Earthing system
Regulations for earthing systems vary among countries, though most follow the recommendations of the International Electrotechnical Commission (IEC).
Radio antennas may require particular grounding for operation, as well as to control static electricity and provide lightning protection.
Static buildup, as induced by friction for example, such as when wind blows onto a radio mast, is dissipated to the Earth.
[4] Having Earth as a common reference point keeps the electrical system's potential difference limited to the supply voltage.
[6] When current flows from a line conductor to an earth wire, as is the case when a line conductor makes contact with an earthed surface in a Class I appliance, an automatic disconnection of supply (ADS) device such as a circuit breaker or a residual-current device (RCD) will automatically open the circuit to clear the fault.
The earthing system, in combination with protective devices such as fuses and residual current devices, must ultimately ensure that a person does not come into contact with a metallic object whose potential relative to the person's potential exceeds a safe threshold, typically set at about 50 V. While there was considerable national variation, most developed countries introduced 220 V, 230 V, or 240 V sockets with earthed contacts either just before or soon after World War II.
However, in the United States and Canada, where the supply voltage is only 120 V, power outlets installed before the mid-1960s generally did not include a ground (earth) pin.
The US National Electrical Code permitted the use of the supply neutral wire as the equipment enclosure connection to ground from 1947 to 1996 for ranges (including separate cooktops and ovens) and from 1953 to 1996 for clothes dryers, whether plug-in or permanently fixed, provided that the circuit originated in the main service panel.
For similar reasons, most countries now mandate dedicated protective earth connections in consumer wiring, a practice that has become nearly universal.
International standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes — TN, TT, and IT.
[9] Three variants of TN systems are distinguished: In the UK, a common practice with TN-C-S is to connect the combined PEN supply conductor to Earth at multiple points along its length between the source transformer and the consumer installation.
The invention of residual current devices (RCDs) provided another means of protection from earth faults, which can be critical for a TT system as an RCD is often the only means of achieving suitable quick disconnection times, but is simply used as a secondary layer of protection in a TN system.
[12] The danger is serious enough that the UK Electricity Safety, Quality and Continuity Regulations 2002 forbids use of PEN conductors to supply caravans and boats where simultaneous contact with Earth is especially high.
The big advantage of the TT earthing system is the reduced conducted interference from other users' connected equipment.
A big disadvantage of TT systems is that the impedance of the earth path is often so high that it can prevent overcurrent protection devices from breaking the supply sufficiently quickly to meet safety regulation.
In the pre-RCD era the TT earthing system was unattractive for general use because of this difficulty of achieving reliable automatic disconnection of supply (ADS).
In some countries (such as the UK) TT is recommended for situations where a low impedance equipotential zone is impractical to maintain by bonding, where there is significant outdoor wiring, such as supplies to mobile homes and some agricultural settings, or where a high fault current could pose other dangers, such as at fuel depots or marinas.
[15] In the UK and some Commonwealth countries, the term "PNE", meaning phase-neutral-earth is used to indicate that three (or more for non-single-phase connections) conductors are used, i.e., PN-S. A resistance earth system is used for mining in India as per Central Electricity Authority Regulations.
Previously, for a short period before the invention of the RCD, voltage-operated earth leakage circuit breaker (VO-ELCB) devices were used.
This type of circuit is a must for portable heavy electric equipment (like LHD (Load, Haul, Dump machine)) being used in underground mines.
In India it is restricted for 50 A for open cast mines according to Central Electricity Authority Regulations, CEAR, 2010, rule 100.
However, in practice, the fault current will not be equal to zero: conductors in the circuit — particularly underground cables — have an inherent capacitance towards the Earth, which provides a path of relatively high impedance.
[28] Systems with isolated neutral may continue operation and provide uninterrupted supply even in presence of a ground fault.
[28] For that reason, they are chiefly limited to underground and submarine networks, and industrial applications, where the reliability need is high and probability of human contact relatively low.
In urban distribution networks with multiple underground feeders, the capacitive current may reach several tens of amperes, posing significant risk for the equipment.
For choosing a grounding rod there are several selection criteria such as: corrosion resistance, diameter depending on the fault current, conductivity and others.
