting installation is blue and the telephone system is green.

What should an installation plan include?

The examples of different installation plans show that there are various ways to answer this question. If you just want a rough overview of the location of the equipment, you can use a very simple installation plan like the first one shown above. Doors and windows should always be included in the plan. As a rule, water installations such as bathtubs and showers are also included because you have to maintain minimum distances from them with sockets. If the installation plan is actually to be used as a guide for an electrical installation, then that is of course not enough either. In this case, further information is important, for example the material and cross-section of the individual wires, the number of cores and the type of installation (e.g. flush-mounted installation). As will be seen in the next section, installation plans for kitchens, for example, are particularly challenging.

Installation plan for kitchen

The kitchen of a property places particular demands on the electrical installation for a number of reasons. In kitchens (as in the bathroom), the electrical and plumbing installations come together and the electricity and water pipes must remain strictly separate. Furthermore, there usually has to be a connection to three-phase alternating current (high-voltage current) for the cooker with an oven. Since cookers are usually relatively large appliances, the placement of the connection should be carefully planned because it determines the placement of the cooker. Separate circuits are recommended for cooling appliances, and these must be planned in. And these are also large appliances that don't fit everywhere in the kitchen floor plan. The installation plan must take this into account. In addition, it should provide for a sufficient number of additional sockets for large appliances, as many smaller appliances also require a power connection. You can use the RAL-RG 678 guideline from the Association for the Efficient Use of Energy (HEA) as a reference. It defines equipment specifications for electrical installations in buildings and divides them into one- to three-star equipment. Even the one-star minimum standard stipulates 15 sockets for the kitchen. Many kitchens in existing buildings have fewer. But if you consider modern kitchen equipment, the specified number makes sense.

The framework for the installation plan: standards and laws

Concealed cables and wires may generally only be laid in certain installation zones. The rules for this can be found in DIN 18015-3: 2016-09. For example, when laying horizontally in the lower area of a wall, an installation zone must be maintained that is 15 to 40 cm above the finished floor (upper edge of screed). In addition to the horizontal installation zones, there are also vertical installation zones, for example at doors. A distance of between 10 and 30 cm must be maintained here.

One standard that is important for the installation plan and for the electrical installation, among other things, is DIN VDE 0298, which defines minimum cross-sections for wires. For example, a cross-section of 2.5 mm² is recommended in the kitchen for larger consumers, while 1.5 mm² is sufficient for sockets and lamps that are not heavily used. In addition, the standards already mentioned in the section on reading circuit diagrams, such as DIN EN 60617, with specifications for the symbols to be used, are relevant.

Electrical symbols in the installation plan

As with any circuit diagram, the readability of the installation plan depends on knowing the symbols. Some of the symbols are therefore listed here, which form the basis for reading installation plans.

The future is also important for the installation plan

When designing installation plans, you should also think about the future. A child's room may be converted into an office at some point. This will then place different demands on the electrical installation. It is possible that the property will become increasingly smart, requiring new wires, or that a charging station for an electric car will be added to the overall installation. For cases like these, it's best to plan for empty conduits from the outset that can easily accommodate new wires. This makes it easier to make subsequent installations. In addition, the better the installation plan is legible, the better the actual state of the electrical installation is documented and the easier it is to find the approach for expanding the system.

Overview circuit diagram

Overview circuit diagrams are sometimes referred to as overview plans. And while the spatial arrangement of the equipment in a floor plan is important for the installation plan, it plays a less significant role for them. Instead, the electrical circuit is in the foreground. The overview circuit diagram has exactly the task that its name suggests: it provides an overview. It is good at this. Other plans are used for detailed information.

What is an overview circuit diagram?

Unlike installation plans, overview plans are not limited to real estate. They also exist for circuits in motor vehicles or other machines. However, for a better understanding of this type of circuit diagram, the focus here will remain on overview plans for electrical installations in real estate. Since these plans belong to the single-pole circuit diagrams, the connecting lines are simplified and represented with a single line.

When is an overview circuit diagram needed?

Overview circuit diagrams are required by various electricity grid operators, for example, when a photovoltaic system that feeds into the grid is registered. The overview is often sufficient as part of the system's documentation. The electricity grid operators usually offer sample overview plans on their website for download.

For schematics that include multiple plans, the overview schematic can provide an initial understanding of how a circuit is structured before the circuit diagrams go into detail. A kind of overview can also form the top level of a hierarchical circuit diagram design. Some software solutions for drawing circuit diagrams enable such a design, in which block symbols are integrated into the top level. They represent lower levels with more detailed circuit diagrams that can be called up.

The designation ‘Q1’ and the symbol represent a switch, the circle with the integrated cross a lamp. The arrow-shaped line at the top left symbolises the supply line and thus the direction in which the current flows. The small dots represent junction boxes and the horizontal lines (with or without numbers) represent the number of wires in the cable. This small example already shows the essential components of a circuit diagram. The following variant is even more detailed.

Here, too, we have a switch labelled Q1. There is no ‘Q2’ in this overview. Why not? Because it contains only a single electrical device that, according to DIN EN 81346-2, is labelled with the letter ‘Q’.
The ‘X’ stands for equipment used to connect objects. This can include the distributors and sockets shown in the drawing. Since three of these devices can be seen in the drawing, the ‘X’ is numbered. The lamp is labelled ‘E’ because it belongs to the group of equipment that supplies radiant or thermal energy.
But this overview circuit diagram shows even more. The symbol under the line symbol indicates that the line is routed on plaster. If the line is routed in the plaster, the same symbol crosses the line that symbolises the line.
The sequence of letters and numbers at the top right indicates the cable material and the cable diameter. In the example, it is an NYM-J cable (N: standard cable; Y: PVC insulation; M: sheathed cable), where the ‘J’ stands for a green-yellow protective conductor. The ‘Cu’ stands for copper and the 1.5 mm² describes the cable diameter, which is the same for all cables in this plan. Finally, there is the small ‘t’ in brackets. This indicates the installation conditions and stands for installation in dry rooms.

What is the current flow?

Here is a simple example to illustrate the flow of electricity: the current flows through the supply line via junction box X2 directly into the earthed socket X3 and back. It also flows via junction box X1 to switch Q1. When switch Q1 is turned on, the current flows from there to lamp ‘E1’ and back. When the switch is turned off, no current flows to the lamp.

Symbols in the overview circuit diagram

The symbols in the overview plan follow the standards applicable in Germany. As an example, here are some symbols for various light sources:

Here are various switch symbols that may appear in an overview plan:

For a better understanding: a series switch is usually a double rocker switch that switches two devices (e.g. lamps) on and off independently of each other. An alternating switch allows a device to be switched on and off by two switches placed independently of each other, and a cross switch allows a device to be switched on and off with more than two switches. In addition to switches, sockets are among the most frequently used symbols in both installation and overview circuit diagrams for domestic installations. Here, too, there are various variants with different symbols.

Light sources, switches and sockets: These three types of equipment appear in the overview circuit diagrams for the electrical installation in all rooms of a house.

Summary: What is in the overview circuit diagram?

As with the installation plan, the same applies to the overview plan: there are different variants with different levels of information content. A detailed overview circuit diagram tells you something about the type and structure of the circuit(s) shown. It also provides information about the material and cross-section of the wires, how they are laid, and the respective laying conditions. What is important in all of this is that the plan is clear, without omitting essential information. When creating the overview circuit diagram, you should always keep in mind: readers with the necessary basic knowledge should be able to grasp it quickly and understand the circuit shown. Only then has the overview circuit diagram fulfilled its purpose.

Circuit diagram

Circuit diagrams are more precise and more detailed than overview circuit diagrams. They are all-pole circuit diagrams, whereas overview and installation plans are single-pole variants. All-pole diagrams show the cable connections in full, whereas single-pole diagrams simplify them using lines and symbols. In the all-pole plan, on the other hand, all the connections and contacts required for the circuit can be seen. In the case of circuit diagrams, a distinction can be made between variants in a resolved and contiguous representation. The variant in resolved representation is easier to grasp, while the variant in contiguous representation is more precise.

What is a circuit diagram?

The following three pictures, showing a very simple circuit, will give you a first impression of the difference between a circuit diagram and an overview plan:

In the overview plan, the connection between the electrical equipment is only shown with individual lines and additional information. In a circuit diagram, on the other hand, the individual cores are drawn. The ‘L’ stands for the phase conductor (also: phase), the ‘N’ for the neutral conductor and ‘PE’ for the protective earth conductor. Current flows through the phase conductor to the equipment and back again via the neutral conductor, while the protective conductor is used for safety in the event of a fault and is intended to prevent electric shocks. The respective cores in an electrical installation are coloured black, brown or grey (phase conductor), blue (neutral conductor) and green-yellow (protective conductor). This colour scheme has only been partially adopted in the coherent representation in the image of the circuit diagram. However, it is not absolutely necessary to colour the individual conductors in the circuit diagram.

Basic rule for circuit diagrams

As a rule, the current path in a circuit diagram runs from left to right and/or from top to bottom.

The circuit diagram in exploded view

In a circuit diagram in exploded view, the phase conductor is usually drawn either on the left or at the top, with the neutral conductor as a parallel on the right or at the bottom. The connection of equipment is drawn as a connecting line between ‘L’ and ‘N’. In the example below, a switch and a lamp have been placed between the phase and the neutral conductor in the plan. The lamp is shown in the off state, which is common for circuit diagrams, whether exploded or contiguous.

Of course, there are also more complicated circuit diagrams. The following example shows an alternating current circuit diagram in a resolved display:

This circuit diagram shows two switches (S1 and S2), which can be used to switch the lamp ‘E1’ on or off. In both example images, you can see another basic principle of the circuit diagram in a more detailed view. Where possible, wires are always drawn so that they do not cross. This is not always possible with more complex circuits. Here it still works, because even the somewhat more complicated representation of a two-way circuit remains relatively simple. The following represented current impulse switch is a little more complex, in which various buttons give a current impulse switch the impulse to switch a lamp on or off.

Important when viewing the circuit diagram in exploded view: This variant of the circuit diagram does not provide any information about the spatial arrangement of the equipment in relation to one another. What can be seen in the circuit diagram is the functional relationship of the circuit. This means that you can understand what happens when the switches are turned on or off.

The circuit diagram in a coherent representation

At first glance, the circuit diagram in a coherent representation appears more complicated than in a exploded view. The phase, neutral and protective conductors are not shown as parallel lines here. You can practically follow the current with your finger on this plan to understand its flow through the circuit.
In the example, the current flows through the phase conductor on the left via a junction box to switch ‘S1’. Since the switch is off in this example (as is usually the case in circuit diagrams), no current flows behind it. If it were switched on, the current would flow to lamp ‘E2’ and from there back via the neutral conductor. The protective earth (PE) is also for safety. The circuit diagrams also show the housings of the individual pieces of equipment in a continuous line. These are the three light blue rectangles in the background. Here is another somewhat more complex circuit:

Here, too, we have the phase conductor, the neutral conductor and the protective conductor. The equipment includes two junction boxes and one socket, labelled ‘X1’ to ‘X3’, as well as a switch (Q1) and a lamp (E1). The current is fed from junction box ‘X1’ to junction box ‘X2’ and to socket ‘X3’ and then flows back via the neutral conductor.
From junction box ‘X1’, however, the current also flows to switch ‘Q1’ and (if the switch is switched on) on to lamp ‘E1’ and from there back. Both devices (lamp and socket) have a protective earth. Important: In the circuit diagram in a coherent representation, care is often taken to ensure that the arrangement of the devices in the plan corresponds to the spatial arrangement. You can use any existing overview circuit diagrams as a guide.

When to use which representation

Let's briefly summarise: the circuit diagram in exploded view shows the interactions of a circuit without information about the spatial arrangement of equipment. It is usually clearer than the circuit diagram in continuous view, which, however, takes spatial arrangements into account. As with the overview circuit diagram, the following applies to circuit diagrams: design them to be as clear as possible. This will work better with a circuit diagram in exploded view than with the variant in continuous view. However, the basic rules defined for the overview plan also help to maintain the greatest possible overview Of course, circuit diagrams are not only used to document domestic installations, but also for automotive electronics, for example. Complex variants often extend over several pages, making cross-references and current path numbers necessary to show the reader on which page the section currently displayed is continued.

Conclusion: Summary of the most important points and why it is worth learning how to read circuit diagrams correctly.

Often, circuit diagrams are enough to understand circuits. However, as mentioned at the beginning, circuit diagrams often consist of several individual diagrams, including others in addition to the circuit diagram. In such cases, it makes sense to first look for a description page with reading rules and information on the labelling system. Then it is best to work your way from the less complex plan to the more complex ones. For domestic installations, the single-pole variants ‘installation plan’ and ‘overview plan’ are a good place to start. In addition to the plans listed here in the guide, there are others such as the terminal diagram, which shows the wiring in the electrical system in detail using terminals and terminal strips. A lot of information comes together here. For complex circuit diagrams, the following applies: First get an overview of the existing documents, the circuit symbols used and the labelling, and only then start reading the plans.

Despite careful editing and review of the contents, Stecker Express cannot guarantee the timeliness, correctness, completeness and quality of the information provided.