Grounding is the deliberate electrical connection of parts and assemblies of electrical equipment to a grounding device. With the help of such a device, protection against electric shock is carried out by reducing the voltage to a safe value when touched by a person or an animal. Measurement of the resistance to spreading of the ground electrode current is necessary to determine the compliance of the protection device with technical standards.
Content
- Measurement principle
- Applied devices
- The order of the work performed
- Presentation of results
Measurement principle
Measurement of the resistance of grounding devices is carried out at the frequency established at the enterprise, but at least once every 12 years. For a more accurate measurement, an artificial electrical network is created.
An auxiliary device, called a current electrode, is embedded in the ground next to the circuit under test, and it is also connected to the network. And also an electrode is installed, which determines the voltage drop in the network.
In order to measure and obtain more reliable data, at the time of the process, there must be optimal weather conditions. That is, the soil resistance at this moment should be maximum. Wherein the following conditions must be met:
- the electrode from which readings will be taken is placed strictly between the grounding structure and the additional electrode;
- the distance between the elements should be equal to five times the depth of the earthing switch;
- when measuring the earthing system, the diagonal with the greatest length is taken into account.
In addition, insulation resistance measurements are additionally carried out.
Applied devices
Due to the fact that a household tester is not capable of providing high voltage, it cannot be used for this procedure. Usually they use devices that have been produced by the industry for a long time, but there are also new models that work on new electronic technologies. All of them are characterized by low current consumption from the built-in power supply. Among them the following models are worth noting:
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F4103-M1 is a popular device for performing work on measuring contours of various geometric shapes and sizes. The instrument measurement error is 4%, and the current frequency is from 265 to 310 Hz. The device is powered by 9 A373 batteries, while the current consumption does not exceed 160 mA. - M-416 - the operation of this measuring device has been carried out for a long time. Differs in high accuracy of readings and reliability in work. In addition to measuring grounding resistance, this meter can determine the specific resistance of the soil. The measurement range is from 0.1 to 1000 ohms.
- The Fluke 1625-2 GEO is a more modern instrument that can measure with just the clamps. In this case, grounding electrodes are not used. In addition to measuring the grounding resistance, you can also check the lightning protection.
In addition, the following models can be noted: MRU-101, IS-20/1, IS-10, etc.
The order of the work performed
To measure the grounding resistance, in addition to the device, you should prepare two pieces of fittings or pipes. They will act as a current and potential electrode. In addition, it is necessary to prepare the wires of the appropriate length. The measurement is carried out, taking into account the peculiarity of the assembly of the contour structure, namely two schemes are used:
- To check a simple grounding scheme, the electrodes are connected linearly. The potential workpiece should be located 20 m from the ground, and the current one - 12 m from the potential electrode.
- In the case of complex circuits, this method is not recommended, since it will not comply with the permitted standards. When measuring the grounding of the circuit, its largest diagonal is determined. The potential one is set at a distance of five diagonals, and a current electrode is hammered 20 m away from it.
The M-416 device is used as an apparatus for measuring, since it is the most widespread and reliable one. His work is based on the principle of the compensation method, he must be verified and have a corresponding entry in the passport.
The instrument must first be adjusted by setting the switch to the 5 ohm position. Then, while operating the slide chord, adjust the arrow closer to zero. Then the circuit is disconnected from the grounding conductor, and the device is connected to the corresponding electrodes.
The end of the earthing switch, which will be checked, is thoroughly cleaned to eliminate extraneous interference during testing, and then the device is connected to it. Depends on receiving resistance readings the device is connected with two or four wires.
In the first case, it is assumed that the resistance is more than 5 ohms, and in the second it should be below this value. How to correctly connect the conductors of the device to ground is shown in its passport.
After connecting the conductors, press the corresponding button, having previously zeroed the readings. As a result, the value of the resistance of the ground electrode will be reflected on the rheochord scale.
Presentation of results
Be sure to draw up an appropriate document after the measurements. All entries are made on a special form of a certain form. It indicates:
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Object name; - installation diagram of grounding electrodes and their connections;
- ground loop plan;
- method of determining resistance.
In addition, the name of the device with which all measurements were carried out is indicated in the corresponding column.
It is imperative that all readings for measuring the resistance of the ground loop are entered in the device's passport. Experts draw up a separate protocol in which they reflect the test results of transient resistances.
They indicate possible losses during the passage of current associated with welding, bolting and other types of connection of the entire ground loop. This procedure is usually performed with a special device - a microohmmeter.
All these measurements and the results of the readings can only be carried out by a special laboratory registered with the standardization bodies. This organization issues a decision on the further use of the grounding device.
