An article to understand and calculate electrical clearance!

Creepage distance and electrical clearance

Creepage distance: the shortest space between two conductive components measured along the surface of an insulating material. Crawling means the shortest distance an ant has to travel to get from one charged body to another.

Electrical gap: The shortest space between two conductive components. You can think of it as an ant with wings, the shortest possible distance. Includes the shortest air distance between live parts and between live parts and ground parts; ​minimum air distance between live parts and vulnerable parts.

Creepage distance and electrical gap are two concepts, which must be satisfied at the same time when making judgments. They cannot be substituted with each other.

Creepage distance and electrical clearance play a very important role in the safety of electrical products. If the distance between live parts in electrical products and the housing is too small, it is easy to short circuit or leakage, so that the housing is charged, endangering personal safety; ​the distance between the live parts of different potentials is too small, also easy to cause inter-electrode short circuit or inter-electrode leakage, fire or insulation function failure, so that the insulation performance of electrical products decline.

In safety inspection, therefore, need to be familiar with electrical products internal structure on the basis of identifying the composition of the insulation structure, distinguish between live parts and a part of, determine their climb electric distance and the clearance between the path, in accordance with the requirements of the standard of measuring equipment quantitative path, determine the product conformed to the standard requirements in the eye.

The creepage distance is determined by the rated voltage of electrical equipment, the leak resistance of insulation material, the surface shape of insulation material and other factors, which determines the level of environmental tolerance of electrical equipment. The creepage distance depends on the effective value of the working voltage and the insulating material has a great influence on it.

The electrical gap is determined by the lightning impulse voltage, which determines the external insulation level of the electrical equipment. Its size depends on the peak of the operating voltage, which is greatly affected by the overvoltage level of the grid.

Note: Both conditions must be met at the same time, so by definition, the creepage distance cannot be less than the electrical gap at any time. Of course, for two charged bodies, it is impossible to design a creepage distance less than the electrical gap.

9fda94dff308b092876553fda768c862Requirements for minimum air clearance:

For METAL CLOSED SWITCHGEAR AND CONTROL EQUIPMENT WITH AIR AS THE SOLE insulating medium, THE minimum AIR gap between phases and relative ground shall meet the requirements of subordinates.

For metal closed switchgear and control equipment with composite insulation made of air and insulation board as insulation medium, the minimum air gap between live body and insulation board should meet the subordinate requirements:

For 3.6kV, 7.2kV and 12kV equipment should not be less than 30mm;

​for 24kV equipment it should not be less than 45mm;

​for 40.5kV equipment, it should not be less than 60mm.

Metal closed switchgear and control equipment using air or air-insulation material as insulation medium shall take into account the thickness, design field strength and aging of the insulation material and shall be subjected to condensation tests in accordance with DL/T 593, 6.2.8. The minimum air clearance may be appropriately smaller than the above distance provided it passes the condensation test.

Steps to determine creepage distance:

1. Determine the effective value or DC value of the working voltage;

2. Determine material groups;

3. Determine the pollution level;

4, determine the type of insulation (basic insulation, additional insulation, functional insulation, insulation).

Steps to determine electrical clearance:

1. Determine the peak value and effective value of the working voltage;

2. Determine the power supply voltage of the equipment and the type of power supply facilities;

3. Determine the transient overvoltage entering the equipment according to the type of overvoltage;

4. Determine the pollution level of the equipment (pollution level 2 for general equipment);

5. Determine the insulation type of electrical gap bonding (basic insulation, additional insulation, functional insulation, strengthened insulation).

Factors to be considered in determining creepage distance and electrical clearance:

Creepage distance: When determining the electrical gap and creepage distance, various service conditions and environmental factors such as rated voltage, pollution status, insulation material, surface shape, position direction, voltage duration and so on should be considered. The creepage distance is directly related to the operating voltage and insulation materials. At the same time, it should be noted that different operating environments will also have an impact, such as air pressure and pollution.

Electrical gap: The electrical gap of an electrical product must be determined according to the insulation fit of the system, which is established when the instantaneous overvoltage is limited to the specified shock tolerance voltage, and the instantaneous overvoltage generated by the electrical appliance or equipment in the system must also be lower than the shock voltage specified by the power supply system.

Therefore:

1. The rated insulation voltage of the electrical appliance shall be greater than or equal to the rated voltage of the power supply system.​

2. The rated impact tolerance voltage of the electrical appliance shall be greater than or equal to the rated impact tolerance voltage of the power supply system.​

3. The transient overvoltage generated by electrical appliances should be less than or equal to the rated impact tolerance voltage of the power supply system.

Determine creepage distance and electrical clearance Note: movable parts should be placed in the most unfavorable position; ​the creepage distance shall not be less than the electrical clearance value; ​withstand mechanical stress test.

Example: How to determine the electrical clearance and creepage distance of high voltage cabinet at high altitude? Based on what criteria?

Normal high voltage cabinets are designed with an altitude of no more than 1000m. The electrical gap at high altitude increases by 10% for every 1000m(on the basis of 1000m) (for example, the electrical gap at the altitude of 10KV switchgear at 3000 should be greater than or equal to 125+125*20% and equal to 150mm). The creepage distance of external insulation can be designed according to class II pollution conditions. That is, porcelain 18mm/KV, organic insulation for 20mm/KV(individual pollution heavier areas can be considered according to 25mm/KV).

Selection of measurement means for creepage distance and electrical clearance:

After determining the path of creepage distance and electrical gap, the creepage distance and electrical gap can be measured by vernier calipers, micrometers, rulers and other measuring tools. Whether the creepage distance and electrical gap are qualified can be judged according to the minimum distance of creepage distance and electrical gap specified in relevant product standards (pay attention to the size of working voltage).

Precautions when measuring creepage distance and electrical gap:

1, measurement equipment must not be charged;

2. When measuring the creepage distance, pay attention to the joint between two insulating materials (such as gluing).

​It is also deemed to be the above surface;

3. For a complete set of equipment containing extractable parts, the electrical clearance and creepage distance should be verified at the test position and the separation position, respectively;

4. When there are relatively moving parts in the complete equipment, the electrical clearance and creepage distance should be measured when the relatively moving parts are in the most unfavorable position.

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