1. Introduction
Creepage distance and clearance are among the safety inspection items for lighting fixtures. Their safety design and testing are crucial to the entire product design process, directly affecting other test items such as dielectric strength. Therefore, they must be correctly understood, considered, and strictly controlled. This paper describes the determination and measurement of creepage distance and clearance for a printed circuit board sample used in CNAS proficiency testing. The process and results are summarized and analyzed, the correct path selection method is identified, and uncertainty assessment and analysis of the experimental data are performed.
2. Basic Concepts and Sample Introduction
There are subtle differences in the definitions of creepage distance and clearance in Chinese standards. The main reason for this is that each standard takes into account the differences in the use of their respective products [3]. However, if we ignore the limiting conditions in their respective standards and combine them with the illustrations, they can be defined as follows:
Creepage distance refers to the shortest distance between two conductive parts along the surface of a solid insulating material; that is, under different usage conditions, due to the polarization of the insulating material around the conductor, the insulating material becomes charged, and the radius of this charged area (when the conductor is circular, the charged area is annular) is the creepage distance, as shown in Figure 1. Clearance refers to the shortest distance between two adjacent conductive parts in the air; that is, the shortest distance through which insulation can be achieved while ensuring stable electrical performance and safety [2]. As shown in Figure 2.
Figure 1. Simplified diagram of creepage distance definition
Figure 2. Simplified diagram of electrical clearance definition
It can be seen that creepage distance and clearance are actually two related parameters, both of which are related to electrical insulation. In particular, when selecting industrial control products such as relays and switches, it is necessary to comply with relevant standards and set appropriate creepage distance and clearance according to the actual usage environment requirements (air pressure, pollution, etc.) in order to ensure the safety of people's lives and property and the stability of electrical performance [4].
The sample used for this creepage distance and clearance capability verification was a printed circuit board, as shown in Figure 3. In the figure, the sample is divided into two sides, A and B; the white area represents the cutout part on the circuit board; T1, T2, and T3 represent the trajectory lines on the front side of the sample, and R1 represents the trajectory line on the back side of the sample.
Figure 3 Schematic diagram of printed circuit board
3. Route selection and measurement
Limitations on creepage distance and clearance are intended to prevent creepage and breakdown accidents that may occur between two conductors through the surface or space of insulating materials. When determining creepage distance and clearance, various usage conditions and environmental factors such as rated voltage, contamination status, insulating material, surface shape, position and orientation, and duration of voltage exposure should be comprehensively considered [5]. Since the contamination level of this sample is set to 3, X = 1.5 mm. According to the CTL590 resolution, angles less than 80° are shorted by X-segment bridging principle. In addition, a small magnifying glass is used for viewing, and vernier calipers, micrometers and tool microscopes (magnification of 30x) are used for measurement.
3.1 Measurement of creepage distance between T1 and T2
5. Conclusion
Analysis of the test results shows that the choice of path plays a very important role in the accuracy of the measurement results. In the daily measurement of creepage distance and clearance, different testers often have different conclusions for the same product. In addition to the influence of path selection on the measurement results, the measurement method is crucial to the measurement results [7]. In most cases, calipers, micrometers, plug gauges and reading microscopes can be used to complete the measurement. In addition, the uncertainty analysis of the corresponding test can effectively reduce the error and obtain satisfactory measurement calculation results [9]. By participating in proficiency testing or comparison tests, the laboratory can find problems, continuously improve, improve technical capabilities, and improve the overall level of electronic and electrical safety performance testing.
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