How to Select the Best Earthing System

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,
4. Neutral point connection to earth + the connection method of the electrical installations exposed conductive parts.

And I explained the first two categories in this Article, showing that the 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 the Types of Earthing Systems according To Its Size can be divided to Two types as follows:

1. simple,
2. complex. 

Also, In Article " Types of Earthing System – Part Two ", I explained the Types of Earthing Systems according To Neutral point connection to earth, which were:

1. The solidly (or directly) earthed neutral,
2. The unearthed neutral, 
3. High impedance-earthed neutral,
4. Resistance earthing,
5. Reactance earthing,
6. Petersen coil earthing.

And the Types of Earthing Systems according To Neutral point connection to earth + the connection method of the electrical installations exposed conductive parts, which were:

1. IT (Unearthed transformer neutral, earthed frame),
2. TT (Transformer neutral earthed, and frame earthed),
3. TNC (If the N and PE neutral conductors are one and the same (PEN))
4. TNS (If the N and PE neutral conductors are separate),
5. TNC-S (Use of a TN-S downstream from a TN-C (the opposite is forbidden).

Today I will explain How to select the best earthing system for a new construction from the above types 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

How to select the best earthing system for a new construction

Who decide which Type of Earthing Systems to be used? For any new construction, three parties share in the Choice of earthing system: Electrical power users, Network operators (electrical service), Installation’s design engineering firms. Experience shows however that the choice is mainly made by the engineering firms designing the installation.

Criteria For Selection Of The Best Earthing System the criteria used to make the best choice will be according to: Country Regulations, Country development, Type of building, Type of networks and building Loads, Availability of Electrical Service, Costs, Complexity of design.

1- Country regulations The regulations or standards applied in the country in some cases impose certain types of earthing system arrangements (SEA). The most common systems in most countries are TT and TN; a few countries, in particular Norway, use the IT system. The table in below figure#1 lists some examples for LV earting systems for public distribution (LV consumers) for different countries. This table shows that Anglo-Saxon countries mainly use the TN-C, whereas the TT is used in the rest of the world.  Fig.1: examples for LV earting systems for public distribution (LV consumers) for different countries

2- Country development The degree of development of the country should also be taken into consideration, as should be national practices, climate, etc. If we plot an axis from North to South, as regards public distribution, we find the IT earthing system in Norway, TN-C in Germany, TT in France and in most African countries. In temperate, industrial countries, all three earthing systems are used in private installations.

3- Type of building 1- In a certain number of countries, for some buildings or parts of a building, the choice is laid down by legislations or standards, e.g. for hospitals, schools, navy, worksites, mines, etc. In other cases, certain earthing systems are strictly prohibited, for example the TN-C in premises with explosion risks. 2- The required level of dependability should determine which earthing system is chosen for a specific building type. Dependability means that electrical power should thus always be available and be completely risk-free, i.e. «out of sight, out of mind». The elements making up installation dependability: Safety; Availability; Reliability; Maintainability, Proper operation of low current communicating systems These elements must therefore be optimised. The table in below figure# 2 reviews the strong and weak points in each earthing system: Fig.2: Comparison of system earthing arrangements Note: In terms of the protection of persons, the three system earthing arrangements (SEA) are equivalent if all installation and operating rules are correctly followed. Consequently, selection does not depend on safety criteria.

4- Type of networks and building Loads The particular characteristics of the network and the loads will affect the selection of earthing system arrangements as in below table in figure#3. Fig.3:  Influence of networks and loads on the selection of system earthing arrangements

5- Availability of Electrical Service The decision of the owner if supply is via a private MV/LV transformer (MV subscription) or the owner has a private energy source (or a separate-winding transformer). If the owner effectively has a choice, the decision on the SEA is taken following discussions with the network designer (design office, contractor), The discussions must cover:  First of all, the operating requirements (the required level of continuity of service) and the operating conditions (maintenance ensured by electrical personnel or not, inhouse personnel or outsourced, etc.) Secondly, the particular characteristics of the network and the loads as in fig.3.

6- Costs The TN-S is the least costly to install, for example if the neutral is neither protected nor switched. But be warned: the cost of curative maintenance can be high. The IT is slightly more costly to install (insulation monitoring and insulation fault tracking devices). Search for maximum availability of electrical power requires the presence of an electrical engineer, whose action will minimize curative maintenance. The TT, if enough discriminating RCDs are installed, is slightly more costly to install than the IT, but fault tracking is simple and curative maintenance less costly than in TN. Note: In terms of complete cost over 10 to 20 years, all three earthing systems are equivalent.

7-Complexity Of Design Designing is simpler in TT, the same for extensions (no calculations). Designing complexity is equivalent in TN-S and IT.

Steps For Choosing The Best Earthing System Step#1: Firstly do not forget that the three system earthings can all be included in the same electrical installation: This guarantees the best possible answer to safety and availability needs (see below figure#4). Fig.4: several earthing system included in the same LV installation. Step#2: check that the choice is not specified or stipulated by standards or legislation (decrees, ministerial decisions) Step#3: dialogue with the user to get to know his requirements and resources, and Generally (see table in figure#5 ): 1- Need for continuity of service and Whether or not there is a maintenance service: Continuity of service and maintenance service: the IT will be chosen. Continuity of service and no maintenance service: No fully satisfactory solution: Prefer the TT whose discrimination on tripping is easier to implement and which minimises damage with respect to the TN. The installation of additionnal output is easily achieved without the necessity of further calculations. Continuity of service not essential and compent maintenance service: prefer the TN-S (rapid repairs and extensions performed according to rules), Continuity of service not essential and no maintenance service: Prefer the TT 2- Fire hazard IT if maintenance service and use of 0.5 A RCD or TT. 3- Safety The TT is the best. 4- Availability The IT is the most suitable. 5- Maintenability Fault tracking is fast in TN (thanks to the SCPD) but repair time is often long. Conversely, in IT, tracking of the first fault may be more difficult, but repairs are quicker and less costly. The TT is a good compromise. 6- Reliability The protection devices used are reliable, but reliability of the installation and loads may be affected: a- In TN-C by the fact that the PEN, not protected, may be damaged by harmonic currents; b- In TN-C and TN-S: By insufficient rigour for extensions, By use of replacement sources with low short-circuit power, By the effects of electrodynamic forces; c- In IT, on a double fault, the risks inherent in TN described above also exist. However if tracking and elimination of the 1st fault are rapid, installation reliability is excellent. d- in TT, by disruptive breakdown by return of the loads due to a fault in the HV/LV transformers. However the likelihood of this fault occurring is small and preventive solutions are available, e.g. use of surge arresters between one of the live conductors and the load earth connection. 7- Disturbance The TT is to be preferred to the TN-S whose high fault currents may be the source of disturbance. Fig.5: Comparison of system earthing arrangements according to its dependability Step#4: Finally allow for the special features of network and loads as follows: Very long network or, even more important, leakage current: Prefer the TN-S, Use of replacement or standby power supplies: Prefer the TT, Loads sensitive to high fault currents (motors): Prefer the TT or IT, Loads with low natural insulation (furnaces) or with large HF filter (large computers): Prefer the TN-S, Supply of control and monitoring systems: Perfer the IT (continuity of service) or the TT (enhanced equipotentiality of communicating devices).

In the next Article, I will explain the Construction and Components of Earthing Systems. Please, keep following.

Back To Course EE-5: Grounding System Design Calculations