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Mains Earthing Systems The Earth is an integral part of our power system. It can be used to improve electrical safety, reduce energy losses or save on the cost of a dedicated conductor. Here, we look at the different Earthing systems used worldwide and how they work. By Brandon Speedie T he Earth’s crust is moderately conductive thanks mainly to the salts of sodium dissolved in water and, to a lesser extent, elements such as calcium, potassium, and magnesium. These charge carriers can move freely through soil and rock as long as they remain dissolved in water. The result is a surprisingly conductive electrolyte – see Fig.1. There are two broad reasons for using the Earth as a conductor. As a functional conductor The most obvious use of the Earth is to save on the cost of a dedicated conductor. One example is the Single Wire Earth Return (SWER) line, a common way to distribute power in rural 0.01 0.1 1 areas. In this case, a significant cost saving can be achieved by only having a single overhead conductor on a power line (see Figs.2 & 3). The return current (for Neutral) flows through the soil back to the substation or generator. This can sometimes be a distance of hundreds of kilometres. The Earth is also commonly used in RF applications. A monopole antenna relies on a ground plane to radiate and receive effectively, a role very commonly allocated to ‘terra firma’ (Latin for “firm land” or perhaps “solid ground”). Another application of Earth is on grid-scale solar farms. Solar panels are effectively three-terminal devices; Resistivity (Ωm) 10 100 1000 10,000 (igneous rocks: igneous and metamorphic rocks mafic felsic) mottled duricrust zone saprolite Safety Perhaps the most prominent function of Earth is to provide electrical 100,000 massive sulfides graphite they have a positive output, a negative output and a frame or chassis. If the frame is left electrically unconnected, it can float to a different voltage from its other two terminals. Charge carriers will then begin to migrate out of the substrate in a process known as Potential Induced Degradation (PID). This leads to reduced yield and eventually, early failure. On commercial solar farms, care is taken to ensure the panel mounting solution is well bonded to Earth and that the array DC voltage does not float too far from the Earth’s potential. shield unweathered rocks weathered layered (metamorphic rocks) clays gravel and sand glacial sediments tills shales sandstone and conglomerate sedimentary rocks dolomite, limestone lignite, coal salt water permafrost fresh water water, aquifers sea ice 100,000 10,000 1000 100 10 1 0.1 0.01 Conductivity (mS/m) Fig.1: resistivity figures for some common components of the Earth’s crust. Note the different units on the top and bottom horizontal axes, which are inversely equivalent; as S (siemens) is the inverse of W (ohms), 1mS is equivalent to 1kW. Source: GeoSci Developers – siliconchip.au/link/abu7 (CCA 4.0). 48 Silicon Chip Australia's electronics magazine Fig.2: SWER line in South Australia. The unusual pole construction is concrete sandwiched between two steel beams, known as a “Stobie pole”. siliconchip.com.au safety. In normal operation of a single-­ phase AC circuit, current flows into or out of the Active conductor, through the load, and returns via the Neutral conductor. In a fault scenario, an Earth connection gives a low impedance path for current to flow, which will usually trip a circuit breaker. In some scenarios, the fault will not draw enough current to trip the circuit breaker, but it should be enough to trigger a Residual Current Device (RCD). In an RCD, the Active and Neutral conductors both pass through a current transformer (CT). In the absence of a fault, the current flow is balanced between Active and Neutral, so the magnetic fields of these two currents cancel, and no net current is detected – see Fig.4. In a fault scenario, current flows through Active, but not all is returned via the Neutral; some flows through the Earth connection. This imbalance is detected in the RCD, which will typically trip once the imbalance exceeds 30mA (although more sensitive RCDs exist, eg, 15mA; the trip current is a balance between sensitivity and nuisance tripping). The Earth can also be used to ensure electrical safety during the normal operation of a grid. The most prominent such application is lightning suppression. If the potential difference between the Earth’s surface and the power lines were left uncontrolled, a direct strike from a lightning bolt would charge up the network to a high voltage, leading to arc-over at the insulators. It is therefore critical that this energy is shunted to Earth to maintain grid tolerances. Earth is also a logical place to shunt this charge as the lightning originates from a static buildup between the ground and the atmosphere. Types of Earthing systems Earthing schemes used in a mains grid are commonly described by a sequence of letters based on where the circuit Earth originates from. “T” (terra; Latin for “Earth”) refers to a direct connection to the soil of the Earth. This is typically achieved by driving a conductive stake into the ground, or perhaps multiple stakes and/or bonding to buried metal pipes. In larger installations, such as substations or generators, a dedicated buried circuit or ‘Earthing ring’ made of bare wire (usually copper) encircles the installation. “I” (insulatum; Latin for “insulated”) means no connection to Earth or a high-impedance connection through an Earthing resistor. “N” (network) means the Earth connection is via the network or grid. Network Earths will still connect to the soil at some point, but this may be some distance away, not at a local Earth stake, as with terra (T). Fig.4: a Residual Current Device (RCD). Usually, current flows to or from the Active conductor through the load and is returned via Neutral. The magnetic fields of the two conductors are cancelled, so the CT detects no net current. A small amount of leakage between Active and Earth, shown as a thin red line here, is enough to trip the RCD. “C” (combined) means the circuit Neutral and Earth are combined into a single conductor in the network. “S” (separate) is where the circuit Neutral and Earth are run as separate conductors in the network. The Earthing system can thus be described by two letters, the first indicating the source Earth, and the other the load Earth. TT (Terra-Terra) Terra-terra networks are physically connected to Earth at both the generator and load (see Fig.5). Typically, this will be at the low-voltage distribution transformer and the customer’s premises. TT networks rely heavily on the Earth connection’s integrity, so care is taken to ensure a sufficiently low Earth loop impedance. This can include tight specifications around Earth stake Terminology Fig.3: the start of the SWER line shown in Fig.2. The three conductors on the right are 33kV phase-to-phase, or 19kV from phase to Neutral/Earth. The SWER line taps off the middle phase and extends to the left. The return Neutral current flows via Earth. Phrases such as Earth, Neutral, common and ground are sometimes used somewhat interchangeably. They can be confusing terms, particularly from our perspective as electronics enthusiasts. Earth: a connection to terra firma, either directly through an Earthing stake, or via a conductor that is bonded at some point with Earth. Neutral: the return current of a single-phase AC supply. Typically, this will be the centre point of a star-connected three-phase circuit. In regular operation, most networks should have very little voltage difference between the Neutral and Earth. Ground: a common node in a circuit, usually at 0V DC potential. Confusingly, ground can be ‘grounded’ by tying to ‘Earth’, but it is uncommon in modern usage. Floating circuits are generally considered to have a ‘ground’, but it could drift relative to Earth; it is usually the negative end of a battery or similar and is used as a local reference and/or current return. In a circuit running directly from the mains, ‘ground’ may even be connected to (or very close to) the Active potential! Some circuits can have multiple grounds (analog, digital etc). Common: a node in a circuit shared by many components. It is sometimes used interchangeably with ‘ground’ but can also be used where multiple signals are tied together. Examples are a common bus in a multiplexed display or a common signal tying multiple opto-isolators together. siliconchip.com.au Australia's electronics magazine September 2024  49 construction and placement, as well as considerations of soil conductivity. This is particularly important in cold areas, where frozen soil dramatically increases resistivity, or in high rainfall regions, where soil electrolytes are diluted, leaving few charge carriers for conduction. Even given these additional requirements, a standard overcurrent breaker is not guaranteed to trip from an Earth fault. As a result, TT customers will almost always need Earth leakage protection in the form of an RCD. Historically, TT networks were not popular due to the difficulty of ensuring safety without Earth leakage protection. The advent of cheap RCD breakers has led to its increasing use worldwide, such that it is now the most common scheme. Many parts of Europe, including France, Denmark, Belgium, Spain, Italy, and Portugal, are now predominantly TT, as well as Japan, Malaysia, Argentina and many others. Germany extensively uses TT outside of metropolitan areas. IT (Insula-Terra) Insula-terra networks are connected to Earth at the customer’s end but not at the generator or distribution transformer (Fig.6). Therefore, the Active and Neutral connections have no reference to Earth, which minimises shock hazards. The reader may recognise this advantage from using an isolation transformer when working on mains-powered electronics. IT networks are often referred to as “first fault free”, as any fault will convert the system into another scheme (usually TN) while the fault is present, and subsequent faults may be dangerous. This is why IT networks are not common worldwide, except in specialised applications. This includes hospitals, where patients are at a higher risk of shock when coupled to medical equipment, and industrial areas where a flammable atmosphere may be present, so the risk of sparking needs to be minimised. Fig.5: the TT Earthing scheme. 50 Silicon Chip Scandinavia is an exception, where frozen ground and rocky geology make Earthing difficult. Norway in particular makes heavy use of IT Earthing. In India, a variation of the IT network called Resistance Earthed Neutral (REN) is used in mining areas. A Neutral grounding resistor limits the Earth current to 750mA. TN-C (Terra-Network-Combined) Terra-network-combined systems get their Earth from the network by combining it with the Neutral conductor (Fig.7). This combined conductor is commonly referred to as the Protective Earth Neutral (PEN). In a TN-C network, the distribution transformer is Earthed at the source end, which is also connected to the circuit Neutral. This PEN conductor then runs along the poles and wires of the grid to customer premises, where it is used as the “Earth”. TN-C networks do not require an Earth stake at the customer premises or RCD breakers as in a TT or IT network, but they are extremely reliant on the integrity of the PEN. If there is a break in this conductor, the customer load will act like a pullup resistor, raising the potential of Neutral/Earth to mains voltage; a hazardous situation (see Fig.11). TN-C networks also suffer from conducted interference. As the circuit Neutral is combined with the Earth, coupled noise from heavy industrial equipment can pass through the network and cause problems with sensitive equipment such as telecommunications broadcast infrastructure. Fig.11: a Neutral fault with the TN-C scheme. The customer load acts like a pullup resistor, raising the Earth to a high voltage and creating a shock hazard. conductor. TN-S is used extensively in India. TN-C-S (Terra-Network-Combined-Separate) Terra-network-combined-separate networks are a hybrid of the TN-C and TN-S systems. The source transformer is Earthed, while a combined PEN conductor emanates onto the network (Fig.9). The PEN is split into dedicated Neutral and Earth conductors at some agreed location (usually the customer’s switchboard). TN-C-S is widely used in the UK, USA, Canada, Israel, Australia and New Zealand – see Fig.14. Germany also predominantly uses TN-C-S in metropolitan areas. MEN (Multiple-Earthed-Neutral) Terra-network-separate networks run a dedicated Earth on the network, separate from the Neutral conductor (Fig.8). The distribution transformer is Earthed at the source end and connected to two conductors. One is the Neutral, while the other is a dedicated Earth – see Fig.13. TN-S networks are the safest configuration but are also more expensive, given the added Australia and New Zealand use the Multiple Earthed Neutral (MEN system) – see Fig.10. It is a TN-C-S system, though TT may be permitted in some situations – usually rural areas. Unusually for TN-C-S, an Earth stake is mandatory at each customer premises. In MEN networks, the source distribution transformer is Earthed, and a combined PEN conductor runs in the grid. The PEN is Earthed at multiple points throughout the network, including at the customer stake. This gives good immunity against a broken Neutral; if the customer is downstream of the fault, their PEN will not rise to a dangerous potential thanks to the Earth stake at their premises and any neighbouring premises or network Earths. Fig.6: the IT Earthing scheme. Fig.7: the TN-C Earthing Scheme. TN-S (Terra-Network-Separate) Australia's electronics magazine siliconchip.com.au Fig.12: this configuration stops the communications cable shield from drifting too far from Earth. It keeps it at a low AC impedance via the capacitor but will not form an ‘Earth loop’. At the customer premises, the combined PEN connects to the Neutral bar in their switchboard. This busbar then distributes the Neutrals to all of the circuits within the installation. Separately, a dedicated Earthing busbar connects to the “Earth” conductor emanating throughout the property, as well as the Earth stake. The Earth busbar and the Neutral busbar are joined by a single strap, known as the “MEN link”. This link is the separation point between the TN-C scheme on the network and the TN-S scheme within the customer premises. Earth integrity The Earth connection is convenient, as it can be assumed to be the same voltage across multiple installations, even if they are geographically separated. But as the soil has a finite resistance, this is not always true. This can create problems where two circuits are bonded to mains Earth and are also linked by conductors, similar to a local ‘ground loop’ or ‘earth loop’ that can cause problems with audio electronics. This is particularly problematic when large conductive structures are located near high-power switching gear. This might be a steel fence around a switchyard or a buried gas pipeline adjacent to power lines. In a fault, a large current might flow into ‘Earth’, raising the local voltage near the Earthing stake/ring. If the metal structure is close enough to this fault, its voltage will rise. Fig.8: the TN-S Earthing scheme. siliconchip.com.au If someone touches this structure, they may receive a lethal voltage despite being far from the actual fault. This is because the metal is a better conductor than the Earth, so the ground they are standing on is at a different voltage than the ground near the fault. This is known as a ‘touch potential’ and is a major hazard in high-power assets. Editor’s Note: in extreme cases, there can be enough potential between workers’ feet to electrocute them. Electrical workers are trained to hop if they suspect such a fault exists! Similarly, industrial Ethernet networks can also suffer from ground loops and unequal Earthing. Ethernet uses differential signalling, so it is commonly run over UTP (Unshielded Twisted Pair) cabling. A high Common Mode Rejection Ratio (CMRR) amplifier cancels any coupled noise or interference at the receiver. Thus, shielded cable is not needed for noise immunity. Despite this, shielded or foiled Cat6 cabling is common in industrial settings. Often, the designer will reason that shielded cable will be better than unshielded, so it is worth the marginal cost increase. However, it can often be more trouble than it is worth. Ethernet uses “magnetics” (signal transformers) at the receiver and transmitter to galvanically isolate the channel, preventing ground loops from forming. Shielded cable effectively breaks this isolation by connecting a conductor directly between the receiver and transmitter. If there is any Earth imbalance between this equipment, large currents can flow, which can cause interference or damage. For this reason, if shielded Ethernet cable is used, it is often only Earthed at one end of the cable, or better still, connected through a parallel RC combination, perhaps 1MW || 100nF (see Fig.12). The resistor weakly holds the shield at a known voltage, while the capacitor offers a low impedance path for AC SC voltages (coupled interference). Fig.9: the TN-C-S Earthing scheme. Australia's electronics magazine Fig.13: the TN-S scheme in Namibia. The five conductors (from bottom to top) are Earth, Neutral & three Active phases. Note how the Earth wire has a smaller diameter than the others. Fig.14: the TN-C-S scheme in Melbourne. The distribution transformer feeds the four horizontal conductors directly above it. The second conductor from the left is the combined Neutral/Earth (PEN), while the other three conductors are the Active phases, 400V line-to-line or 230V line-to-Neutral. An Earth stake (out of shot) connects to the conductor running up the left of the pole, partially covered by a white conduit. The transformer is fed on the primary side by the three conductors at the top with 22kV between phases. Fig.10: the MEN Earthing scheme. September 2024  51