How to choose a ground resistance tester

The normal operation of a circuit is inseparable from ground resistance, so what are its standards? This article first explains in detail the standard requirements for ground resistance. Secondly, it introduces how to correctly choose a ground resistance tester. Follow the editor to learn more about it.

Standard ground resistance specification requirements:

1. The independent lightning protection grounding resistance should be less than or equal to 10 ohms;

2. The independent safety protection grounding resistance should be less than or equal to 4 ohms;

3. The independent AC working ground resistance should be less than or equal to 4 ohms;

4. The independent DC working ground resistance should be less than or equal to 4 ohms;

5. Anti-static grounding resistance is generally required to be less than or equal to 100 ohms.

6 The common grounding body (joint grounding) should not be greater than the grounding resistance 1 ohm.

According to national regulations: "For all power equipment below 1KV using the same grounding device, when the total capacity is ≥100kVA, the grounding impedance should not be greater than 4Ω; if the total capacity is <100kVA, the grounding impedance is allowed to be greater than 4Ω, but not greater than 10Ω.

There are many ground resistance values ​​for electrical devices. Different systems specify different resistance values ​​according to the different distribution systems and the magnitude of the ground fault current. Here is an excerpt of some of the specified values ​​in the current specifications. Two of the specifications have been updated or invalidated according to the 2009 Ministry of Construction documents. But still available for reference.

(1) Signal grounding - the grounding set up to ensure that the signal has a stable reference potential.

Unless otherwise specified, the grounding resistance value of electronic equipment should generally not be greater than 4Ω and a one-point grounding method should be used.

Special attention should also be paid to: To determine the presence or absence of electronic, information and computer equipment in a building, the presence or absence of equipment rooms and host equipment is generally used as an important indicator of measurement.

(2) Use shielded wiring, and all shielding layers should maintain continuity. The height of the lightning rod should be enough to protect the antenna facilities. The vertical ground body should be made of angle steel, steel pipe or smooth round steel, and rebar should not be used.

There are three types of grounding

Protective grounding: The metal shell of electrical equipment, concrete, poles, etc. may be charged due to insulation damage. This grounding is designed to prevent this situation from endangering personal safety. Below 1Ω

Anti-static grounding: Ground flammable oil and natural gas storage tanks and pipelines, electronic equipment, etc. to prevent the dangerous effects of static electricity.

Lightning protection grounding: In order to introduce lightning into the ground, the grounding end of the lightning protection equipment (lightning rod, etc.) is connected to the earth to eliminate the harm of lightning overvoltage to electrical equipment, personal property, and grounding. It is also called overvoltage protection grounding.

How to correctly choose a ground resistance tester

Currently, there are hundreds of ground resistance testers on the market, both imported and domestic. To sum up, there are only three types of measurement methods: ground pile method, clamp method, and ground pile and clamp method. .

1. Ground pile driving method: Ground pile method can be divided into two-line method, three-line method and four-line method.

(2) Two-wire method: This is the initial measurement method: one wire is connected to the ground body under test, and the other wire is connected to the auxiliary ground electrode.

The measurement result of this method is R = grounding resistance + ground pile resistance + lead and contact resistance, so the error is large and is generally not used now.

(2) Three-wire method: This is an improved version of the two-wire method, that is, two auxiliary ground electrodes are used. Calculated by the formula, when the middle auxiliary ground electrode is 0.62 times the total length, the error caused by the resistance of the ground pile can be basically eliminated. ;This method is still used today. However, this method still cannot eliminate the error in contact resistance caused by weathering and corrosion of the measured ground body.

(3) Four-line method: This is an improved method based on the three-line method. This method eliminates errors caused by auxiliary ground resistance, test leads, and contact resistance. Instrument selection: Currently, there are many instruments on the market that support this method, among which Kyoritsu 4105A-H ground resistance tester is the representative.

2. Clamp method: The clamp method is divided into single clamp method and double clamp method.

(1) Double clamp method: Utilizing the principle that a conductor in a changing magnetic field will generate an induced voltage, a clamp is used to pass a changing current, thereby generating an alternating magnetic field. This magnetic field causes the conductor within it to generate a certain induced voltage. Use another clamp to measure the induced current generated by this voltage, and finally use Ohm's law to calculate the loop circuit value. The applicable conditions are that a loop must be formed, and the resistance at the other end is negligible.

(2) Single clamp method: The essence of the single clamp method is to integrate the two pliers of the double clamp method into one. However, if mechanical damage occurs, the two adjacent pliers will inevitably interfere with each other, thus affecting the measurement accuracy.

3. Ground pile and clamp combination method: This method is also called the selected electrode method

The measurement principle of this method is the same as that of the four-wire method. Since when Ohm's law is used to calculate the results, the current value is measured by an external current clamp instead of the internal circuit like the four-wire method, thus greatly increasing the The applicable scope of the measurement. In particular, it solves the problem of multi-point grounding of transmission towers and metal connections underground.

4. Measurement of soil resistivity

With four ground stakes, the distance between the ground stakes can be set, which is a standard soil resistivity measurement method. The above are the standard requirements for ground resistance. I hope it can help you.