Ground measurement

The earth measurement is a measurement process that serves the effectiveness of grounding check. A carefully carried out earthing measurement serves the safety of people, animals and technical facilities.

Basics

When checking earthing systems, earthing measurement is a measure that must be carried out in accordance with the guidelines. The measurement is used to check a specific earth resistance or a specific earth impedance. Using earth measurements, protective earths and operational earths must be checked for compliance with the values ​​required by the standards. These earthing measurements are carried out for protective earthing in low-voltage networks as well as in transformer stations if the low-voltage operational earth and the high-voltage operational earth share a common operational earth . In addition, in medium-voltage networks, the earthing measurement is carried out on the earthing systems for the earthing reactor. The earthing measurement must be carried out on newly constructed or modified earthing systems. According to the regulations, a new earth measurement must be carried out every ten years. In the case of lightning protection earthing, an earthing measurement must be carried out even if no specific measured values ​​are required.

Standard measurement method

There are several different measurement methods for earth measurement. Which measurement method is used depends on the respective local conditions. There are the following measuring methods:

• Current / voltage measurement
• Voltage comparison measurement
• Bridge method
• Loop resistance measurement

The earth measurements are carried out with alternating current.

Current / voltage measurement

Basic representation of a current / voltage measuring arrangement

With this measuring method, the mains voltage of a network with an earthed star point is used. The measurement process is carried out in accordance with DIN VDE 0413 Part 7. First, a probe is struck into the ground at a distance of 50 to 100 m from the earth electrode. An adjustable test resistor is used to conduct a current into the ground via an earth electrode. The resistance value of the test resistor can be between 20 and 1000  Ω . The current through the earth electrode is determined with an ammeter . The lowest resistance value must not fall below 10 Ω. A voltmeter is connected between the earth electrode and the probe . This voltage measuring device must have the highest possible resistance so that the voltage drop between the earth electrode and the probe can be precisely determined. When using a high-resistance voltmeter, the earth resistance of the probe, which is usually between 500 and 1000 Ω, can be largely compensated. The earthing resistance is calculated from the voltage between the earth electrode and the probe and the current that is introduced via the earth electrode in accordance with Ohm's law .

${\displaystyle R_{x}={\frac {U}{I}}}$

Since this measuring method is carried out using mains voltage, an impermissibly high voltage can drop across the earth electrode. For this reason, the test current must be limited. Due to stray currents, the voltmeter may already indicate a voltage, although no test current is flowing yet. However, these measurement errors are only minor and can usually be neglected. The prerequisite for this is that the voltage when the test current is switched on is significantly higher than the stray voltage .

Voltage comparison measurement

A measuring device with its own voltage source is used with this measuring method. The DC voltage of the battery is a inverter into an AC voltage converted. This alternating voltage has a frequency which is outside the mains alternating voltage. A second auxiliary earth electrode must also be used. This second auxiliary earth electrode is used for the return flow of the test current. To ensure that none of the earth electrodes is in the voltage funnel of the other earth electrode, a minimum distance of 20 meters must be maintained between the earth electrodes. The voltmeter is earthed via the first auxiliary earth electrode. The resistance can be bridged with a switch . The measuring device is connected to the earth electrode to be checked via a resistor . The value of the resistance is known, the resistance to earth is unknown. A variable resistor is connected between the resistor and the voltmeter . The voltage drop on can be changed by changing the resistance . To carry out the measurement, the resistance is first changed until the pointer of the measuring device is at the end stop. By switching, the resistance is bridged and the earth electrode is switched on directly. This means that the voltage drop on the earth electrode can now be determined. The scale of the measuring device can also be calibrated in resistance units. ${\displaystyle R_{H}}$ ${\displaystyle R_{M}}$ ${\displaystyle R_{M}}$ ${\displaystyle R_{M}}$ ${\displaystyle R_{X}}$ ${\displaystyle R_{M}}$ ${\displaystyle W}$ ${\displaystyle W}$ ${\displaystyle R_{M}+R_{X}}$ ${\displaystyle W}$ ${\displaystyle R_{M}}$

Bridge method

Basic representation of a bridge measuring arrangement (according to Behrend)

The measuring procedure using an earthing bridge is regulated in DIN VDE 0413 Part 5. With this measuring method, the influence of the earth resistance of the measuring probe is completely compensated. An auxiliary earth rod and a grounding probe are also required here. The measuring current is generated by a small generator and conducted into the ground via a measuring transformer via the auxiliary earth electrode and the earth electrode to be tested. The measuring device is connected to the transducer via a potentiometer and is grounded via the grounding probe. The current in the secondary circuit of the transducer depends on the current in the primary circuit. The current in the secondary circuit causes a voltage drop on the potentiometer; this voltage is compared with the voltage on the earth electrode via a switch. There are various measuring devices for measurement, which are either equipped with a display or, in the case of commercially available devices, with three light-emitting diodes. For devices with a display, this is calibrated in ohms, which enables the measured earth resistance to be read off directly.

Loop resistance measurement

This measuring method is used when it is not possible to insert an auxiliary earth rod or an earthing probe. A loop resistance meter is required for the measurement. This method provides an approximate measurement of the earth resistance. A total resistance is measured. This total resistance is formed from the resistance of the operational earth electrode, the resistance of the outer conductor and the unknown resistance of the earth electrode to be measured. Since the resistances of the outer conductor and the operating earth are usually very small, they can generally be neglected. The resistance determined in this way corresponds with slight deviations to the resistance of the earth electrode to be measured.

Simplified measurement

In densely built-up areas, it is often not possible to carry out an earth measurement with the aid of probes and auxiliary earth electrodes. The earth measurement is carried out with a clamp meter. When measuring with a clamp meter (earthing test clamp), no auxiliary earth electrodes have to be plugged in and the system does not need to be separated from the earth electrode. Both the currents flowing to earth and the earthing resistance can be determined with the earthing test clamp, for this only the earthing cable has to be reconnected.

This measuring method uses two current transformers . Both current transformers are located in a clamp housing and are magnetically shielded from one another . In addition, the electronics required and the supply voltage for the electronics are also located in the same gun housing. The prerequisite for a correct measurement is that only the earth loop that is to be checked is connected to the mains with the earthing clamp via the neutral conductor or the PEN conductor. A voltage is induced in the loop to be tested by one of the two current transformers. This voltage generates a current in the loop. This current is measured with the second current transformer. The total resistance of the earth loop can be calculated using the measurement data for voltage and current.

Extended Earth Systems

The four-probe method was developed so that extensive earth systems can also be measured. With this method, which is also known as the four-electrode method, the measurement is carried out using an electrical flow field generated in the ground. The potential difference of this flow field is recorded on the earth's surface between two points by means of an earthing probe. Since the method only provides accurate measurement results in homogeneous soil, the measured earth resistance must be calculated back to a value that would occur under the worst conditions. There are two methods of arranging the measuring probes, the Wenner method and the Schlumberger method. Both methods are identical in principle, they only differ essentially in the different distances between the grounding probes.

The Wenner's method uses a fixed center point as the starting point. This center point is also retained for further measurements. Then the four earthing probes are hammered into the ground in a line with equal distances from one another. The distance between two earthing probes is decisive for the depth to which the earth resistance is measured. Incorrect measurements can result from pipes and underground cables laid parallel to the earthing probes. Water veins and roots can also falsify the measured value. To avoid major measurement errors , the resistance of the external measuring probes must be less than 500 Ω. In order to be able to adhere to these values ​​in poor soil conditions, four grounding electrodes are distributed equally spaced around the circumference of a circle at each outer end and screwed or hammered into the ground as deep as possible. The four grounding probes are connected to one another in an electrically conductive manner. The circle may have a maximum diameter of 20 meters.

The Schlumberger method also uses a fixed center point as the starting point. Starting from this center point, the inner earthing probes are driven into the ground at a certain distance from one another. This distance between the two inner earthing stakes is less than the distance to the outer earthing stakes. The inner earthing stakes also serve as measuring probes. The distance between the inner probes is not changed during the measurement. The distance between the outer grounding stakes and the inner grounding stakes can be changed as required. The distance between the inner and outer earthing probes corresponds to the depth to which the resistance of the soil is determined.

Lightning protection earth measurement

The measurement of lightning protection earthing must never be carried out during a thunderstorm or when a thunderstorm is approaching. The two-pole measuring method has established itself as the most widely used measuring method. No auxiliary earth electrodes or probes are required. A metal water pipe or the potential earth connection of the protective earth can be used as reference earth. When measuring with reference earth, the resistance of the measuring line must be compensated. The main connection room of the house offers a good possibility for measurement. To avoid incorrect measurements, all connections on the main earthing rail must be disconnected and only one earth electrode must be measured. The three-point measuring method with an auxiliary earth electrode can also be used to measure lightning protection earths.

Railway grounding

The ground measurement of railway grounding is often particularly difficult and extensive. In order to obtain sufficiently accurate measurement results, a series of measurements must be carried out. For this purpose, grounding stakes are inserted into the ground as a reference point at different distances from the railroad ground and used as a measuring point. The problem with rail earthing is the use of the earth conductor in the traction network . Not only fault currents flow in it, but also operating currents, which greatly affects the earthing measurement. Due to the frequency of the traction current (16 2/3 Hertz), the earth currents spread differently than with normal mains electricity of 50 Hertz. The earth current can penetrate deeper into the ground, which means that in the event of a fault, the earth fault current also strays outside the railway line. Due to the disturbance variables, the measurement requires several specially developed measuring devices, including the use of oscilloscopes . Although specially developed and precise measuring devices have to be used to measure the grounding of railway grounding systems, precise grounding measurement is hardly possible when railway systems are in operation. Nevertheless, errors and weak points in the earthing system can be localized and then eliminated.

report

For earthing in low-voltage systems and lightning protection earthing systems, a test report must be prepared for the earthing measurement. The measurements carried out and the accompanying measures carried out must be specified therein. The test report must be written precisely and clearly so that the measurement can later be traced back to others. The measurement method used and the type of measurement device used must be specified in the test report. In addition, the switch positions of the existing switches must be specified. If connections had to be made or disconnected for the measurement, this must also be mentioned in the test report. The measurement result must be clearly written in the test report with all weather-related measurement allowances.

literature

• Gerhard Kiefer: VDE 0100 and practice. 1st edition. VDE-Verlag, Berlin / Offenbach 1984, ISBN 3-8007-1359-4 .
• Klaus Peter Weber, Herbert Sack, Manfred Leischner: Measure more - know more. 5th revised edition. Dr. Alfred Hüthing Verlag, Heidelberg 1986, ISBN 3-7785-1167-X .

Individual evidence

1. ↑ ^{a b} Reinhold Bräunlich: The metrological review of large earthing systems. Online ( Memento from April 30, 2015 in the Internet Archive ) (accessed June 30, 2011; PDF; 120 kB)
2. ↑ ^{a b c d} Enno Hering: Measurements and tests on earthing systems. Online (PDF; 364 kB) German Copper Institute (accessed on June 30, 2011)
3. ↑ ^{a b c d} Hans-Günter Boy, Uwe Dunkhase: The master's examination in electrical installation technology. 12th edition. Vogel Buchverlag, Oldenburg / Würzburg 2007, ISBN 978-3-8343-3079-6 .
4. ^ ^{A b c} Heinz Nienhaus, Dieter Vogt: Tests before commissioning high-voltage systems. VDE-Verlag, Berlin / Offenbach 1995, ISBN 3-8007-2071-X .
5. ↑ Dehn + Söhne: Earthing measurement online ( Memento from May 26, 2011 in the Internet Archive ) (PDF; 583 kB).
6. ↑ Special report on earthing measurements. Online (accessed June 30, 2011).
7. ↑ Michael Hirsch: The determination of the loop impedance in electrical systems. Online ( Memento from February 24, 2014 in the Internet Archive ) (PDF; 224 kB).
8. ↑ ^{a b} Johann Frei: Measurement of the impedance of extensive earth systems and their calculation. Online (PDF; 2.9 MB) Diploma thesis, Graz University of Technology (accessed July 1, 2011)
9. ↑ Useful information about earth measurement online (accessed July 1, 2011; PDF; 238 kB).
10. ↑ Vojtech Kopecky: Testing of lightning protection systems. Online ( Memento of July 12, 2014 in the Internet Archive ) (accessed July 1, 2011; PDF; 402 kB).
11. ↑ Reinhold Bräunlich, Günther Storf, Max Sigg: Earthing measurements in substations of the Swiss Federal Railways. Online ( Memento of April 3, 2015 in the Internet Archive ) (accessed July 1, 2011; PDF; 482 kB).