Distribution-System-Earthing-Guidelines.pdf

Distribution System Earthing Guidelines

DISTRIBUTION SYSTEM EARTHING GUIDELINES

TABLE OF CONTENTS TABLE OF CONTENTS...............................................................................................2 1. Introduction............................................................................................................3 2. The multiple earthed rural system (MEN) .............................................................3

High voltage earthing system.................................................................................4 Low voltage earthing system .................................................................................4

3. Separately earthed high voltage and low voltage MEN system ............................4 4. The common multiple earthed system (cmen).......................................................5 5. How to earth successfully ......................................................................................5 6. Measurement uncertainties ....................................................................................6 7. Resistance of an earthing system ...........................................................................6 8. Measuring resistance..............................................................................................8 9. Making resistance measurements of isolated earthing systems.............................8 10. Making resistance measurements of a MEN system .........................................9 11. Effective resistance areas...................................................................................9 12. Soil resistivity measurements ..........................................................................10

The Wenner method.............................................................................................12 13. Making soil resistivity measurements..............................................................13


Distribution System Earthing Guidelines Page 3 of 15

DISTRIBUTION EARTHING GUIDELINES

1. INTRODUCTION An earthing system is installed as an integral part of an electricity distribution system. The earthing system has the following purposes;

a) To minimise the potential rise of earthed metallic parts that may become energised due to the passage of fault currents.

b) To provide a return path for fault currents and to allow these fault currents to be sensed and rapidly disconnected from the system.

c) In the case of SWER earths, to provide a reliable path for earth return currents without generation of significant standing voltages on the earth system.

The earthing systems adopted by the majority of power authorities in Australia are the multiple earthed neutral (MEN) system and the common multiple earth system (CMEN). In the MEN system, the high voltage and low voltage earth systems are kept separate. The CMEN system is a variation in which HV earths are also bonded to the LV system.

Figure 1 – Multiple Earthed Neutral System

2. THE MULTIPLE EARTHED RURAL SYSTEM (MEN) The multiple earthed neutral (MEN) system of earthing is one in which the low voltage neutral conductor is used as the low resistance return path for fault currents and where its potential rise is kept low by having it connected to earth at a number of locations along its length. The neutral conductor is connected to earth at the distribution transformer, at each consumer's installation and at specified poles or underground pillars. The resistance between the neutral conductor of the distribution system and the earth must not exceed 10 ohms at any location.



HIGH VOLTAGE EARTHING SYSTEM At distribution transformers, the high voltage earthing system should have the following items connected to it:

• Transformer tank; • HV switchgear; • HV surge arresters; • Exposed metalwork associated with HV cables and • HV cable sheath

The design value of resistance to ground that should be achieved by the earthing system should not exceed 30 ohms. The lower the resistance, the greater the fault current and hence the easier the fault may be detected and cleared. External metalwork (including concrete poles) within 2.4m of the ground which may become energised through the failure of high voltage insulation or conductor support must be earthed to a value low enough to ensure safe touch and step potentials. To assist in lowering the hazards, equipotential earth mats may be installed. If it is not economical or practical to install the desired earthing systems, the metalwork between ground level and 2.4m should be insulated.

LOW VOLTAGE EARTHING SYSTEM The low voltage earthing system at a distribution transformer should have the following connected to it:

• The neutral terminal of the transformer and • Any low voltage surge arresters

The value of resistance for this earthing system is recommended to be the same as that of the low voltage neutral. That is if the low voltage neutral system is to have a maximum neutral to ground resistance of 10 ohms, the low voltage earthing at the transformer should also be 10 ohms. This will ensure that the 10 ohm value for the low voltage neutral is achieved.

3. SEPARATELY EARTHED HIGH VOLTAGE AND LOW VOLTAGE MEN SYSTEM

This system has traditionally been used in the rural and some urban areas and is to continue to be used in areas where it is impractical to achieve a CMEN system. At distribution transformers two separate and distinct earthing systems must be provided. One system is used for the earthing of all exposed metal work associated with the high voltage including the transformer tank and the high voltage surge arresters. The second system is to be used for earthing the low voltage neutral, exposed metalwork associated with the low voltage and the low voltage surge arresters. At locations other than distribution transformers, all metal parts which are within 2.7 metres above ground and which may become energised through contact with high voltage conductors shall be insulated. In remote and isolated locations outside urban areas such metal work shall be locally earthed and is not required to be insulated. If the metal work can only be energised through contact with low voltage conductors, then the metal work shall be earthed and bonded to the low voltage neutral and is not require to be insulated.



4. THE COMMON MULTIPLE EARTHED SYSTEM (CMEN) The CMEN system is an extension of the MEN system whereby the low voltage neutral conductor and hence also the low voltage earthing system is extended and connected to the high voltage earthing system of substations, transformer stations and at poles carrying exposed metal work associated with voltages up to and including 33kV. This system provides a low impedance metallic return path for high voltage fault currents. Where a common system of earthing is used, the resistance between the neutral conductor of the distribution system and the earth at any location is not to exceed 1.0 ohm. Requirements for the adoption of a CMEN system are detailed in the “CMEN Guidelines” Under no circumstances is the low voltage neutral to be bonded to:

a) Overhead earth wires used for lightning protection. b) Metalwork or earthing conductors which are associated with voltages

exceeding 33kV.

5. HOW TO EARTH SUCCESSFULLY 1. The Overhead construction manual Earthing Section contains options for both

driven electrode and deep drilled earthing installations. 2. The standard earthing system is a driven electrode system using 1.5 m long 13

mm diameter flush coupled copper clad steel electrodes. 3. The alternative system is the deep drilled option which aims to establish the

earth in an area of constant soil moisture and will provide more consistent earth values over time. This earthing system is therefore the standard for SWER isolator installations and should be specified for other installations where difficulty is expected in obtaining specified values using a standard driven electrode system. The installation of deep drilled earths requires the use of a boring plant which requires prior organisation.

4. The decision as to which system to specify would best be based on soil resistivity tests carried out to determine the soil depths at which the lowest values of soil resistivity exist.

5. Alternatively local experience may be a good guide. 6. In areas with good soil resistivity, the resistance values stipulated in the

manuals can be easily obtained. In rocky or dry sandy locations, the required resistance values can be difficult to achieve. When this occurs, carry out a soil resistivity test as described in sections 12 and 13.

7. If the soil becomes more conductive with depth, drive the earth rods in deeper until the desired value is obtained.

8. If the soil is less conductive with depth, install additional earth rods at some distance away until the desired value is reached (refer relevant earthing drawings from the Overhead construction manual).

The following points may also be useful:

a) In general, driven electrodes should be driven as deep as possible using extension sections in order to penetrate to depth where moisture content is likely to be more consistent, unless indicated otherwise by the results of soil resistivity tests. Additional earth rods should be installed at a distance of no closer than 2 earth rod lengths. In multi-layered soils where an underlying



layer of low resistivity soil is encountered, the length penetrating this layer may be taken as the effective earth rod length. In some cases it may be advantageous to run a buried earth or a suspended cable to a location in a lower lying area where better earth values may be expected. This may particularly be the case for installations located on rocky ridges.

b) When trying to obtain a 1.0-ohm system, connect the new area to an existing 1.0-ohm area whenever possible.

c) In very high soil resistivity areas, earth the low voltage neutral at every pole/pillar to obtain the desired resistance to ground.

DEEP DRILLED ELECTRODES d) Deep drilled electrodes should be taken down to depth where constant soil

moisture is achieved or soil resistivity measurements indicate an area of low soil resistivity. This may require depths of 30 m or more. The hole should not extended into the watertable however as this would result in dissipation of the earthing compound. A bentonite – gypsum mix or similar product should be placed in the hole with the earth rod. The mixture should be inserted dry and allowed to take up moisture from the surrounding soil. The earth resistance value will then reduce over time. This may require some several days before a reliable earth reading can be achieved. If the soil moisture is very low, the application of water between layers of earthing mix may assist in obtaining a satisfactory earth resistance reading at an early stage, however the mix should not be added as a slurry as while this may achieve a low immediate reading, the reading will deteriorate over time as the earthing compound dries out and shrinks. Successive deep drilled electrodes should be separated by sufficient distance such that their potential gradient patterns do not overlap significantly. This separation should be at least twice the depth of the good conducting layer of soil and in any case not less than 5 metres.

6. MEASUREMENT UNCERTAINTIES The conduction of current through soil is non-linear. Consequently, the resistance to ground of earthing systems will differ for different test voltages, currents and frequencies. The resistance to ground of earthing systems will also vary on a daily basis due to climatic conditions. As a consequence of these problems an uncertainty of ± 20% must be given to all measurements.

7. RESISTANCE OF AN EARTHING SYSTEM

Figure 2 – Sectional View of Ground Electrode



In homogeneous soils the pattern of current flow and the values of potential and gradients for any earthing system approximate that of an equivalent hemispherical electrode (of the same ground resistance) at points remote from the earthing system. Figure 2 shows a sectional view of ground through an electrode showing hemispherical equal voltage fields. In an homogeneous soil with an earth resistivity of ‘ρ’ Ohm-metres, the resistance ‘R’ between a hemispherical electrode of radius ‘r’ and a point on the surface ‘P’ metres away is:

⎟⎠⎞

⎜⎝⎛ −=

PrR 11

2πρ

Ohms As ‘P’ goes to infinity,

rR

πρ

2=

Ohms

Figure 3 – Resistance as a Function of ‘P’. A rod electrode of length ‘l’ metres and of diameter ‘d’ metres may be modelled as a large number of spheres placed on top of each other. The resistance of the rod to the general mass of ground (ie ‘P’ at infinity) is given by:

⎥⎦

⎤⎢⎣

⎡−⎟

⎠⎞

⎜⎝⎛= 18ln

2 dl

lR

πρ

Ohms The rod electrode may be replaced by an equivalent hemisphere having the same resistance the radius ‘r’ can be obtained by equating the last two equations. Thus the radius is given by:

18ln −⎟⎠⎞

⎜⎝⎛

=

dllr

metres



8. MEASURING RESISTANCE The most common method of measuring earthing resistance is “the fall of potential method”. In this method, a test current is injected into the earthing system under test ‘E’ and returns via a temporary current probe ‘C’ at some distance away. The potential rise of the system under test is measured using a roving probe ‘P’ placed in the ground between the two current probes (refer figure 4). Resistance is calculated from the values of potential rise and test current. For an electrode system with an equivalent hemisphere radius of ‘r’ and in soil of resistivity ‘ρ’ Ohm-meters, the resistance equation:

⎟⎠⎞

⎜⎝⎛ −=

PrR 11

2πρ

was for the case when the other current electrode was at infinity (ie C = ∞). Since ‘C’ is at a finite distance, the value of resistance measured at any point ‘P’ is given by:

⎥⎦

⎤⎢⎣

⎡⎟⎠⎞

⎜⎝⎛

−−−⎟

⎠⎞

⎜⎝⎛

−−==

PCPrCrIVR 1111

2πρ

Ohms

Figure 4 – Fall of Potential Method Resistance measurements are made using an Earth Resistance Tester. The tester calculates the resistance from the values of potential rise and current and displays value of resistance of the earthing system.

9. MAKING RESISTANCE MEASUREMENTS OF ISOLATED EARTHING SYSTEMS

1. Connect up the equipment as shown in figure 5. Use test leads with a cross sectional area of at least 2.5mm.

2. Locate the current probe ‘C’ at 50m away from the earthing system under test.

Ideally try and place the current probe at right angles to the footpath to avoid any buried earthing conductors, metallic water pipes, metal fences, etc. Where access is restricted, ‘C’ may be placed at 20m.



3. Ensure all electrodes are in an approximate straight line and buried to a depth of 300mm to 500mm. It is very important that the current probe is driven deep enough into the soil to ensure a low resistance connection to ground.

4. Take resistance readings with the potential probe ‘P’ at 50%, 60% and 70% of the

distance to current probe ‘C’ (ie. at 25 m, 30 m and 35 m for ‘C’ at 50 m).

Figure 5 – Test Set-Up for Earth Resistance Measurements There should be no more than 10% difference between the three (3) readings. If there is, then reposition the current electrode ‘C’ at 100 m as the resistance area of the earthing system must be quite large and is interfering with the measurement (Refer to following section). Repeat the resistance measurements with the potential probe ‘P’ at 50%, 60% and 70% of the distance to ‘C’.

10. MAKING RESISTANCE MEASUREMENTS OF A MEN SYSTEM The method used is similar to that for an isolated earthing system but with the following changes:

1. The current probe ‘C’ can no longer be situated along the footpath but must be located in a remote area removed from consumer and electricity network earth rods, metallic water pipes, railway lines, metal fences, etc. Sporting fields, paddocks and wasteland may be used.

2. The current probe should be at least 100 m away.

11. EFFECTIVE RESISTANCE AREAS Figure 6 shows the effective resistance areas (concentric shells) of the earthing system under test ‘E’ and of the current probe ‘C’. The resistance areas overlap. If readings were taken by moving the potential probe ‘P’ towards either ‘E’ or ‘C’ the readings would differ significantly.



Figure 6 – Overlapping Resistance Areas

Figure 7 – Separate Resistance Areas In Figure 7, the earthing system under test ‘E’ and the current probe ‘C’ are sufficiently spaced so that the areas of effective resistance do not overlap. Readings taken with ‘P’ located at 50%, 60%, and 70% of the distance to current probe ‘C’ will not be significantly different.

12. SOIL RESISTIVITY MEASUREMENTS Specific resistance or resistivity is defined as the resistance between opposite faces of a cube of unit length, and it is usually measured in ohm-metres. The resistivity will vary with moisture content and since the earth is not likely to be homogeneous, strata of different resistivity materials such as rock will occur at varying depths.



However, generally the resistivity of soil is determined by the quantity of water held in the soil and on the resistivity of the water itself, since most soils are non-conductors when completely dry. Conduction through soil therefore becomes conduction through the water held in the soil and so the conduction is mainly electrolytic. The main factors which determine the resistivity of soil are:-

a) Type of soil b) Salt dissolved in the contained water c) Moisture content d) Temperature e) Grain size f) Closeness of packing and pressure

The most common method of measuring the resistivity of large volumes of undisturbed earth is the Wenner four (4) probe method.

Type of Soil Typical Resistivity (Ohm-metres)

Ashes of Cinders 6-70 Clay (Damp) 14-30 Clay (Dry/Compacted) 100-200 Granite 2000-3000 Limestone 1000-5000 Loam (Humus) 200-400 Loam (with Sand and Gravel) 30-50 Loam (with some stones) 100-300 Loam (with stones and poor vegetation) 200-350 Marshy Soils 300-400 Mountain Rocks (with little or not soil) 5-40 Mountain Soil (Damp Peat over Rock Base) 1000-5000 Mountain Soil (Over Rock Base) 150-300 Salt Pans 300-1000 Sand (Below Water Table) 6-70 Sand (Damp) 60-130 Sand (Dry) 1000-5000 Sand (Leached) 1000-5000 Sandstone 120-7000 Shales 100-160

Table 1: Typical Resistivity of various soil types



THE WENNER METHOD

Figure 8 – The Wenner Method In this method four (4) electrodes are positioned at equal intervals of ‘a’ metres and driven in to a depth of ‘b’ metres. The four electrodes must be in a straight line (refer figure 8). A test current ‘I’ is passed between the two outer electrodes and the potential ‘V’ between the two inner electrodes is measured. The ratio ‘V/I’ gives the mutual resistance ‘R’ in ohms. For homogeneous soil of resistivity ρ, Wenner developed the equation:

⎥⎦

⎤⎢⎣

⎡

+−

++=

2222 44221

4 baa

baa

aR

πρ

From the equation the resistivity ‘ρ’ can be extracted.

2222 44221

4

baa

baa

aR

+−

++

=πρ

The equation, however, applies to four small point electrodes buried at a depth ‘b’ and not to conducting rods driven into the earth. In practice, four rods are driven into the earth. If the rods are driven to a depth of less than 1/20th of their spacing (ie b < a/20) then:

aRπρ 2= Ohm-metres The derivation of the equation is based on the assumption that the soil resistivity is uniform; usually the soil is not. Thus the resistivity calculated for various spacings of ‘a’ will vary. The resistivity calculated is known as the apparent resistivity for electrode spacing ‘a’ - due to non-uniform soil resistivity. It is suggested that the effective penetration of the current is equal to the electrode spacing ‘a’. It can be demonstrated that this is not so. However, use of the apparent resistivity at electrode spacing ‘a’ metres as the average resistivity to a depth of ‘a’ metres is good enough approximation for most circumstances.



13. MAKING SOIL RESISTIVITY MEASUREMENTS 1. Connect up the equipment as shown in figure 9. A four terminal earth resistance

tester is required as it will indicate directly the value of mutual resistance 'R' in ohms. Use test leads with a cross sectional area of at least 2.5mm2.

2. Ensure the rods are in a straight line with an equal spacing of ‘a’ metres and

inserted to a depth of not more than 1/20th their spacing. 3. Keeping the centre position the same, take resistance measurements at various

rod spacings. Always ensure that the spacing between individual rods are identical.

Figure 9 – Test Setup for Soil Resistivity Measurements

4. For each spacing, calculate the apparent soil resistivity using the equation given above. The depth to which the soil resistivity is measured is approximately the same as the spacing.

Figure 10 gives typical curves of apparent soil resistivity for multi-layered soils. A blank test results sheet is given at the end of this section.



Figure 10 – Typical Curves of Apparent Soil resistivity



SOIL RESISTIVITY TEST

Date: ___/___/______ Time: _______:________ LOCATION: __________________________________________________________

SPACING

‘a’ METRES

RESISTANCE READING

‘R’ ohms

APPARENT RESISTIVITY

ρ = 2πaR ohm-metres

1 2 5 10 15 20 25 30 35 40 45 50 60 70

1 2 3 4 5 6 7 8 9 10 11 12 13

Spacing (m)

App

aren

t Res

istiv

ity (o

hm-m

etre

s)

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Distribution-System-Earthing-Guidelines.pdf