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EMC darkroom measurement conditions are determined by the measurement method

2021-02-12 58
It must be pointed out that taking the environment as the relevant condition for immunity measurement is an important feature of immunity measurement. Because if you ignore these correlations, regardless of the application working environment conditions of the device, and think that the device should be "independent" and should be suitable for insertion into any combination device (or system), it will result that all devices under test must be accepted Interference experiment of all items, and to reach the wrong conclusion of the highest severity level. This not only imposes excessively high and unreasonable strict restrictions on the equipment to be used, but also has to bear a great economic burden due to the need for a large number of experiments.

 
In addition, immunity measurement involves high-voltage signals. In addition to strict compliance with relevant safety regulations, it is also necessary to perform safety measurements on equipment after the immunity test.

 
For high-power electrical products such as AC stabilized power supplies, it is necessary to select anti-interference items characterized by high frequency and high energy imported from the mains, and to choose a higher severity level than other electrical and electronic products. .
Another important feature of immunity measurement is to make strict and clear regulations on the technical parameters of the experimental generator. In order to compare the immunity performance of the equipment, it is necessary to talk about an experimental device that can produce more consistent and reproducible, which is the interference simulation generator. Obviously, the output internal resistance, output waveform rules, open-circuit voltage amplitude and error of the generator must be specified; to ensure the consistency of the experimental results and good repeatability. Otherwise, due to the different source impedances of different devices under test, the impedance matching to the generator is different, and the generator cannot output the same waveform or amplitude under load. In fact, impedance mismatch is an effective means to suppress electromagnetic disturbance.

 
The electromagnetic disturbance measurement items of AC power supply to the outside (through the mains network) include: harmonic conduction interference measurement, high frequency conduction interference measurement.

 
Harmonic conduction interference measurement is to measure the power frequency current harmonics of the equipment's power supply input end; measure the maximum current harmonics below the 40th order, and measure the current harmonics of the neutral line for the three-phase power supply. In the AC power supply performance project, the relative harmonic content of the source current is used to evaluate this project.

 
Radio frequency radiation interference measurement is more complicated, involving measurement techniques such as measurement site, antenna, and measurement line connection. The measurement site is open and the background electromagnetic noise level is at least 6dB lower than the allowable limit value. This kind of rule is difficult to achieve, and the standard also recommends that it can be replaced with an electromagnetic shielding room (and an anechoic chamber, etc.). When measuring the radiation field strength, the device under test should be wired in strict accordance with the actual working method, and the power cord and signal line are not allowed to be crimped or contracted to reflect the authenticity.

 

The measurement depends on three factors: method, technology and equipment. The method is determined by both the measurement principle and the use method of the measurement equipment. Technology is all measurement methods used to obtain correct measurement results (higher accuracy), and equipment reflects all the above two factors for measurement services. Technical device. These must be standardized to ensure that the measurement is reproducible and authentic.



EMC darkroom measurement conditions are determined by the measurement method
 
The EMC measurement conditions are determined by the measurement method. The specific measurement methods are divided into the experimental bench method under laboratory conditions and the field method under actual use conditions. It is impossible to simulate all the interference phenomena that may be encountered in the field, especially the field method has insurmountable limitations. However, through standardized measurement, information about the EMC performance of the device under test can be obtained more comprehensively. For this reason, it is recommended internationally to first adopt the experimental bench method, unless it cannot be carried out in the laboratory, generally not the field method.

 
The main method of immunity measurement is to choose the appropriate severity level according to the electromagnetic environment conditions of the equipment, combined with the method adopted by the user on the equipment, measure according to the relevant measurement methods, and finally evaluate the measurement according to the qualified judgment conditions proposed by the product standard Whether the result is qualified. This is the main difference between immunity measurement and other measurements.

 
The electromagnetic disturbance source in the electromagnetic environment, the coupling method of the electromagnetic disturbance source to the equipment, the sensitivity of the equipment to electromagnetic disturbance, and the protection method of the user on the work site are directly related to the severity level. Even if the use environment determines the form of interference, the installation protection conditions determine the severity of interference. GB/T13926.4 specifies the electrical environmental conditions in the electromagnetic environment and the equipment corresponding to the severity level:

 
Level 1, a well-protected environment, such as a computer room;

 
Level 2, the protected environment, such as the control room or terminal room of factories and power plants;

 
Level 3, typical industrial environment, such as industrial process installations, power plants and relay rooms in open high-voltage substations;

 
Level 4, harsh industrial environments, such as power stations, industrial process equipment without special installation methods, outdoor areas, etc.

 
According to IEC801-5, the source of the surge is the power switching transient or the indirect lightning transient. The installation conditions and protective facilities of the equipment are classified as follows (applicable surge):

 
Category 0: A well-protected electrical environment with primary and secondary overvoltage protection, usually in a special room, and the surge voltage will not exceed 25V;

 
Category 1: Partially protected electrical environment with one-time overvoltage protection, and the surge voltage does not exceed 500V;

 
Category 2: Electrical environment where the power cord is separated from other lines, the cable is well isolated, and the surge voltage does not exceed 1kV;

 
Category 3: Electrical environment where power cables and signal cables are laid in parallel, with surge voltage not exceeding 2kV;

 
Type 4: The interconnection line is laid along the power cable like the outdoor, and the electronic circuit and the electrical circuit are used in the electrical environment of the cable, and the surge voltage does not exceed 4kV;

 
Category 5: Electrical environment where electronic devices are connected to telecommunication cables and overhead power lines in non-populated areas.

 
No surge measurement is made for category 0. Generally, the power supply products are in a category 1 or 2 electrical environment, and the severity level can be purchased as 1 or 2.

 
It must be pointed out that taking the environment as the relevant condition for immunity measurement is an important feature of immunity measurement. Because if you ignore these correlations, regardless of the application working environment conditions of the device, and think that the device should be "independent" and should be suitable for insertion into any combination device (or system), it will result that all devices under test must be accepted Interference experiment of all items, and to reach the wrong conclusion of the highest severity level. This not only imposes excessively high and unreasonable strict restrictions on the equipment to be used, but also has to bear a great economic burden due to the need for a large number of experiments.

 
In addition, immunity measurement involves high-voltage signals. In addition to strict compliance with relevant safety regulations, it is also necessary to perform safety measurements on equipment after the immunity test. ]

 
For high-power electrical products such as AC stabilized power supplies, it is necessary to select an immunity item characterized by high frequency and high energy imported from the mains, and to select a higher severity level than other electrical and electronic products.

 
Another important feature of immunity measurement is to make strict and clear regulations on the technical parameters of the experimental generator. In order to compare the immunity performance of the equipment, it is necessary to talk about an experimental device that can produce more consistent and reproducible, which is the interference simulation generator. Obviously, the output internal resistance, output waveform rules, open-circuit voltage amplitude and error of the generator must be specified; to ensure the consistency of the experimental results and good repeatability. Otherwise, due to the different source impedances of different devices under test, the impedance matching to the generator is different, and the generator cannot output the same waveform or amplitude under load. In fact, impedance mismatch is an effective means to suppress electromagnetic disturbance.

 
The electromagnetic disturbance measurement items of AC power supply to the outside (through the mains network) include: harmonic conduction interference measurement, high frequency conduction interference measurement.

 
Harmonic conduction interference measurement is to measure the power frequency current harmonics of the equipment's power supply input end; measure the maximum current harmonics below the 40th order, and measure the current harmonics of the neutral line for the three-phase power supply. In the AC power supply performance project, the relative harmonic content of the source current is used to evaluate this project.
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