Created by: Glen Zhu | Updated Date: July 29th, 2024
What is the creepage distance?
The shortest path between two conductive components is measured on the insulated surface or between a conductive element and the equipment protective interface. That is, under different usage conditions, due to the polarization of the insulation material around the conductor, the insulation material exhibits a charged phenomenon. The radius of this charged area (when the conductor is circular, the charged area is annular) is known as creepage distance.
A leakage current path will form on the surface of the insulating material. If these leakage current paths constitute a conductive path, surface flashover or breakdown phenomena will occur. This change in insulating materials takes time and is caused by the prolonged application of operating voltage to the device; environmental pollution around the device can accelerate this change.
Therefore, when determining the creepage distance of terminals, one must consider the magnitude of the operating voltage, pollution level, and the anti-creepage characteristics of the insulation material used. The creepage distance should be selected based on the reference voltage, pollution level, and insulation material category. The reference voltage value is derived from the rated voltage of the power supply grid.
What is the clearance distance?
Clearance distance refers to the shortest distance measured between two conductive parts or between a conductive part and the equipment protective interface. That is, under the condition of ensuring stable and safe electrical performance, the shortest distance for insulation can be achieved through air.
The size of the electrical air gap is not related to aging phenomena. The electrical air gap can withstand very high overvoltages, but when the overvoltage value exceeds a certain critical value, this voltage quickly causes breakdown, so when confirming the size of the electrical air gap, it must be based on the maximum internal and external overvoltages that may occur in the equipment (based on impulse withstand voltage). The magnitude of overvoltage that occurs when using the same electrical equipment in different situations or using overvoltage protectors varies.
Creepage distance is measured along the surface of an insulating material in SPDs and MCBs, while clearance distance is measured through the air. This means that creepage distance is more affected by environmental factors such as surface contamination and humidity, whereas clearance distance is influenced mainly by air quality and pressure.
Creepage distance primarily prevents leakage current or arcing along the surface of the insulator in SPDs and MCBs, ensuring long-term safe operation of electrical equipment. Clearance distance, on the other hand, prevents electrical breakdown between conductive parts, avoiding arcing and short circuits.
3) Different Design Considerations
When designing creepage distance for SPDs and MCBs, one must consider the choice of insulating materials, humidity, and pollution levels in the operating environment. In contrast, designing clearance distance requires consideration of voltage levels, air quality, and air pressure.
1) Ensuring Safety
Creepage and clearance distances are critical parameters in the design of SPDs and MCBs. Their correct design prevents electrical arcing, short circuits, and equipment failure, thereby ensuring the safety of electrical systems.
2) Enhancing Reliability
Proper design of creepage and clearance distances enhances the reliability of SPDs and MCBs, reduces maintenance requirements, and extends the equipment’s service life.
3) Compliance with Standards and Regulations
The design of creepage and clearance distances in SPDs and MCBs must adhere to relevant international standards and regulations (such as IEC and UL) to ensure that equipment meets safety and performance requirements, thereby avoiding legal and quality issues.
The crawling distance is not only related to the insulation type (basic insulation, double insulation, reinforced insulation), but also closely related to the pollution level of the microenvironment (pollution level 1, pollution level 2, pollution level 3), the insulation performance of materials (i.e. CTI value), and the operating voltage.
Insulation materials are damaged by the combined effects of pollution, leakage current, and flashover discharge, gradually forming conductive channels on their surfaces, known as “tracking.” Materials are divided into four groups based on their Comparative Tracking Index (CTI) values as follows: Group I with CTI ≥ 600; Group II with 400 ≤ CTI < 600; Group IIIa with 175 ≤ CTI < 400; and Group IIIb with 100 ≤ CTI < 175.
The CTI value mentioned above refers to the value obtained from testing a sample prepared according to GB/T4207 using Solution A (the highest voltage that the material surface can withstand without forming tracking after exposure to 50 drops of electrolyte). For glass, ceramics, or other inorganic insulation materials that do not produce tracking marks, the creepage distance does not need to be greater than their relevant clearance distance. This is one of the reasons why such insulation materials are chosen for power transmission equipment.
When designing creepage distances, it is advisable to place some transverse ribs and grooves on solid insulating surfaces as much as possible to block leakage current paths and delay the process of tracking formation.
In addition to being related to the type of insulation, clearance distance is also related to factors such as micro-environment pollution level, altitude, electric field conditions, operating frequency voltage, etc.
1) Pollution Level
Pollution is caused by foreign substances, including solids, liquids, and gases. The result is the bridging of small clearance distances, reducing the surface resistance of insulating materials, making them unable to withstand the maximum transient overvoltage that may occur in the circuit.
Therefore, using a shell, including sealing can effectively reduce pollution. According to GB479311, pollution levels are divided into: Pollution Level 1, Pollution Level 2, and Pollution Level 3. For measurement control and laboratory equipment use because of better environmental conditions with only non-conductive pollution occasionally short-term conductivity due to condensation if devices and components adopt fully enclosed shells generally adopting Pollution Level 2.
2) Altitude
Due to the different atmospheric pressure (density), the impact voltage values that the same clearance distance can withstand at different altitudes are also different. If equipment is specified to operate at an altitude higher than 2000m, then its clearance distance should be multiplied by the coefficient obtained from Table 1. This coefficient does not apply to creepage distance, but the creepage distance should always be at least equal to the specified value of clearance distance.
Table 1 Electrical gap multiplication factor within 5000m altitude | |
Rated working altitude (m) | Doubling factor |
≤2000 | 1.00 |
2001 ~ 3000 | 1.14 |
3001 ~ 4000 | 1.29 |
4001 ~ 5000 | 1.48 |
The shape and configuration of conductive components will affect the uniformity of the electric field intensity, so they are divided into two types: uniform electric field and non-uniform electric field. The clearance distance required for a non-uniform electric field is larger than that for a uniform electric field. It is difficult to achieve the conditions of a uniform electric field in measurement, control, and laboratory equipment power circuits. Table 2 lists the values of non-uniform electric fields.
| Table 2 clearance distance and creepage distance of power supply circuit in the grid | ||||||||||
| Phase-to-phase or line-to-line AC effective value or DC value (V) | clearance distance value (see Note 1) | creepage distance value | ||||||||
| Pollution Level 1 | Pollution Level 2 | Pollution Level 3 | ||||||||
| CTI≥100mm | CTI≥100mm | CTI≥600mm | CTI≥100mm | CTI≥600mm | CTI≥100mm | |||||
| Printed circuit board | All material groups | Printed circuit board | Material Group I | Material Group II | Material Group III | Material Group I | Material Group II | Material Group III | ||
| >50 ~ ≤100 | 0.1 | 0.1 | 0.25 | 0.16 | 0.71 | 1.0 | 1.4 | 1.8 | 2.0 | 2.2 |
| >100 ~ ≤150 | 0.5 | 0.5 | 0.5 | 0.5 | 0.8 | 1.1 | 1.6 | 2.0 | 2.2 | 2.5 |
| >150 ~ ≤300 | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 | 2.1 | 3.0 | 3.8 | 4.1 | 4.7 |
| >300 ~ ≤600 | 3.0 | 3.0 | 3.0 | 3.0 | 3.0 | 4.3 | 6.0 | 7.5 | 8.3 | 9.4 |
| Note 1: The minimum clearance distance values for different pollution levels are: Pollution level 2: 0.2mm; Pollution level 3: 0.8mm; Note 2: The specified values | ||||||||||
4) Working frequency voltage
Table 2 lists the clearance distances applicable to power supply circuits of the power frequency grid.
5) Insulation type
The values listed in Table 2 are applicable to basic insulation and supplementary insulation (i.e. auxiliary insulation). For reinforced insulation and double insulation, the values are twice those of basic insulation.
Measuring Creepage Distance
1) Tools and Equipment
Calipers or Rulers: Precise measuring tools such as calipers or rulers are used to measure the creepage distance along the surface of the insulating material.
Microscope: For small and intricate components, a microscope may be used to ensure accurate measurement of the creepage distance.
2) Procedure
Identify the Path: Determine the shortest path along the surface of the insulating material between two conductive parts. This path should follow the contour of the insulator surface.
Measure the Distance: Using a caliper or ruler, measure the distance along the identified path. Ensure that the measuring tool is aligned with the surface to get an accurate reading.
Record and Verify: Record the measured creepage distance and verify it against the required standards and specifications for the specific application in SPDs and MCBs.
3) Standards and Specifications
Refer to relevant standards such as IEC 60664-1, which provides guidelines on the required creepage distances based on voltage levels, material properties, and environmental conditions.
Measuring Clearance Distance
1) Tools and Equipment
Calipers or Rulers: Precision measuring tools such as calipers or rulers are used to measure the clearance distance through the air between conductive parts.
Laser Distance Meters: For larger distances, laser distance meters can provide more accurate measurements.
2) Procedure
Identify the Points: Determine the two closest conductive parts between which the clearance distance needs to be measured.
Measure the Distance: Using a caliper, ruler, or laser distance meter, measure the shortest distance through the air between the identified points.
Record and Verify: Record the measured clearance distance and verify it against the required standards and specifications for the specific application in SPDs and MCBs.
3) Standards and Specifications
Refer to relevant standards such as IEC 60950-1, which outlines the minimum clearance distances required based on voltage levels and other influencing factors like altitude.
Importance in Electrical Product Safety Design
1) Preventing Electrical Arcing and Short Circuits
Properly designed creepage and clearance distances are essential for preventing electrical arcing and short circuits, which can lead to equipment failure, fire hazards, and safety risks.
2) Ensuring Compliance with Safety Standards
Electrical products must comply with various international safety standards such as IEC, UL, and other local regulations. These standards specify the minimum creepage and clearance distances required to ensure product safety and reliability.
3) Enhancing Product Reliability and Longevity
Adequate creepage and clearance distances help in maintaining the reliability and longevity of electrical products by reducing the risk of electrical breakdown and ensuring consistent performance under different environmental conditions.
1) Material Selection and Design
In SPDs, the choice of insulating materials with high dielectric strength and pollution resistance is critical for achieving the required creepage distances. The design must also account for environmental factors such as humidity and pollution levels to maintain the effectiveness of the creepage distance.
2) Voltage Considerations
Surge protective device is designed to protect electrical systems from voltage surges. Ensuring proper creepage and clearance distances helps in preventing arcing and breakdown during high voltage events, thereby protecting the system.
3) Compliance with Standards
SPDs must adhere to standards like IEC 61643, which specify the minimum creepage and clearance distances based on the device’s operating voltage and environmental conditions. Ensuring compliance with these standards is crucial for product certification and safety.
1）Design for Safety
MCBs are designed to protect electrical circuits from overcurrent and short circuits. Adequate creepage and clearance distances are vital for ensuring that the breaker operates safely under fault conditions without causing electrical arcing or breakdown.
2) Environmental Considerations
MCBs are often used in various environmental conditions. Designers must consider factors such as temperature, humidity, and pollution when determining the required creepage and clearance distances to ensure reliable operation.
3) Standards Compliance
MCBs must meet standards such as IEC 60898, which outline the required creepage and clearance distances based on the breaker’s rated voltage and environmental conditions. Compliance with these standards ensures the safety and reliability of the MCBs.
LSP’s reliable surge protection devices (SPDs) are designed to meet the protection needs of installations against lightning and surges. Contact our Experts!
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