Definition of standardised earthing schemes Earthing schemes Earthing connections Definition of standardised earthing schemes Characteristics of TT, TN and IT systems Selection criteria for the TT, TN and IT systems Choice of earthing method - implementation Installation and measurements of earth electrodes The installation system Distribution switchboards Cables and busways External influences (IEC 60364-5-51) Definition and reference standards Classification List of external influences The different earthing schemes (often referred to as the type of power system or system earthing arrangements) described characterise the method of earthing the installation downstream of the secondary winding of a MV/LV transformer and the means used for earthing the exposed conductive-parts of the LV installation supplied from it The choice of these methods governs the measures necessary for protection against indirect-contact hazards. The earthing system qualifies three originally independent choices made by the designer of an electrical distribution system or installation: The type of connection of the electrical system (that is generally of the neutral conductor) and of the exposed parts to earth electrod (s) A separate protective conductor or protective conductor and neutral conductor being a single conductor The use of earth fault protection of overcurrent protective switchgear which clear only relatively high fault currents or the use of additional relays able to detect and clear small insulation fault currents to earth In practice, these choices have been grouped and standardised as explained below. Each of these choices provides standardised earthing systems with three advantages and drawbacks:
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Definition of standardised earthing schemes
Earthing schemes
Earthing connections
Definition of standardised earthing schemes
Characteristics of TT, TN and IT systems
Selection criteria for the TT, TN and IT systems
Choice of earthing method - implementation
Installation and measurements of earth electrodes
The installation system
Distribution switchboards
Cables and busways
External influences (IEC 60364-5-51)
Definition and reference standards
Classification
List of external influences
The different earthing schemes (often referred to as the type of power system or system earthing arrangements) described characterise the method of earthing the
installation downstream of the secondary winding of a MV/LV transformer and the means used for earthing the exposed conductive-parts of the LV installation
supplied from it
The choice of these methods governs the measures necessary for protection against indirect-contact hazards.
The earthing system qualifies three originally independent choices made by the designer of an electrical distribution system or installation:
The type of connection of the electrical system (that is generally of the neutral conductor) and of the exposed parts to earth electrod (s)
A separate protective conductor or protective conductor and neutral conductor being a single conductor
The use of earth fault protection of overcurrent protective switchgear which clear only relatively high fault currents or the use of additional
relays able to detect and clear small insulation fault currents to earth
In practice, these choices have been grouped and standardised as explained below.
Each of these choices provides standardised earthing systems with three advantages and drawbacks:
Connection of the exposed conductive parts of the equipment and of the neutral conductor to the PE conductor results in equipotentiality and
lower overvoltages but increases earth fault currents
A separate protective conductor is costly even if it has a small cross-sectional area but it is much more unlikely to be polluted by voltage
drops and harmonics, etc. than a neutral conductor is. Leakage currents are also avoided in extraneous conductive parts
Exposed- and extraneous-conductive-parts of the installation are connected to an earth electrode.
In practice all circuits have a leakage impedance to earth, since no insulation is perfect. In parallel with this (distributed) resistive leakage path,
there is the distributed capacitive current path, the two paths together constituting the normal leakage impedance to earth (see Fig. E9).
Fig. E9: IT system (isolated neutral)
Example (see Fig. E10)
In a LV 3-phase 3-wire system, 1 km of cable will have a leakage impedance due to C1, C2, C3 and R1, R2 and R3 equivalent to a neutral earth
impedance Zct of 3,000 to 4,000 Ω, without counting the filtering capacitances of electronic devices.
Fig. E10: Impedance equivalent to leakage impedances in an IT system
IT system (impedance-earthed neutral)
An impedance Zs (in the order of 1,000 to 2,000 Ω) is connected permanently between the neutral point of the transformer LV winding and earth
(see Fig. E11). All exposed- and extraneous-conductive-parts are connected to an earth electrode. The reasons for this form of power-source
earthing are to fix the potential of a small network with respect to earth (Zs is small compared to the leakage impedance) and to reduce the level
of overvoltages, such as transmitted surges from the MV windings, static charges, etc. with respect to earth. It has, however, the effect of slightly
increasing the first-fault current level.
Fig. E11: IT system (impedance-earthed neutral)
Earthing system
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In electricity supply systems, an earthing system defines the electrical potential of the conductors relative to the Earth's conductive surface. The
choice of earthing system can affect the safety andelectromagnetic compatibility of the power supply, and regulations can vary considerably
As described below, most electrical systems connect one supply conductor to earth (or ground). If a fault within an electrical device connects a
"hot" (unearthed) supply conductor to an exposed conductive surface, anyone touching it while electrically connected to the earth (e.g., by
standing on it, or touching an earthed sink) will complete a circuit back to the earthed supply conductor and receive an electric shock.
A protective earth, known as an equipment grounding conductor in the US National Electrical Code, avoids this hazard by keeping the exposed
conductive surfaces of a device at earth potential. To avoid possible voltage drop no current is allowed to flow in this conductor under normal
circumstances, but fault currents will usually trip or blow the fuse or circuit breaker protecting the circuit. A highimpedance line-to-ground fault
insufficient to trip the overcurrent protection may still trip a residual-current device (ground fault circuit interrupter or GFCI in North America) if one
is present.
In contrast, a functional earth connection serves a purpose other than shock protection, and may normally carry current. Examples of devices that
use functional earth connections include surge suppressors and electromagnetic interference filters, certain antennas and measurement
instruments. But the most important example of a functional earth is the neutral in an electrical supply system. It is a current-carrying conductor
connected to earth, often but not always at only one point to avoid earth currents. The NEC calls it a groundED supply conductor to distinguish it
from the equipment groundING conductor.
Until the mid 1900s, power outlets generally lacked protective earth terminals. Devices needing an earth connection often used the supply neutral.
Some used dedicated ground rods. Many appliances had polarized plugs to maintain a distinction between "live" and "neutral", but using the
supply neutral for equipment earthing was highly problematical. "Live" and "neutral" might be accidentally reversed in the outlet or plug, or the
neutral-to-earth connection might fail or be improperly installed. Even normal load currents in the neutral might generate hazardous voltage drops.
For these reasons, most countries mandated dedicated protective earth connections that are now almost universal.
A combined PEN conductor fulfills the functions of both a PE and an N conductor. Rarely used.
TN−C−S
Part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N lines. The combined PEN conductor
typically occurs between the substation and the entry point into the building, and separated in the service head. In the UK, this system is also
known as protective multiple earthing (PME), because of the practice of connecting the combined neutral-and-earth conductor to real earth at
many locations, to reduce the risk of broken neutrals - with a similar system in Australia being designated as multiple earthed neutral (MEN).
TN-S: separate protective earth (PE) and neutral (N) conductors from transformer to consuming device, which are not connected together at any point after the building distribution point.
TN-C: combined PE and N conductor all the way from the transformer to the consuming device.
TN-C-S earthing system: combined PEN conductor from transformer to building distribution point, but separate PE and N conductors in fixed indoor wiring and flexible power cords.
It is possible to have both TN-S and TN-C-S supplies from the same transformer. For example, the sheaths on some underground cables corrode
and stop providing good earth connections, and so homes where "bad earths" are found get converted to TN-C-S.
TT network
In a TT earthing system, the protective earth connection of the consumer is provided by a local connection to earth, independent of any earth
connection at the generator.
The big advantage of the TT earthing system is the fact that it is clear of high and low frequency noises that come through the neutral wire from
various electrical equipment connected to it. This is why TT has always been preferable for special applications like telecommunication sites that
benefit from the interference-free earthing. Also, TT does not have the risk of a broken neutral.
In locations where power is distributed overhead and TT is used, installation earth conductors are not at risk should any overhead distribution
conductor be fractured by, say, a fallen tree or branch.
In pre-RCD era, the TT earthing system was unattractive for general use because of its worse capability of accepting high currents in case of a
live-to-PE short circuit (in comparison with TN systems). But as residual current devices mitigate this disadvantage, the TT earthing system
becomes attractive for premises where all AC power circuits are RCD-protected.