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:
- System size,
- Neutral point connection to earth,
- 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:
- Static grounding,
- Equipment grounding,
- System grounding,
- Lightning grounding,
- Electronic (including computer) grounding,
- Maintenance safety grounding.
And the Types of Earthing Systems according To Its Size can be divided to Two types as follows:
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:
- The solidly (or directly) earthed neutral,
- The unearthed neutral,
- High impedance-earthed neutral,
- Resistance earthing,
- Reactance earthing,
- Petersen coil earthing.
- IT (Unearthed transformer neutral, earthed frame),
- TT (Transformer neutral earthed, and frame earthed),
- 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).
Today I will explain How to select the best earthing system for a new construction from the above types as follows.
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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:
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:
2- Country development
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:
These elements must therefore be optimised. The table in below figure# 2 reviews the strong and weak points in each earthing system:
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.
5- Availability of Electrical Service
In terms of complete cost over 10 to 20 years, all three earthing systems are equivalent.
7-Complexity Of Design
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).
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:
2- Fire hazard
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:
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.
Step#4: Finally allow for the special features of network and loads as follows:
In the next Article, I will explain the Construction and Components of Earthing Systems. Please, keep following.