Electrical grounding Techniques, Resumos de Medição Eletrônica e Instrumentação

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Electrical grounding techniques from

Why Ground ?....................................................

What is a ground and what does it do ?................................

Ground Resistance Values.........................................

Ground Electrodes................................................

Types of Ground Systems..........................................

Ground Resistance Testing - Soil Resistivity...........................

Measuring Soil Resistivity 4 - Pole Method............................

Ground Resisting Testing 3 - Pole Fall of Potential......................

Ground Resistance Testing Existing Systems Selective Clamp - On........

Ground Resistance Testing Existing Systems "Stakeless"................

Ground Resistance Testing 2 - Pole..................................

Ground Impedance Measurements..................................

Measuring Ground Resistance at Substations..........................

Measuring Ground Resistance at Central Offices.......................

Measuring Ground Resistance at Cellular Sites/Microwave and Radio Towers

Measuring Ground Resistance at Remote Switching Sights..............

Measuring Ground Resistance for Lightning Protection Commercial/Industrial

Ground Testing Instruments - UNILAP GEO / GEO X....................

Ground Testing Instruments - SATURN GEO / Handy GEO...............

Ground Resistance -

Principles, Testing,

Techniques & Applications

Table of Contents

LEM Instruments

Palmersstrasse 2

A-2351 Wr. Neudorf

Tel: +43/2236/691-

Fax: +43/2236/691 415

email: lno@lem.com

http://www.lem.com

Publication A 99415 E

Printed in Austria

round Electrodes

1 2

Sphere of influence:

The resistance of a ground

e le c tro de ha s 3 ba sic

components:

A) The resistance of the ground electrode itself and the connections to the electrode.

B) The contact resistance of the surrounding earth to the electrode.

C) The resistance of the surrounding body of earth around the ground electrode.

consist of three basic components:

ground conductor the connection/bonding of the conductor to the ground electrode (^3) the ground electrode itself

A) The resistance of the ground electrode and it's connection is generally very low, ground rods are generally made of highly conductive/ low resistance material such as copper of copper clad.

B) The contact resistance of the earth to the electrode: The Bureau of Standards has shown this resistance to be almost negligible providing that the ground electrode is free from paint, grease etc. and that the ground electrode is in firm contact with the earth.

C) The resistance of the surrounding earth: The ground electrode is surrounded by earth which is made up of concentric shells all having the same thickness. Those shells closest to the ground electrode have the smallest amount of area resulting in the greatest degree of resistance. Each subsequent shell incorporates a greater area resulting in lower resistance. This finally reaches a point where the additional shells offer little resistance to the ground surrounding the ground electrode.

The NEC specifies that the ground electrode shall be installed so that it is at least 2,4 m in length and in contact with the soil. There are 3 variables that affect the resistance of a ground electrode:

1. The ground Itself
2. The length/depth of the ground electrode
3. Diameter of the ground electrode.

Increasing the diameter of the ground electrode has very little effect in lowering the resistance. For example you could double the diameter of a ground electrode and your resistance would only decrease by as much as 10 %. One very effective way of lowering resistance is to drive ground electrodes deeper. Because the earth is in layers resistivity changes and varies considerably on the layer and the depth within that layer. Soil is not consistent in its resistivity but highly unpredictable. With that in mind it is of critical importance that when installing the ground electrode that it is below the frost line so that the resistance to ground will not be greatly increased by the freezing of the surrounding soil. Generally speaking by doubling the length of the ground electrode you can reduce the resistance level by an additional 40 %. There are occasions where it is physically impossible to drive ground rods deeper, areas that are composed of rock, granite etc. In these instances alternative methods such as grounding cement are a viable alternative.

To assist you in installing a ground rod that will meet your specific resistance require- ments you can use the table of ground resistances on page

5. Remember this is to be used only as a rule of thumb, because soil is in layers and is rarely homogenous, so the resistance values will vary greatly.

Another system to lowering ground resistance is through the use of multiple ground electrodes. In this system more than one electrode is driven into the ground and connected in parallel to lower the resi- stance. Each ground electrode has it's own sphere of influence and for additional electrodes to be effective the spacing of additional rods needs to be at least equal to the depth of the driven rod. Without proper spa- cing of the ground electrodes the spheres of influence will intersect and the lowering of the resistance will be minimal and of little value.

Multiple ground electrodes - Interaction

Ω m

Moist humus soil, moor soil, swamp (^30) Farming soil loamy and clay soils (^100) Sandy clay soil 150 Moisty sandy soi 300 Dry sand soil 1000 Concrete 1: 5 400 Moist gravel 500 Dry gravel 1000 Stoney soil 30, Rock 10 7

Soil resistivity

3 6 10 5 10 20

10 5 3 12 6 3

33 17 10 40 20 10 50 25 15 60 30 15 66 33 20 80 40 20 330 165 100 400 200 100

• • • 160 80 40 160 80 48 200 100 50 330 165 100 400 200 100 1000 500 300 1200 600 300

---

Earthing resistance ( Ω) Type of Soil RE Earthing rod m depth Earthing strip m

Ground Resistance Values

Soil resistivity depends on soil composition, moisture and temperature. It stands to reason that soil resistivity will vary through out the year in those areas of the country where seasonal changes bring about a change in the moisture and temperature content of the soil. For a grounding system to be effective it should be designed to withstand the worst possible conditions.

Since soil and water are generally more stable at deeper strata it is recommended that the ground rods be placed as deep as possible into the earth, the water table if possible. Ground rods should also be installed where there is a stable temperature i.e. below the frost line.

Caution! Soil that is low in resistivity is often highly corrosive because of the presence of water and salts, and this soil can eat away at ground rods and their connections. That is why it is highly recommended that grounds and ground fields be checked at least annually. Although resistance to ground will change seasonally and over time any increase of resistance >20% or more should be investigated and corrective action taken to lower the resistance.

Soil Resistivity

Measuring Soil Resistivity 4 - Pole Method

The measuring procedure described below uses the universally accepted Wenner method developed by Dr. Frank Wenner of the US Bureau of Standards in 1915. (F. Wenner, A Method of Measuring Earth Resistivity;Bull, National Bureau of Standards, Bull 12(4) 258, p. 478-496; 1915/16.

The formula is as follows: ρ = 2 π A R Where: ρ = the average soil resistivity to depth A in ohm - cm π = is the constant 3. A = the distance between the electrodes in cm R = the measured resistance value in ohms from the test instrument

The calculation of this measurement can be simplified by converting distance in cm to distance in feet giving you the following equation: ρ = 191.5 A R Where: ρ = the average soil resistivity to depth A in ohm - cm A = the distance between electrodes in feet R = the measured resistance value in ohms from the test instrument

Note: Divide ohm - centimeters by 100 to convert to meter - ohms. For example, you have decided to install 3 m ground rods as part of your grounding system. To measure the soil resistivity at a depth of 3 m requires that the spacing between the test electrodes is 3 m. The depth that the test electrodes is to be driven is A/20. To measure the soil resistivity start the GEO and read the resistance value in ohms. Now if your resistance reading is 100 ohms the soil resistivity for one cubic meter would be: ρ = 2 x π x 3 x 100 ρ = 1885 Ωm

The ground stakes are positioned in a straight line equidistant from one another and at a distance between one another reflecting the depth to be measured. The ground stakes should be screwed in no deeper than 1/3 the distance from one another. A known fixed current is generated by the GEO between the two outer ground stakes and a drop in potential (which is a result of the resistance) is then measured automatically between the two inner ground stakes. The GEO then displays this resistance value in ohms.

Because measurement results are often distorted and invalidated by underground pieces of metal, underground aquifers etc. additional measurements in which the stakes axis is turned 90 degrees is always recommended. By changing the depth and distance several times a profile is produced that can determine a suitable ground resistance system.

Soil resistivity measurements are of- ten corrupted and/or prevented by the existence of ground currents and their harmonics. To prevent this from occurring the GEO uses an Automatic Frequency Control System (AFC), that automatically selects the testing frequency with the least amount of noise enabling you to get a clear reading.

To test soil resistivity connect the ground

tester as indicated.

oil Resistivity Testing

round Resistance Testing

Existing Systems 'Selective' Clamp-on

This unique exclusive LEM method has been created to measure resistances of individual ground electrodes in all types of grounded systems including ground grids and wired meshes as are common in substations, high voltage pylons with ground cabling, and commercial settings with multiple grounds. By using a specialized clamp-on current transformer the effects of parallel resistances are

eliminated from the measuring process and therefore do not influence the measuring results. A special rectification method is used to isolate or 'digitally filter' out other currents to significantly increase accuracy. As with the standard 3-Pole Fall of Potential testing the rules / guidelines for the setting of ground stakes apply for both simple and complex grounds.

NO DISCONNECTION REQUIRED!

The "Ground Under Test" does not have to be disconnected!

Diameter of Ground Grid, or Field in m -

Distance to Probe P2/S in m

Distance to Probe C2/H in m

Connect the ground tester as shown in the picture above. Press START, and read out the RE value. This is the actual value of the ground electrode under test.

To test the accuracy of the results and to ensure that the ground stakes are outside the 'spheres of influence' reposition ground stake P2/S 1 m in either direction and take a new measurement. If the measured value remains fairly constant the distance between the ground stakes is sufficient. If there is a significant chan- ge in the reading (30 %) you need to increase the distance between the ground rod under test and P2/S and C2/H until the measured values re ma in f ai rl y c o ns ta nt w he n repositioning the P2/S ground stake 1 m or so. Ground resistance measurements are often corrupted and or prevented by the existence of ground currents and their harmonics. To prevent this from occurring the UNILAP GEO uses an Automatic Frequency Control System (AFC), that automatically selects the testing frequency with the least amount of noise enabling you to get a clear and accurate reading.

Testing individual ground electrode resistances of high voltage transmission towers with overhead ground or static wire requires that these wires be disconnected. If a tower has more than one ground at it's base, these must also be disconnected one by one and te- sted.The UNILAP GEO X with the 31 cm diameter clamp-on current transformer can measure the individual resistances of each leg without disconnecting any ground leads or overhead static/ground wires.

'Selective' Measuring of

High Voltage Transmission

Towers

From the picture above we can see that the total resistance of an individual tower is the parallel sum of all grounds. The tower has 4 individual grounds you must measure all 4, generating the individual resistance and then calculate according to the formula above.

There are 3 separate t y p e s o f g r o u n d measurements that are necessar y t o conduct when doing a grounding audit of a substation.

M e a s u r i n g G r o u n d R e s i s ta n c e a t

Substations

First determine the nature of the ground system, i.e. mat, rods, water system, combination etc. Substations generally consist of high voltage transmission towers and transformers that are connected and grounded to a ground grid. Follow the rules / guidelines for stake setting to ensure that the measurement is accurate and has not been influenced by the effects of the grid. Reposition P2/S a yard or so and take a new measurement. If there is a deviation >30 % of measured value, reposition both P and C2 further from the ground under test and repeat. This value should then be recorded. These measurements should be repeated at least annually to detect any change within the ground grid.

After having completed the 3-Pole test for the entire grid we to measure individual ground rods and their connections in the grid using the selective clamp-on method. We measure each connection separately without having to disconnect. The purpose of the selective clamp on is to ensure that the resistances within the grid are fairly uniform.

To conduct a selective clamp-on test, keep in mind that the spacing requirements for the reference stakes are the same as with a standard fall of potential test. Make sure that you leave enough slack in your leads so you can move easily from connection to connection. The results of this test should be recorded and the test repeated at least annually.

G r o u n d I m p e d a n c e M e a s u r e m e n ts

When attempting to calculate possible short circuit currents in power plants and other high voltage/current situations, determining the complex grounding impedance is important since both inductivity and resistivity are present. Because inductivity and resistivity are known in most cases actual impedance can be determined using a complex compu- tation. Since impedance is frequency dependent, GEO uses a 55 Hz signal for this calculation to be close to mains as possible without corrupting the measurement. Accurate direct measurements of grounding impedance are possible.

Power utilities testing high voltage transmission lines are interested in two things. The ground resistance in case of a lightning strike and the impedance of the entire system in case of a short circuit on a specific point in the line. Short circuit in this case means an active wire breaks loose and touches the metal grid of a tower.

The first measurement to be taken is a "Stakeless" measurement. Use the GEO X to clamp around all grounding connections.

A measurement that showed a great deviation to the other measurements is probably indicative of a problem that should be investigated.

Depth of electrode

Approximate distance to auxiliary probes using the 62% method in m

2 3 6 10

25 30 40 50

15 20 25 30

Distance to probe C1/E

Distance to auxilliary probe C2/H

a ground lead going to t h e M G N ( M u l t i - Grounded Neutral) or incoming service, a se- parate ground lead from the MGB to the ground field, another ground lead from the MGB connected to the water pipe and a ground lead connected to structural or building steel. The first measurement to take is stakeless measurement of all the

by as much as a mile.

Record the measurement and this test should be repeated at least annually.

When conducting a grounding audit of a central office there are 3 or 4 different measurements required. First locate the MGB (Master Ground Bar) within the central office to determine the type of ground. The MGB will have

easuring Ground Resistance

At Central Offices ...

Measurements would be accurate but would not show how the system behaves as a network, because in real life in the event of a lightning strike or fault current everything is connected. To prove this out you can measure each leg separately disconnected via the 3-pole method and record each measurement. Using Ohm's law again these measure- ments should be equal to the resi- stance of the entire system. From the calculations you will see that you are from 20 - 30 % off the total RE value. The final way to measure the resistances of the various legs of the MGB is the 'Selective Stakeless Method'. It works similat to the Stakeless Method but it differs in the way we use the two separate CT's. We clamp the inducing voltage CT around the cable going to the MGB, and since the MGB is connected to the incoming power which is straight parallel to earth system we have achieved that requirement. Take the sensing CT and clamp it around the ground cable leading out to the ground field (see below). When we measure the resistance this is the actual resistance of the ground field plus the parallel path of the MGB which because it should be very low ohmically should have no real effecton the measured reading. This process can be repeated for the other legs of the Ground Bar i.e. water pipe and structural steel. To measure the MGB via the Stakeless Selective method

individual grounds coming off of the MGB. The purpose is to ensure that all the grounds are connected especially the MGN. It is important to note that you are not measuring the individual resistance rather the loop resistance of what you are clamped around. Connect the GEO X and measure the loop resistance of the MGN, the ground field the water pipe and the building steel.

clamp the inducing CT around the lead going to the water pipe as the water pipe should have very low resistance, your reading will be for the MGN only.

There are 3 key measurements when conducting tests at remote switching sites also known as slick sites, digital line concentrators and probably more. The remote site is generally grounded at either end of the cabinet and then will have a series of ground stakes around the cabinet connected by copper wire.

easuring Ground Resistance at

Remote Switching Sites ...

The second measurement to be

... and for Lightning Protection

Commercial / Industrial

There are 3 ground resistance measurements required when conducting an audit of a lightning/fault current protection system. Most lightning fault current protection systems follow the design of having all 4 corners of the building grounded and these are usually connected via a copper cable. Depending on the size of the building and the resistance value that was tried to be obtained number of ground rods will vary.

Keep in mind the rules for stake setting. This measurement should be recorded and measure- ments should take place at least semi-annually.

SATURN GEO

Universal earth tester 4-pole ground measurement 3-pole ground measurement 2-pole resistance measurements Resistance measurements with AC Specific earth resistance Digital display Earth impedance of high voltage pylons Extremely rugged housing thanks to protective cover and carrying strap.

Handy GEO

Small, handy ground tester 3-pole ground measurement 2-pole resistance measurements Digital display, bar graph Logging of the measured values via an optional RS 232 interface and PC software WinVIEW or directly via a printer.

Ground Testing Instruments

Designation Order No.

Handy GEO base instrument A 1885 03110 Handy GEO incl. ground measuring kit

• 2 ground stakes, 2cable reels (25m, 50m), carrying case A 1885 03111 Handy GEO base instrument
• RS 232 interface and PC-software WinVIEW A 1885 03112 Ground measuring kit 2 stakes, 2 reels (25m, 50m), case A 6030 03100

SATURN GEO without accessories A 1885 06411

UNILAP GEO without accessories A 1885 06110 UNILAP GEO incl. ground measuring set: A 1885 06111 4 ground stakes, 2 cable reels (25 m), 1 cable ree (50 m), case

UNILAP GEO X incl. ground measuring set A 1885 06211 UNILAP GEO X incl. ground measuring set and RS232 A 1885 06212 UNILAP GEO X incl. ground measuring set and stakeless kit A 1885 06213 UNILAP GEOX with ground measuring set and DOCU-PACK A 1885 06215

WinGEO PC software for UNILAP GEO X with RS232/DOCU-PACK A 1885 00172 12 1/2" split core transformer A 6805 06211 Earth/Ground measuring kit: 2 stakes, 2 reels (25m, 50m) in carrying case A 6045 10301

LEM Instruments Palmersstrasse 2 A-2351 Wiener Neudorf

TEL.: 0043 (0) 2236 691- FAX: 0043 (0) 2236 691 415

Printed in Austria Technical modifications reserved. Publication A 99415 E (12.99)