No, this article is not about gardening! Ground and earth are so well used and will mean different things to different people, just like electronic vs. electrical users may differ from east to west.
When doing electronic design, ground, earth, and chassis are important concepts. All three imply a point of theoretical zero volts, but this may not always be true or even possible. There are also differences in the use of symbols, and shown below are the most accepted ones.
Definition of Terms
- Earth ground: The point where the grounding system will run to the planet, the 3rd pin on a mains socket, a copper rod hammered into the soil, or a connection to a water pipe (ideally copper). Places that rely on good earth (telephone exchanges, radio stations, power stations) often have large buried copper mesh or plates or, in the case of transmitters, buried radials under the system. Lightning protection also requires not only the good ground but substantial current carrying capacity.
- Chassis ground: In a system like a computer or radio, it is impractical to connect common 0V points to the earth, so these connect to the chassis, casing, or rack of the mother system. The ideal is one common point. Systems where a danger of shocking is possible, for example, a kettle or toaster, will have this chassis connected to mains earth. This means that in the event of a fault or broken live wire touching the casing, it gets shunted to earth and hopefully trips a breaker.
- Virtual ground: This is a point on a circuit, for example, an inverting op-amp at the -ve pin. It cannot be used as a ground but behaves like a ground point.
- Floating ground: In systems where the power supply is isolated from the mains by a transformer or where the device is portable and running off its own battery, like a cell phone, there are, of course, grounds in the circuit. These are called floating. Mains-powered devices coming off a two-pin plug, such as a hairdryer, are also floating ground.
- Signal ground: In an electronic system such as an amplifier, there are many points where components are to be connected to zero volts. Because the system may have substantial amplification and, in some places, large currents, tiny voltages can be set up in the ground tracks, which will cause oscillation. In RF systems, a ground plane or large area of copper in one of the PCB sides can help screen parts from each other.
In the circuit below, two 9V cells are connected in series, and the common point is connected to create a ground. This effectively creates a dual supply rail where an op-amp is needed.
We hope that all grounds and earth sit at a theoretical zero voltage, but this is never quite true.
On a ship, for example, there may be a potential difference of tens of volts between ‘grounds’ on different decks or parts of the ship. A similar problem occurs on aircraft surfaces as they cut through the earth’s magnetic field at super high speeds generating voltages like in an electric generator, creating small differences between ground points.
Conductors, wiring, joints, and even the soil itself have finite impedance, and as current flows, voltage is generated by Ohm’s law V = I x R. Someone on a ship using a long extension cable notices significant voltages between the deck where he is working and the earth on the extension cord.
Any loop created by multiple PCB tracks or connecting wiring will act as a single turn on a transformer in an audio or high-speed digital system if any magnetic field is present. This is very evident in the presence of mains wiring or transformers.
Where possible, designers strive to avoid loops and multiple earth loops and bring all grounded points back to one common point, and this is often the point at where the power supply enters, or the main smoothing/decoupling capacitors are connected to the chassis.
In systems with high gains, such as amplifiers, large currents will be present in the ground wiring. It becomes paramount to see that these grounds are not shared in series with the input stages. Otherwise, oscillation is inevitable. The input and output stages should only meet at one common ‘star’ point.
Ground loops are a problem when connecting systems with their own mains supply and some length of connections in a high-gain system.
For example, a mixing desk is connected to a PA system, each having its own mains power supply. The screen on the screened cables connecting them forms a loopback with the mains earth. Substantial hum problems may occur. There may be people connected to the ground at one end, such as someone holding a mic or guitar. Thus, simply disconnecting the main’s earth is not permissible.
Here’s another example. In a mains power supply system that is a three-phase system, the neutral point is supposed to have zero potential when the loads on each phase are balanced. This is seldom the case. Large currents on one phase will cause the neutral to ‘float’ up, causing voltage fluctuations on the other phases. For this reason, the neutral is taken to earth with good quality earth such as a grounding rod.
In many countries, the neutral point is also earthed at the consumer’s supply entry point.
Earth Leakage Relay
This is a relay that has two coil windings in opposite directions. The live and neutral are fed through these windings to the consumer. Because the outgoing and incoming currents are the same, there is no flux in the relay coil, and it does not operate.
However, in the event of a leakage fault—something/someone gets connected between life and earth, the returning current is less, as it is now flowing to earth, and the relay will operate, disconnecting the supply.
Note that an earth leakage relay cannot protect you if you connect yourself to life and neutral while being insulated from earth (rubber shoes). In this case, the earth leakage relay thinks you are a light bulb and will make you light up!
For example, many electronic components, like MOSFETs and CMOS, can be damaged in handling and transport. Assemblers are often connected to earth through a wrist or ankle strap or a partially conducting floor mat (carbon impregnated). Note that wrist and ankle straps must go to earth via a 1M resistor to prevent shock in case of accidental mains contact.
Radio devices are a special case. The length of the wire/conductor from a ground point to a signal ground may have enough inductance to present a higher impedance than expected. This is even worse, if, in addition to its inductance, it has capacitance to real ground. It could also become parallel resonant and now be electrically disconnected to ground at all.
Utmost care must be taken to avoid lengths of tracks where the length is a multiple of a ¼ wavelength as these may now start to radiate. 1/8 is better. So if a wavelength is 300/f in MHz, that should be 3GHz. If a wavelength is 1/10m or 100mm, and a ¼ would be 25mm, a genuine possibility in the wiring.
A lightning protection system also presents an RF type problem in the case of a high building. Even with substantial current-carrying and, thus, low resistance copper wire/strap, the rise time of lightning is very high, behaving like an RF pulse. The inductance of the long wire will cause substantial voltages to appear across it.
|Bonding straps of chassis and door||Good PCB ground layout|
Earth and grounding are very interesting and important concepts and strict rules need to be followed for safety and functional reasons.