Standard Grounding Methods
The international standard IEC 60364 distinguishes ways to ground a system using the two-letter codes TN, TT and IT.
The first letter indicates the connection between the power supply equipment and the ground (generator or transformer):
• - T - Direct connection of a ground point (Latin: terra).
• - I - No point is connected to ground (from English isolated), or it is but passing through a high impedance connection.
The second letter indicates the connection between ground and the supplied electrical device:.
• - N - Direct connection to neutral at the origin of the installation, which is connected to ground.
In TN schemes, an S (separate) or a C (set) is added to define whether the neutral conductor and the protection conductor are a single conductor.
• - T - The equipment has its own grounding, not connected to the supply ground.
There is indirect contact if a person touches a conductive part of electrical equipment that has been made live by an insulation fault. Said person would complete the circuit to ground by receiving an electric shock.
All schemes, in combination with other protection devices, guarantee the safety of people against indirect contacts due to insulation failures. Its main difference lies in the continuity of the electrical supply.
It is the most used in most installations because it has excellent personal protection characteristics and a good operational cost.
In Spain, 95% of installations use this neutral regime, including, for example, public lighting installations.[3].
In this scheme, the neutral of the transformer and the metal masses of the receivers are connected directly, and without any protection element, to separate ground outlets.
In the event of a ground fault, a current circulates through the ground to the neutral point of the transformer, causing a current difference between the phase and neutral conductors, which when detected by the differential switch causes the automatic disconnection of the power supply.....
During the fault, the fault voltage is limited by the receiver ground, to a value equal to the resistance of the ground (protective conductor + ground) times the fault current.
In this system, the use of differential switches is essential to ensure small fault voltages and thus reduce the risk in the event of electrical contact of people or animals and to reduce the possibility of an electrical fire occurring.
It is used in public distribution networks, since having two different ground connections, defects are not transmitted between different receivers.[4].
• - Operation technique: disconnection at the first defect.
• - Protection technique: interconnection and grounding of metal masses. Disconnection by differential switches.
• - Uses: general, public distribution network.
It is the least used scheme, being relegated almost exclusively to temporary uses with generating sets (diesel generators). It is a system with a significantly higher operational cost than the TT scheme, since it requires periodic reviews.
The biggest disadvantage of this system is the need to calculate the impedances at all points of the line and design the protections individually for each receiver. In the case of very long or small lines, it may be the case that the fault current is not sufficient to trigger the protections.
In the TN-C scheme, the protection conductors are connected directly to the neutral conductor. In Spain, this scheme is not allowed to be used if the section of the neutral conductor is less than 16 mm².[5].
In the TN-S scheme, the protective conductors are connected to a protective conductor distributed next to the line, and connected to the neutral conductor in the transformer.
It is a combination of the previous two, used when the section of the neutral conductor is insufficient to serve as a protective conductor.
• - Operation technique: disconnection at the first defect.
• - Protection technique: interconnection and grounding of metal masses. Evenly distributed grounding. Disconnection due to overcurrent protections.
• - Uses: temporary and relief facilities.
It is preferred in applications where service continuity is critical, such as in operating rooms or industries with processes sensitive to interruption.
In this, the neutral of the transformer is isolated from ground (or connected through a high value impedance) and the metal masses connected to an exclusive ground.
This is the scheme that offers greater continuity of service, since it cuts off the supply to the second defect, unlike the others that do so to the first. This is because in a first defect the current encounters a very large resistance to return to the transformer and can be considered an open circuit. A second contact will cause current circulation and the protection devices will act.
In the event of a first defect, an insulation meter constantly monitors the installation, triggering an alarm in the event of insulation failure.
The IT scheme requires grounding completely independent of other installations, since otherwise the current could return to the transformer and cause the first defect to be truly dangerous. Likewise, the metal masses must not be connected to others from different installations.
Installations carried out according to this scheme are called floating or island installations.
In this type of scheme it is recommended not to distribute the neutral.[6] A light bulb can be added to warn that there is an electrical fault. Normally, it is placed above the resistance of the ground line.
• - Operation technique: signaling of the first defect. Disconnection at second defect.
• - Protection technique: interconnection and grounding of metal masses. Disconnection at the second fault due to overcurrent protection.
• - Uses: operating rooms and sensitive industrial processes.
The wiring standard establishes a color code for the electrical installation. In this way, each cable has a specific color according to its function.
The neutral conductor is blue, the phase wire is brown, gray or black, and the ground wire is always green and yellow.
In the high voltage lines of the electrical energy transport network, the ground wire is placed on the top of the conductor support towers and electrically connected to their structure, which, in turn, are equipped with a ground connection as described above. In this case, the ground wire performs a double function: on the one hand, it protects people from accidental discharge of the high-voltage conductors, and on the other, as it is higher than the aforementioned conductors, it acts as a lightning rod, protecting everyone from atmospheric discharges, which in this way are diverted to the ground, causing the minimum possible damage to the electrical installations.
• - Wiktionary has definitions and other information about tellurometer.
• - Alternative Transient Program.
• - Ground loop.
• - Electrical conduit.
• - Equipotential connection.
• - Circuit breaker.
• - Electromagnetism.
• - Plug.
• - Ground connection diagram.
• - Differential switch.
• - Faraday cage.
• - Electrotechnical Regulation for Low Voltage (Spain) "Electrotechnical Regulation for Low Voltage (Spain)") (REBT).
• - Single-phase system.
• - Three-phase system.