Earthing Systems Design steps – Part One

In Article " Electrical Properties of the Earthing System ", I indicated that the electrical properties of earthing system depend essentially on the following parameters: 

  • Earthing resistance,
  • Configuration of the earth electrode and favourable earth surface potential distribution,
  • Adequate current carrying capacity,
  • Long durability. 

Theses parameters must be considered in earthing system design.

Today I will explain Steps of Earthing Systems’ Design Process as follows.

You can preview the following Articles for more info:

Earthing Systems Design Steps

A grounding system design process has (3) main steps:

  1. Data Collection,
  2. Data Analysis,
  3. Grounding Design Calculations.

1- Data Collection

Once a need for grounding system design is established, data collection must begin. These needed data provide the basis for all grounding design and will be obtained from:

  1. Facility official data,
  2. Facility characteristics,
  3. Nearby area data,
  4. Electric Utility Data,
  5. Engineering data,
  6. Geographical data of the area,
  7. Geological surveys.

1- Facility Official Data

These data include, but not limited to, the following:

  1. Facility Name,
  2. Facility owner,
  3. Facility A/E Consultant (for new construction),
  4. Facility address.

2- Facility Characteristics

The facility characteristics are represented by the following  points:

A- Purpose Of Facility:

  • Grounding needs vary according to function of the facility. The grounding requirements of a power system will vary from those of electrical equipment, lightning protection or for the proper function of electronic equipment.
  • Proper installation of appropriate grounding systems requires knowledge of the needs and layout of the facility. A list of common functions for general and special facilities is as follows:

  1. Commercial building,
  2. Industrial building,
  3. Power Substation,
  4. Airports,
  5. Railways,
  6. Fences and Gates,
  7. Satellite station,
  8. Marine communications centre,
  9. Data centers,
  10. Cellular radio site,
  11. Oil and gas site,
  12. Others.

B- Design Life Of Facility:

  • The life time of the earthing system (which called The durability of the earthing system) is from construction up to the time when, due to the corrosion of metallic parts, electrical continuity is lost. The life time of the earthing system should exceed the expected lifetime of the installation.
  • For the majority of power installations, lifetime can exceed 25 years and for power lines, 35 – 50 years. The earthing system should be included in repair and maintenance cycles.
  • Many installations are over 50 years old and during that time fault levels have, in some cases, more than tripled. Therefore, the old design may not be electrically safe or sufficiently robust to withstand the increased fault levels.

C- Shape And Available Area Of Facility Site

  • The earthing design practices for small dimension sites like small substation have been quite well established.
  • Earthing system for large installation like power stations and coal processing plants require more sophisticated calculations to achieve safe cost-effective designs.

D- Future Uses, Additions, Equipment For Facility

  • For example, adding extra spare numbers of grounding and bonding bars for bonding and grounding any future additions and equipment for the facility.

3- Nearby Area Data

The area surrounding the facility usually has a reasonable effect on the earthing system of that facility, this effect can be represented by the following points:

A- Existing Structures And Their Grounding Systems:

  • Many older facilities present the possibility of hazardous situations existing during fault conditions for the following reasons:

  1. The older facilities may not comply with the currently accepted international standards.
  2. Uncertainty concerning both the condition and effectiveness of the existing eathing system.
  3. In old facilities, in many instances the electrical hardware becomes inadequate to handle the increasing fault currents with the required degree of safety.
  4. Many installations are over 50 years old and during that time fault levels have, in some cases, more than tripled. Therefore, the old design may not be electrically safe or sufficiently robust to withstand the increased fault levels.

B- Adjacent Electrical Systems:

  • Because of the increasing area of industrial installations, the increasing density of power generating, transmission and distribution equipment, the effect of power equipment on industrial equipment (such as voltage rise) must be assessed, controlled and coordinated in the interest of personnel and equipment safety in a cost-effective manner.

C- Nearby Metallic Structures:

  • If any nearby metallic structures are exist in the vicinity, the earthing details and locations are required for these structures which including:

  1. Pipelines (e.g. water, gas, oil) stating the method of installation in the soil (insulated or not insulated, on pipe supports and bridges),
  2. Fences (e.g. bare metal fences, bare wires or insulated wire),
  3. Building construction details (e.g. Steel or reinforced concrete),
  4. Railway tracks, stating the foundation (e.g. Ballast or directly embedded in paved soil) and isolation details,
  5. Poles and other steel structures in immediate contact with the soil or water or connected with the soil or water through concrete,
  6. Rivers, streams, lakes, headwaters and soil water ponds of hydro-electric power stations or pumped storage stations,
  7. Communication lines,
  8. Disused buried metal works.

D- Existing Ground Systems Located At The Site:

  • Additional data is always helpful and can be collected from existing ground systems located at the site. For example, driven rods at the location can be tested using the 3-point fall-of-potential method or an induced frequency test using a clamp-on ground resistance meter.

4- Electric Utility Data

The electric utility company needs to provide electrical data regarding the facility, tower or substation under consideration.

A- For General Facility: The required data will be as follows:

  • The maximum prospective short circuit current at the supply terminals;
  • For low voltage connections, the maximum earth loop impedance of the earth fault path outside the installation;
  • The type and rating of the distributor’s protective device or devices nearest to the supply terminals;
  • The type of earthing system applicable to the connection; and
  • Fault clearing time.

B- For Substation And Overhead Transmission Lines Towers: The required data will be as follows:

  • The name of the substation or the number of the tower,
  • The voltage level,
  • The subtransient X/R ratio,
  • In the case of towers, the line names of the substations involved, the amount of current contributed by each substation in the event of a fault,
  • Phase-to-ground fault current contributed by each power line circuit
  • Fault clearing time,
  • The make/type/number of overhead ground wires on each tower/pole line and position with respect to the phase conductors,
  • Ground wire continuity and bonding configuration back to the tower and substation,
  • The average distance from tower-to-tower and tower-to-substation,
  • Typical tower/pole ground resistance: measured or design values,
  • As-built drawings are often acquired and are useful for towers with existing grounding systems. They are also useful in the case of modifications and upgrades to existing substations, which will have extensive grounding systems already installed.

5- Engineering Data

A- Site Drawings:

  • The property map and general location plan of the substation should provide good estimates of the area to be grounded.
  • The proposed site drawings should show the layout of the high-voltage tower or substation, and any additional construction for new equipment that may be occurring on the site, including fencing and gate radius. Incoming power and Telecom runs should also be included. In the case of high-voltage towers, the height and spacing of the conductors carried on the tower, and any overhead ground wires that may be installed on the tower, need to be detailed during the survey. This information is needed to properly address all the touch and step voltage concerns that may occur on the site.

B- Applicable Standards And Codes:

  • Many codes and standards contain different grounding and bonding requirements, it is important to know the standard or code requirements for each installation’s earthing systems under design.
  • A list of the most common Applicable Standards and codes for earthing systems are represented in the below image.

6- Geographical Data Of The Area

A- Rainfall Data:

By comparing Recent rainfall data against the seasonal average, maxima and minima for the area it may be ascertained whether the results are realistic or not.

B- Seasonal Variations:

Recent weather patterns, moisture relative to maximum and minimum and the magnitude of effect of seasonal variations. While difficult to quantify such information does provide a useful context in which the resistivity test results may be interpreted and a set of design data determined.

7- Geological Surveys

Geological data regarding strata types and thicknesses will give an indication of the water retention properties of the upper layers and also the variation in resistivity to be expected due to water content.

A- Geological Data:

  • Topography, nature of soil material, presence of varouis layers, water table, prevouis test data and civil earthworks (eg. Cut and fill).
  • Where there is an option, a site should be chosen in one of the following types of situations in the order of preference given:

  1. wet marshy ground;
  2. clay, loamy soil, arable land, clayey soil, clayey soil or loam mixed with small quantities of sand;
  3. clay and loam mixed with varying proportions of sand, gravel, and stones;
  4. Damp and wet sand, peat.

  • Dry sand, gravel, chalk, limestone, whinstone, granite, any very stony ground, and all locations where virgin rock is very close to the surface should be avoided if possible.
  •  A site should be chosen where the moisture content is ideally continuously within the range of 15% to 20%. A waterlogged location is not essential unless the soil is sand or gravel.
  • Care should be taken to avoid a site where water flows over it (e.g. the bed of a stream) for the beneficial salts can be entirely removed from the soil in such situations.

B- Corrosion Properties Of The Soil:

Determine Corrosion properties of the soil. Ascertain performance of any existing earth electrode by inspection.

C- Soil Resistivity Survey:

  • A comprehensive soil resistivity survey is key to creating an effective earthing system, as inadequate or erroneous soil resistivity readings are likely to result in a flawed design.
  • Multiple accurate soil resistivity readings should be done at various depths (usually 3 depths) across the site. As these results form the basis of the whole earthing design, the experience of the designer is critical in ensuring correct implementation of the test data.

Important Note:

Soil resistivity can be calculated from soil resistivity syrvey or can be estimated from tables. In case that soil resistivity will be estimated from tables the no need to perform the Geological Surveys.

In the next Article, I will explain How to Perform Soil Resistivity Survey. Please, keep following.

No comments:

Post a Comment