Introduction
The fundamental rule for protection against electric shock is that hazardous live parts must not be accessible, and likewise, accessible conductive parts must not be hazardous—neither under normal conditions nor under single fault conditions.
Protection under normal conditions is provided by basic protective provisions, while fault protective provisions handle single fault conditions.
Alternatively, an enhanced protective provision can offer protection against electric shock under both normal and single fault conditions.
Earthing systems play a huge role in safeguarding personnel, equipment, and animals during fault conditions. In this blog post, we will explore the various earthing systems defined worldwide.
The earthing standards in most parts of the world are derived from the IEC 60364 standard, which will serve as our guide in this blog post.
Earthing System Nomenclature
Before we go any further, we need to understand the nomenclature of the earthing system as defined in IEC 60364.
The naming convention is based on the French language, so the letters used may not have an obvious meaning for non-French speakers.
Essentially, the identification of system types is denoted by two letters.
The first letter indicates the transformer’s neutral connection, for which there are two options: ‘T’ signifies ‘connected to the earth,’ and ‘I’ stands for ‘isolated from the earth.’ The two cases are shown below,
The second letter specifies the type of application frame connection. Here, too, there are two options: ‘T’ means ‘directly connected to the earth,’ and ‘N’ represents ‘connected to the neutral’ at the origin of the installation, which is in turn connected to the earth.
Putting the letters together
Combining the first and second letters yields three possible configurations for earthing systems: TT, TN, and IT.
In the TT system, both the transformer’s neutral and the load frame are connected to the earth.
In contrast, the TN system features an earthed transformer neutral, while the frame is connected to the neutral conductor at the origin of the installation.
The IT system is characterized by an isolated or unearthed transformer neutral but includes an earthed load frame.
It’s worth noting that the TN system has several sub-categories. First is the TN-C system, which uses a single conductor for both the neutral and protective earth (PEN).
Second is the TN-S system, in which the neutral (N) and protective earth (PE) conductors are separate.
Lastly, there’s the TN-C-S system, a hybrid configuration that employs a TN-S setup downstream from a TN-C arrangement; the reverse combination is not permitted.
TN Earth Sytem
In TN systems, having a solid and trustworthy ground connection for your PEN or PE wires is important for safety.
You need to connect the neutral or midpoint of your power source to the ground. If you can’t access or find this point, then ground one of the line wires instead.
Also, any exposed metal parts of your setup should be wired to the main ground terminal, which itself should be grounded.
If there are other reliable ground points, it’s a good idea to connect to those as well.
Adding more ground connections spread out evenly can help make sure that, if something goes wrong, the potentials of protective conductors remain, as near as possible to that of earth.
Referring to the diagram above, when an insulating fault is present, the fault current �� is only limited by the impedance of the fault loop cables. �� can be represented by the equation;
Based on this premise, the voltage ��experienced by a person touching a load frame with current �� flowing through it is given by the formula:
For 230/400 V networks, a voltage around Uo/2 (if RPE = Rph) is dangerous, as it exceeds the safety limit voltage listed in the table below, which is an excerpt from IEC 60364, Tables 41 and 48A.
As a result, the installation, or a part of it, must be automatically and quickly de-energized.
To ensure that the protection device will trip, the current Id must be greater than the protection device’s operating threshold Ia (Id > Ia), regardless of the fault’s location.
This condition should be verified during the installation design stage by calculating the fault currents for all distribution circuits.
Another method to guarantee this is by imposing a maximum impedance value on the fault loops, based on the type and rating of the protective device chosen.
This approach may mean increasing the cross-section of the live and/or protective conductors or switching to a circuit protective device with a more suitable trip curve or breaker type.
Another method to ensure that the device provides adequate protection is to calculate the maximum permissible length for each feeder, based on a given protection threshold Ia.
For more information on this method, refer to our maximum cable length calculator.
TN Earthing system variations
As previously mentioned, the TN earthing system encompasses several variations, which we will now discuss.
Firstly, there is the TN-S system, which involves separate conductors for Protective Earth (PE) and Neutral.
These conductors are run to consumer loads from a site’s power supply. In this system, the PE and N conductors are separate in almost all parts of the network, only joining together at the supply source. A typical arrangement is shown below.
Secondly, we have the TN-C system, which employs a combined Protective Earth-Neutral (PEN) conductor, connected to the earth at the source.
However, this type of earthing is rarely used, mainly due to fire risks in hazardous environments and the presence of harmonic currents, which make it unsuitable for electronic equipment.
Additionally, as per IEC 60364-4-41 (Protection for safety – Protection against electric shock), RCDs cannot be used in a TN-C system. A typical arrangement is shown below.
Finally, there is the TN-C-S system, a hybrid where the supply side uses a combined PEN conductor for earthing, and the load side employs separate conductors for PE and N.
This system is commonly used in distribution networks in both Australia and New Zealand, often referred to as multiple earth-neutral (MEN).
In this setup, a TN-C system runs between the site transformer and the premises, with the neutral earthed multiple times along this segment.
Inside the property, a TN-S system is utilized from the Main Switchboard downstream. Overall, such a system is categorized as TN-C-S. A typical arrangement is shown below.
TT Earth Sytem
When an insulation fault occurs, the fault current Id as shown in the figure below is mainly limited by the earth resistances. The fault current is given by the formula;
The voltage experienced by a person touching a load frame with current Id flowing through it is given by the formula.
This induced voltage which is almost half the supply voltage is very dangerous and the affected circuit must be quickly isolated.
In this earthing system, the fault current, beyond which a risk is present, is usually far lower than the settings of the overcurrent protection devices, and at least one RCD must be installed at the supply end of the installation.
To increase the availability of electrical power, the use of several RCDs ensures time and current discrimination in tripping.
The standard stipulates that de-energizing by the RCDs must occur in less than 1 second. It’s important to note that protection provided by RCDs does not depend on cable length.
IT Earth system
The IT earthing system: A critical part of uninterrupted power supply in hospitals
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The basic feature of the IT earthing system is that, in the event of a fault between phases and earth, the system can continue to operate without interruption. Such a fault is referred to as a ‘first fault.’
The first fault current value depends on the neutral impedance and on downstream network capacitances.
The first fault current, Id, should be low enough to meet the rule that Ud ≤ 50 V, ensuring that no dangerous fault voltages can occur.
The system may continue to operate normally until it is convenient to isolate the faulty section for repair, enhancing continuity of service.
In practice, the IT system requires specific measures for satisfactory operation:
- Permanent monitoring of the insulation with respect to earth, which must signal the occurrence of the first fault.
- A device for limiting the voltage that the neutral point of the supply transformer can reach with respect to earth.
- A ‘first-fault’ location routine by an efficient maintenance staff. Fault location is greatly facilitated by automatic devices currently available.
- Automatic high-speed tripping of appropriate circuit breakers in the event of a ‘second fault’ occurring before the first fault is repaired. The second fault (by definition) is an earth fault affecting a different live conductor than that of the first fault.
Rounding Up!
Studying low voltage earthing systems reveals the importance of earthing for electrical safety and ensuring reliable system operation. Different types, such as TN, TT, and IT, each require specific design, setup, and maintenance.
Knowing and adhering to standards is crucial for preventing electric shocks and maintaining power system reliability. If you enjoyed this article, you’ll definitely find our other featured articles interesting.
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