The basic concept so simple, the practical details both interesting, but also a conflated mess that means a lot of semi-experts (and smartarses like me) don't fully grasp.
And on larger scales things become more interesting - yet most of us deal with smaller scales, where other engineers have spent time making things simple and safe for you.
- 1 Terms: Earth, ground, common, signal, chassis, shield, guard, virtual ground, etc.
- 2 Ground as in (not) making noise go elsewhere
- 3 On sharing ground
- 4 Safety discussions (mostly) related to ground
- 5 More safety and/or noise stuff
Terms: Earth, ground, common, signal, chassis, shield, guard, virtual ground, etc.
|This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)|
The main symbols are:
- classically a conductive pole poking into the literal earth
- practically any of these
- if the outside of a device is metal, that's this.
- the reference used for a signal
- in PCBs there may be dedicated trace(s) (or not)
- in cabling this is often one of the wires in the cable
When and/or are also present, those specific distinctions are made, yet earth may sometimes still refer to any of the remaining. (Even when the physical design is very well considered (isolation, shielding, trace weight, order of connections, ground planes), the circuit diagram may not show each, as it may be considered a simplified functional summary.)
Further related concepts (most of which have no symbols):
- common current return path
- many components on a PCB that draw power tend to return it via a trace, often shared by many
- which we tend to call ground. This our main intuition in circuit design
- ...though it has no direct relation to earth (e.g. not all wired ones connect it to ground, no portable devices do)
- Even when this is connected to ground, it is not at quite the same potential, which becomes important in some cases with noise
- Ground plane
- in PCBs design: ground is often made as a large area of copper.
- This is a PCB design thing, and a nontrivial topic in itself, as it relates to coupling and more, and ends up being a balance of design considerations
- in antenna theory: a surface large, comparable to the relevant wavelength. Earth is an easy choice.
- Safety ground/earth
- our main intuition in house wiring
- a subject in itself
- itself a bit of a confusing one / misnomer.
- Also not often relevant to noise or safety discussions, because it's by nature internal to a circuit, so often there by design
- a strategy to alleviate some sorts of coupling
Earth/ground as in 'pole in the earth'
Earth can refer to a conductive pole in the earth near your front door.
Usually it isn't visibly exposed, but you have one. (And usually exactly one - if you have multiple, electrical code often says they must be connected to the main utility earth with a beefy cable, so there's usually little point to more) (One reason to have more is lightning rods, to ensure it conducts enough for a strike(verify), but there are more significant footnotes to this, electrical-code-wise)
The main purpose of this pole is (only) to discharge static electricity, avoiding buildup of it.
Which includes dealing with lightning. Seemingly mainly lightning from outside(verify), because your house is a higher-resistance path than the ground pole in front of it. (this setup also implies lighting can never spread over the grid much, which is nice design city-wise)
Earth is not for safety grounding of chassis. that safety measure would work without the pole (because it's the job of the wallplug wiring and main breaker - see notes below).
It is however required to detect ground faults, e.g. with an RCD.