Types of Earthing System – Part Two

In Article " Types of Earthing System – Part One ", I listed the Different Types of Earthing Systems which can be divided according to the following factors:

1. Function,
2. System size,
3. Neutral point connection to earth (Neutral Grounding),
4. Neutral point connection to earth(Neutral Grounding) + the connection method of the electrical installations exposed conductive parts (Frame Grounding).

And I explained the first two categories in this Article, showing that:

• Types of Earthing Systems according to its Function can be divided to Six types as follows:

1. Static grounding, 
2. Equipment grounding, 
3. System grounding, 
4. Lightning grounding, 
5. Electronic (including computer) grounding, 
6. Maintenance safety grounding. 

• And Types of Earthing Systems according To Its Size can be divided to Two types as follows:

1. simple.
2. complex. 

Today I will explain the Last Two Types of Earthing Systems as follows.

You can preview the following Articles for more info:

• Introduction to Grounding System Design – Part One

• Introduction to Grounding System Design – Part Two

Types of Earthing System (on LV Systems and Within Premises)

Different Types Of Earthing Systems the Earthing systems can be divided according to the following factors: Function, System size, Neutral point connection to earth, Neutral point connection to earth + the connection method of the electrical installations exposed conductive parts.

Types Of Earthing Systems According To Neutral Point Connection To Earth (Neutral Grounding) The Earthing systems can be divided to Five Types according to neutral point connection to earth as follows: The solidly (or directly) earthed neutral, The unearthed neutral, High impedance-earthed neutral, Resistance earthing, Reactance earthing, Petersen coil earthing.

Types of Neutral Point Connection To Earth 1- Solidly earthed neutral An electrical connection is intentionally made between the neutral point and earth. 2- Unearthed neutral There is no electrical connection between the neutral point and earth, except for measuring and protective devices. 3- High impedance earthing A high impedance is inserted between the neutral point and earth. 4- Resistance earthing A resistor is inserted between the neutral point and earth. 5- Reactance earthing A reactor is inserted between the neutral point and earth. 6- Petersen coil earthing A reactor tuned to the network capacitances is inserted between the neutral point and earth so that if an earth fault occurs, the fault current is zero.

Comparison between Neutral Grounding Methods The performance characteristics of each neutral grounding method discussed above  can be compared as in below table:

Types Of Earthing Systems According To Neutral Point Connection To Earth + The Connection Method Of The Electrical Installations Exposed Conductive Parts. According to Neutral point connection to earth + the connection method of the electrical installations exposed conductive parts, The Earthing systems can be divided to Five schemes as follows: 3 Main Earthing Schemes, 3 Sub-Main Earthing Schemes.

(3) Main Earthing Schemes Each one of the 3 Main Types is defined by two letters as follows: The First Letter: It defines the situation of the neutral point in relation to earth, like: T : solidly earthed neutral I: unearthed or high impedance earthed neutral. The Second Letter: It defines the connection method of the electrical installation's exposed conductive parts, like: T : the exposed conductive parts are interconnected and solidly earthed, regardless of whether the neutral point is earthed or not N: the exposed conductive parts are directly connected to the neutral conductor. So the three main earthing systems will be: IT (Unearthed transformer neutral, earthed frame), TT (Transformer neutral earthed, and frame earthed), TN (Transformer neutral earthed, frame connected to neutral).

(3) Sub-Main Earthing Schemes The Three sub-main earthing system are derived from the main  TN earthing system as follows: TNC (If the N and PE neutral conductors are one and the same (PEN)) TNS (If the N and PE neutral conductors are separate), TNC-S (Use of a TN-S downstream from a TN-C (the opposite is forbidden). Where: C : N and PE Combined, S : N and PE Separate, PEN: (Protective Earth and Neutral) conductor. Note:  Each system earthing can be applied to an entire LV electrical installation; however several system earthings may be included in the same installation.

IT earthing system First letter I: The neutral is unearthed or earthed via a high impedance, An impedance between 1000 and 2000 Ω is frequently used. Second letter T: The exposed conductive parts of loads are interconnected, either altogether, or in groups. Each interconnected group is connected to an earth electrode. It is possible for one or several exposed conductive parts to be separately earthed. Notes: Where possible, it is advisable to interconnect all the exposed conductive parts of the same installation and connect them to the same earth electrode. It is nonetheless possible for exposed conductive parts which are far away from each other, or located in different buildings, not to be. In this case, each group of exposed conductive parts connected to the same electrode and each individually earthed exposed conductive part must be protected by a residual current device. The earth electrodes of the exposed conductive parts and the neutral may or may not be interconnected or the same. It is not advantageous to distribute the neutral which results in the maximum length of wiring systems being reduced. Installing an overvoltage limiter between the neutral point of the MV/LV transformer and earth is compulsory. If the neutral is not accessible, the overvoltage limiter is installed between one phase and earth. It protects the low voltage network against rises in voltage due to flashover between the transformer medium voltage and low voltage windings.

TT earthing system First letter T: The neutral point is directly earthed. Second letter T: The exposed conductive parts of the loads are interconnected, either altogether, or in groups, or individually, and are earthed. Protection is ensured by residual current devices. All the exposed conductive parts protected by the same protective device must be connected to the same earth electrode. Note: The neutral earth electrode and that of the exposed conductive parts may or may not be interconnected or the same. The neutral may or may not be distributed.

TN earthing system First letter T: The neutral point is directly earthed. Second letter N: The exposed conductive parts of the loads are connected to the neutral conductor. the TN earthing system can be divided to 3 sub-main earthing schemes as follows: 1- TNC earthing system Third letter C: The neutral and protective conductors form a single conductor called the PEN. Notes: It is advisable to regularly connect the PEN to earth. This system must not to be used for cross-sectional areas below 10 mm² for copper or 16 mm² for aluminium, as well as for mobile wiring systems. It is also forbidden downstream of a TNS system.  2- TNS earthing system Third letter S: The neutral conductor and protective conductor are separate. 3- TNC-S earthing system In TNC-S earthing system, both TNC and TNS earthing systems can be used in the same installation. But the TNC earthing system (4 wires) must never be downstream of the TNS earthing system (5 wires).

In the next Article, I will explain the Selection Criteria for the Best Earthing System. Please, keep following.

Back To Course EE-5: Grounding System Design Calculations