Electromagnetic compatibility standards for information technology equipment

Since the 1980s, my country has accelerated the pace of formulating national standards for electromagnetic compatibility. In June 1988, the national standard "GB9254 1988" equivalent to "CISPR22:1985" of the International Special Committee on Radio Interference was issued. The name of the standard is "Radio Interference Limits and Measurement Methods for Information Technology Equipment", and it was implemented on November 1, 1998. In 1993, the standard became a mandatory national standard. Afterwards, considering the applicability and timeliness of the standard, GB9254 1988 was revised according to the third edition of "CISPR22:1997", and the implementation date was December 1, 1999. This standard replaced GB9254 1988 from the date of implementation, but the content related to telecommunications ports was implemented half a year later. The standard is still implemented as a mandatory standard. At the same time, GB/T17618-1998 "Limits and Measurement Methods for Immunity of Information Technology Equipment" was also released. This standard is equivalent to "CISPR24:1997" and is implemented as a recommended standard. The release of these two national standards for electromagnetic compatibility products will provide a technical basis for the formulation of domestic information technology equipment product standards.

1. Definition of Information Technology Equipment

Information technology equipment is one of the most important terms in GB9254, which defines the scope of application of the standard. Section 3.1 of GB9254 gives a clear definition:
Information technology equipment is any equipment that satisfies both conditions (a) and (b);

(a) Capable of entering, storing, displaying, retrieving, transmitting, exchanging or controlling (or a combination of several functions) data and telecommunication messages; the equipment may be equipped with one or more terminal ports normally used for information transmission;

(b). Rated voltage not exceeding 600V.

According to the product's use environment, information technology equipment is divided into Class A and Class B. They must meet Class A and Class B electromagnetic compatibility standards respectively. Class B standards are stricter than Class A standards. Generally speaking, information technology equipment used in the following places belongs to Class B;

1. Residential areas, such as courtyard houses, apartments, etc.;
2. Commercial areas, such as shops, supermarkets, etc.;
3. Business districts, such as office buildings, banks, etc.;
4. Public entertainment venues, such as cinemas, restaurants, discos, etc.;
5. Outdoor places, such as gas stations, parking lots and sports centers;
6. Light industrial areas, such as workshops, laboratories, etc.

Products that meet Class A electromagnetic compatibility standards usually make the following statements in the instruction manual or product label:

"This product meets the electromagnetic compatibility level A. In the living environment, this product may cause radio interference. In this case, the user may need to take practical measures to deal with the interference."

2. Limits

2.1 Conducted disturbance limits at power terminals

Electromagnetic disturbance can be conducted and emitted through the power terminals of the equipment, causing pollution to the power grid. Therefore, the electromagnetic compatibility standard limits the conducted disturbance emission of the power terminals, which are the conducted emission limits of the power terminals. Table 2 gives the conducted disturbance limits of Class A and Class B power terminals.

Table 2 Conducted disturbance limits for power terminals

2.2 Common-mode conducted disturbance limits for telecommunication ports

The common mode current on the cable will produce strong electromagnetic radiation. Most devices can pass the relevant standards smoothly when not connected to the telecommunication cable, but they no longer meet the requirements of the standard after connecting to the cable. This is because the common mode current in the cable produces common mode radiation. Therefore, this item puts forward restrictions on the common mode conducted emission of telecommunication ports. Table 3 gives the common mode conducted disturbance limits for Class A and Class B telecommunication ports.

2.3 Radiated emission disturbance limits

When information equipment is working, it will radiate electromagnetic waves into space, which will cause interference to other equipment, especially wireless receiving equipment. Therefore, this item sets a limit on the intensity of electromagnetic waves radiated by the equipment. Table 4 gives the limits of Class A and Class B radiated interference.

Table 4: Radiated disturbance limits

2.4 Immunity limits of chassis

The chassis must have a certain degree of resistance to various external interferences. According to the interferences existing in the actual environment, they are divided into three categories as shown in Table 5.

Table 5: Chassis Immunity Test Limits

2.5 Signal ports and immunity limits

The interference on the signal port comes from the current induced on the cable by electromagnetic waves in space. According to the electromagnetic interference phenomenon in the actual environment, there are three types as shown in Table 6.

Table 6: Signal Port and Telecom Port Immunity Limits

2.6 DC power port immunity limit

The immunity requirements for the DC power port are shown in Table 7.

Table 7: Immunity Limits for Power Input Ports

2.7 AC power port immunity limit

The immunity requirements for the AC power port are shown in Table 8.

Table 8: AC power port immunity requirements

3. Measurement method

3.1 Power terminal conducted disturbance measurement method

3.1.1 Measurement equipment

Three types of equipment are required to perform power terminal conducted emission measurements:

• Interference measurement equipment: Equipment used to quantitatively measure interference intensity, which can be an EMI measurement receiver or a spectrum analyzer. The frequency range should cover 150KHz~30MHz, and it should have peak, quasi-peak and average detection functions, meeting the requirements of GB/T6113.1.

• Line Impedance Stabilization Network (LISN): Since the intensity of the conducted emission at the power terminal is related to the impedance of the power grid, in order to make the measurement unique, it must be measured under specific impedance conditions. LISN provides such an environment. The LISN used in the GB9254 standard is 50Ω/50μH, which must meet the requirements of Chapter 8 of GB/T6113.1.

• Grounding plate: The equipment under test should be placed on a grounded metal plate for testing. The metal plate should be 0.5 meters larger than the frame of the equipment under test and have a minimum size of 2m×2m;

3.1.2 Measurement method

The power terminal conducted interference measurement mainly measures the interference voltage emitted by the device under test along the power line to the power grid. When doing this test, it is important to adjust the working state of the device under test and find the working state corresponding to the maximum interference as the test result.

3.2 Telecom port common mode disturbance measurement method

3.2.1 Measurement equipment

Four types of equipment are required to perform power terminal conducted emission measurements:

• Interference measurement equipment: equipment used to quantitatively measure the intensity of interference, with the same requirements as the power port conducted emission measurement.

• Impedance network: Common mode terminal impedance is 150Ω±20Ω, phase angle 0°±20°; isolation is 35-55dB at 150KHz 1.5MHz, increases linearly with frequency, and is greater than 55dB at 15-30MHz; longitudinal conversion loss is 80-55dB±3dB for Category 3 cable at 150KHz 1.5MHz, greater than 50-25dB±3dB at 1.5-30MHz, and decreases linearly with frequency;

• Capacitive voltage probe: impedance > 1MΩ, parallel capacitance < 5pF;

• Current probe: insertion impedance ≤ 1Ω;

• The ground plane is 0.5m larger than the frame of the device under test and has a minimum size of 2m×2m.

3.2.1 Measurement method

When the telecommunications port is configured with more than two balanced pairs or unshielded cables, the voltage method and current method are used;

When the telecommunications port is configured with balanced cable or coaxial (shielded) cable, the current method is used.

For details, see Appendix C of GB9254.

3.3 Radiated disturbance measurement method

3.3.1 Measurement equipment and site

• For elliptical open field or semi-anechoic chamber, the difference between the horizontal and vertical field attenuation measured values ​​and the theoretical values ​​of the ideal field shall not exceed ±4dB;

• Measurement receiver: 30 1000MHz, meeting GB/T6113.1;

• Antenna: Logarithmic dipole antenna or biconical antenna, meeting GB/T6113.1;

• Grounding plate: in accordance with GB/T6113.1;

3.3.2 Measurement method

Place the antenna in a horizontal polarization direction at an appropriate height, and the turntable at an appropriate angle, and perform an initial measurement in the range of 30 1000MHz using peak detection.

Rotate the turntable between 0° and 360°, and find the maximum interference level (quasi-peak value) of the device under test at the frequency point with greater interference during the initial measurement;

Raise and lower the antenna within a height range of 1 to 4 m to find the electrical disturbance level at that frequency point;

Change the polarization direction of the antenna to vertical polarization and repeat the above measurement.

3.4 Electrostatic discharge immunity test

Discharge times and locations: 200 discharges are conducted on the equipment under test, of which 150 are conducted at three test points for direct contact discharges, and another test point is selected at the center of the front edge of the horizontal coupling plate for 50 indirect discharges. At least 10 single air discharges are conducted at each test point of holes and gaps.

Discharge polarity: Perform positive and negative electrostatic discharge.

3.5 Radio frequency electromagnetic field radiation immunity test

Frequency range: 80 1000MHz;

Test signal: Use 1KHz sine wave to modulate the test signal by 80% amplitude;

3.6 Electrical fast transient burst immunity test

When the equipment has multiple identical ports, only one of them is tested; multi-core cables are tested as single cables; data cable ports that are not longer than 3m are not subject to this test.

3.7 Voltage sag, short interruption and voltage variation immunity test

The test voltage sag is specified as two levels: cycle > 95% and 30%, corresponding to two different test cycles: 0.5 and 25.

3.8 Radio frequency field induced conducted disturbance immunity test

_{Only the 3 V rms} (unmodulated) test voltage is applied to the EUT .

3.9 Surge (impact) immunity test

Apply 0.5KV (line-to-ground) disturbance voltage to the DC power port (only suitable for ports directly connected to outdoor cables);

The AC power port applies 1KV (line-to-line) and 2KV (line-to-ground) interference voltages; the signal and telecommunication ports directly connected to the outdoors apply 1.5KV and 4KV interference voltages.

The above immunity test methods can be found in GB/T17618 1998 and GB/T17626.