Earth and Ground Symbols: Protective Earth, Chassis and Signal Ground
Main symbols at a glance
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Earth and ground symbols look alike at a glance, three short lines or a small triangle, but they mean different things on a drawing, and mixing them up leads to real wiring mistakes. The symbols above are drawn to IEC 60417 with the common US signal ground added. Below is what each one tells the person reading the schematic.
A note on words first. “Earth” is the IEC and British term, “ground” the North American one. In the IEC vocabulary they mean the same thing: the conductive mass of the earth, whose potential is taken as zero, and the connection to it.
Earth (ground), general
IEC 60417-5017. A vertical line ending in three horizontal lines of decreasing length. It shows a connection to earth without saying why the connection is there. On older drawings it is used for everything; on a modern drawing that follows IEC 60617 you should expect the more specific symbols below wherever the purpose matters.
Protective earth
IEC 60417-5019. The earth symbol inside a circle. It marks the terminal or conductor that exists for safety: the connection that keeps exposed metalwork at earth potential so that a fault causes automatic disconnection instead of a live enclosure. On equipment you will find it next to the green-and-yellow terminal, and it is the symbol that must never be used for anything else.
On a drawing the conductor connected to it is the protective conductor (PE). If you see this symbol on a terminal, the conductor landing there must be green-and-yellow and must not be switched or fused.
Frame or chassis
IEC 60417-5020. A line with short diagonal strokes below it, like a rake. It means a connection to the metal frame or chassis of the equipment, which may or may not be earthed. In a car the chassis is the negative return of the 12 V system and is not connected to earth at all. In mains equipment the chassis is normally connected to protective earth, but the symbol by itself does not say so.
Signal ground
A downward-pointing triangle. This is the common North American way of marking the 0 V reference of an electronic circuit, the point all signal voltages are measured from. It is not a safety earth and may be isolated from mains earth completely, for instance in battery equipment or behind an isolating transformer.
Other earthing symbols you may meet
IEC 60417 has further symbols for functional earthing, used where an earth connection is needed for the equipment to work properly rather than for safety, and for equipotentiality, which marks terminals that bring parts to the same potential without implying a connection to earth. They appear mostly on instrumentation, IT and medical equipment. The functional earthing article explains where that kind of connection is used, and functional earthing conductor (FE) covers its colour and marking.
Reading them correctly on site
| Symbol | Safety connection? | Where you see it |
|---|---|---|
| Earth, general | Not stated | Older drawings, single-line diagrams, earth electrodes |
| Protective earth | Yes | PE terminals of equipment and enclosures |
| Frame or chassis | Not by itself | Vehicle electrics, equipment frames |
| Signal ground | No | Electronic schematics, 0 V reference |
Two mistakes come up again and again. The first is connecting a signal ground or a chassis connection to protective earth “because they are all earth”, which can create earth loops in audio and measurement systems, or bypass the isolation that a design depends on. The second is the reverse: lifting a protective earth to cure a hum loop. That cures the hum by removing the protection against electric shock, and it is never acceptable. Break the loop on the signal side instead.
For the earthing arrangements these symbols appear in, see types of system earthing and what is electrical earthing. Other symbol charts: wires and connections, AC and DC symbols.