Electrical engineering – The circuit diagram

With electrification in the 19th century a new language emerged: the Circuit diagram.

• It describes components, connections, current paths and functions.

• Internationally standardized symbols (IEC, DIN) ensure comprehensibility.

• Special role: Until 1968 the circuit diagram also contained the Logic – documented directly via relays, contacts and contactors.

👉 The circuit diagram made electrical function visible, traceable and safe.

Historical development

• 1830s: First depictions of simple circuits (Faraday, Ohm)

• 1850s–1900: Expansion of telegraphy and power grids → need for standardized circuit diagrams

• 20th century: DIN/IEC symbols, circuit diagrams as a universal language

• Until 1968: Circuit diagram = electrical + logic (relays, contactors, contacts)

• After 1968: Logic migrated into PLC programs, circuit diagram remained for electrical engineering

The circuit diagram – universal language of electrical engineering

Since the mid-19th century circuit diagrams have been the language of electrical engineering. They depict the functioning of electrical systems precisely and comprehensibly worldwide.

One Circuit diagram is the central form of representation of electrical and electronic systems. It follows clear standards and regulations (e.g. IEC, DIN, EN) and is, like the technical drawing in mechanical engineering, a language, which is understood worldwide. The circuit diagram is not only documentation, but the exact translation of function and purpose of an electrical installation.

Circuit diagram as language

In the circuit diagram wires, components and their interconnections are represented by standardized symbols Each symbol has a unique meaning that is valid internationally. This allows engineers, electricians and manufacturers to work together regardless of language and origin.

A circuit diagram describes:

• What is electrically connected (components, wires, terminals)

• How it works (switching sequences, current paths, signal routes)

• What for it is used for (protection, control, drive, communication)

Thus the circuit diagram is the central means of communication between planning, manufacturing, commissioning and maintenance.

Historical development

The history of circuit diagrams is closely linked to the development of electrical engineering:

• Early 19th century: With the work of Georg Simon Ohm (1827) and Michael Faraday the foundations of electrical engineering were laid. Early depictions of circuits arose as simple sketches.

• Mid 19th century: With the expansion of telegraphy and later power plants circuit drawings became necessary. Engineers developed the first symbols for switches, wires and sources.

• Late 19th century: With the spread of electric power supply and electric motors standardized circuit diagrams became indispensable.

• 20th century: International standards (e.g. DIN 40700, later IEC 60617) established uniform symbols for components. This turned the circuit diagram into the universal technical language of electrical engineering.

• Today: Modern CAE programs (Computer Aided Engineering) enable the digital creation, management and simulation of circuit diagrams. Nevertheless the circuit diagram remains the legally binding document for construction, function and testing of electrical systems.

Importance in electrical engineering and mechanical engineering

The circuit diagram is in mechanical and electrical engineering indispensable:

1. Clarity – it shows the function clearly and according to standards.

2. safety – only through circuit diagrams can electrical systems be tested and operated reliably.

3. Communication – all parties involved (design, assembly, commissioning, service) speak the same language.

4. Legal certainty – the circuit diagram is an official document that is decisive for approvals, tests and liability issues.

👉 In summary: Since the mid-19th century circuit diagrams have been the language of electrical engineering. They depict the functioning of electrical systems precisely and comprehensibly worldwide. Like the technical drawing in mechanical engineering, they are a cornerstone of modern technology – without them neither safe operation nor international collaboration would be possible.