What is earthing?
Earthing is the conductive connection of metal parts of an electrical installation to earth, so that no dangerous touch voltage can arise in the event of a fault and protective devices disconnect the current quickly. In the Netherlands the requirements of NEN 1010 apply to design and installation, and NEN 3140 to safe operation and periodic inspection; internationally, IEC 60364 is the basis. In industrial installations earthing has a second function: a good earthing and equipotential bonding network is the basis for EMC and therefore for interference-free PLCs, measurement signals and networks. In American English, earthing is called grounding.
🧠 What types of earth are there?
| Type | Purpose | Example |
|---|---|---|
| Protective earth (PE) | Safety: prevents dangerous touch voltage | Green-yellow earth conductor to a metal cabinet or motor |
| Functional earth (FE) | Correct operation of equipment, reference potential | Earth of a control cabinet, shield of signal cables |
| Equipotential bonding | Bringing all metal parts to the same potential | Bonding of pipes, cable trays, structures |
| Lightning protection earth | Safely discharging lightning current to ground | Earth electrodes of the lightning protection system under IEC 62305 |
| Earthing in hazardous areas | Preventing sparks and static discharge | Earthing of tanks and intrinsically safe barriers |
🔧 What earthing systems are there?
| System | Characteristic | Typical application |
|---|---|---|
| TN-S | Separate neutral (N) and protective earth (PE) from the source | Industry with its own transformer; preferred for EMC |
| TN-C-S | Combined PEN conductor up to a point, then separate N and PE | Many connections to the public grid |
| TT | Installation has its own earth electrode, separate from the grid | Older buildings and installations, outdoor sites |
| IT | Supply not earthed or earthed through high impedance; first fault does not trip | Hospitals, process plants that must not stop |
For EMC, TN-S is preferred: because no current flows over the protective earth, there is less interference in signal and data cables.
🏭 How do you install earthing in an industrial plant?
- Main earthing terminal (MET) — central collection point where all protective and bonding conductors come together
- Meshed bonding network — connect structures, cable trays and cabinets at multiple points (IEC 61000-5-2), creating a low-impedance reference plane at high frequencies
- Cabinets and mounting plates — bond doors, mounting plate and cabinet frame with short, wide conductors
- Cable shields — connect to the earth bar or mounting plate; at both ends with a well-bonded network
- Separate circuits where needed — for example a dedicated instrument earth for intrinsically safe circuits in hazardous areas
- Measure and document — measure earth resistance and continuity at commissioning and periodically (in the Netherlands under NEN 3140)
⚠️ What is a ground loop?
A ground loop occurs when two devices are connected to earth via multiple paths and there is a potential difference between those points. A current then flows through, for example, the shield of a signal cable, causing interference in the measurement or communication. Solutions are good equipotential bonding, so that there is no difference, and galvanic isolation of signals, for example with isolating amplifiers or fibre optics between buildings.
❓ Frequently asked questions
What is the difference between earthing and equipotential bonding?
Earthing connects an installation to the earth itself, via an earth electrode. Equipotential bonding connects all touchable metal parts to each other so that they are at the same potential, even if that potential is not zero. For safety, equipotential bonding is at least as important as earthing: without a potential difference, no dangerous current can flow through a body.
What earth resistance is required?
The required earth resistance depends on the earthing system. In a TT system, the product of the earth resistance and the operating current of the residual current device must not exceed 50 volts under IEC 60364 and NEN 1010. With a 30 mA RCD, the earth resistance may therefore be at most about 1667 ohms; in practice a much lower value is targeted.
Why is earthing important for PLCs and networks?
Earthing is important for PLCs and networks because controllers, measurement signals and data cables need a stable reference potential. Poor earthing leads to faults such as fluctuating 4-20 mA readings, communication errors on PROFINET and damaged input cards. A meshed bonding network and good shielding prevent most of these problems.
What is the difference between PE and FE?
PE (protective earth) is the protective conductor that protects people against electric shock and is always marked green-yellow. FE (functional earth) is an earth needed for the correct operation of equipment, such as a reference potential or discharge of interference. An FE connection never replaces a PE connection.
How often should earthing be checked?
Under NEN 3140, the electrical installation, including earthing and equipotential bonding, is inspected periodically. The interval is based on a risk assessment and in industry is often between one and five years. An additional check is advisable after changes to the installation and after a lightning strike.
Should a cable shield be earthed at one end or both ends?
In an installation with a well-meshed equipotential bonding network, the shield of data and signal cables is preferably earthed at both ends, as that gives the best protection against high-frequency interference. Without good bonding, earthing at one end can prevent a ground loop, but offers less protection. The guidelines of the bus system manufacturer, such as PROFINET or PROFIBUS, take precedence.
📌 In summary
Earthing connects metal parts of an installation to earth so that no dangerous voltage arises in the event of a fault; in the Netherlands NEN 1010 governs installation and NEN 3140 operation. In industry, a meshed earthing and bonding network also forms the basis for EMC and interference-free control systems.