Accurately Understanding Creepage Distance and Clearance
Abstract Gives an overview of correctly understanding creepage distance and clearance,It also presents the right measure to fulfill the safety requiremnet. Gives as overvirw of correctly measure fulfill safety.
1) Correct understanding of creepage distance and clearance
In the national mandatory standards for electrical products, the terms "creepage distance" and "electrical clearance" are involved, conceptually speaking, the creepage distance is "the shortest distance along the surface of the insulating material between two conductive parts, or between a conductive part and the easily accessible surface of the appliance". It exists at the junction of two parallel insulating materials, and it may exist between solid or gas insulation. The clearance is "the shortest distance between two conductive parts or one conductive part and the surface of the appliance that is easily accessible".
Between different live parts or between the charged parts and the earth, when their air gap is small to a certain extent, under the action of the electric field, the air medium will be broken, the insulation will fail or temporarily fail, so the air between the two conductive parts should maintain a safe distance so that it will not break down, which is the electrical clearance.
The creepage distance is actually a boundary plane, and an important feature of this boundary is that it spans two materials with distinct electrical rated strengths (the voltage value of each unit distance), so the distance between the two conductors should be determined by the insulation material with the weakest rated electrical strength. Because in general, the rated electrical strength of the air is the weakest, the creepage distance between the two conductors should be determined according to the insulating properties of the air.
Air is one of the most common, reliable, cheap electrical insulation medium, usually, for 1mm of air gap, below 1200V rms voltage can maintain insulation performance, but the voltage rises to more than 2900V rms value is no longer an insulating material. The insulation properties of the air are proportional to the size of the air gap. Different from air insulation, solid insulation material is an irreversible insulating medium, and unfavorable factors such as electric field strength, heat, humidity, etc. cause continuous aging of insulation properties. For example, prolonged heat generation will cause a decrease in insulation performance.
Assuming that the connection is a plane, when the voltage difference continues to increase, eventually the air will be broken down and then turned on, producing a spark (arc), the energy dissipated in this arc, if the arc is generated on the surface of the solid insulation, the heat of the arc will burn the surface of the solid insulation, and finally lead to a carbonization path on the solid insulation material.
The way to prevent the formation of carbonization paths is to keep the arc away from the solid insulating surface, and we have two methods that can be used, one is to shorten the clearance value until it is less than the creepage distance, so that the arc spark will occur on the path of the electrical gap. Another way is to greatly increase the value of the interstitial gap, so that the rated electrical strength of the air is close to the solid insulation performance, (we can add a baffle on the solid insulation), the path of the interstitial gap increases part of the creepage distance, so that the path of the interstitial gap is extended, and the extended path increases the electrical strength performance, thereby achieving the purpose of improving the overall insulation performance.
The housing of an appliance is not only used to prevent dangerous contact with live parts, but another function of the housing is to maintain the "amount" of air used as insulation around the live parts. That is, creepage distance and clearance. Isolation of charged bodies with sufficient space is an effective means of electrical insulation, for example, the overhead means of high-voltage cables is to prevent conductors of different polarity or different potential differences from coming into contact. Curved ridged ceramic insulators are an effective tool for providing sufficient creepage distance.
2) Application of creepage distance and clearance in the safety design of electrical products
The same means are also widely used in electrical products, the isolation space in the air is the "electrical gap", and the creepage distance is used to reduce (prevent) leakage scars or arc discharge. (Obviously, the lower the voltage, the smaller the space between the conductors, and the creepage distance and clearance values can be reduced accordingly.) ）
The most basic purpose of conductor space isolation is to ensure the normal operation of the appliance. Conductors with different polarity or potential differences in contact with each other will cause the appliance to work abnormally. However, if the factor of preventing the conductor short circuit is simply considered, then the distance requirement is very small, and the functional insulation only needs to meet a few fractions of the standard requirements. In fact, the main purpose of the insulation system is to prevent electric shock, and the functional requirements of anti-electric shock are greater than the requirements of functional insulation.
Another factor considering the distance of the charged conductor is to compensate for the reduction of the electrical insulation gap property after long-term use. For example, the accumulation of dust or other particles will cause leakage marks or even electrical conduction on the printed circuit board. Solid particles, dust and water in the atmosphere can bridge small electrical gaps, and in humid environments, non-conductive pollution can also be converted into conductive pollution. When the insulation surface is polluted to a certain extent, the leakage current between the live parts is large, it will form a flicker, and the released energy will cause damage to the insulation surface, and under the action of a long time, the insulation performance will gradually deteriorate, forming a live channel (leakage trace).
We can take the method of coating or adding housing to prevent dust accumulation to meet the requirements of the relevant electrical product standards.
The requirements for the clearance also take into account the factor of instantaneous overvoltage of the power supply, and there are many sources of instantaneous overvoltage: lightning overvoltage, lightning induction, operating overvoltage (switch operation, line failure), functional increase voltage inside the appliance, grid fluctuations, etc. The main content involves the following: ● Insulation grade: functional, basic, attached, reinforced. ● The rated working voltage of the circuit. ● Insulation type, class I, class II. ● Pollution level (design to prevent dust, pollution).
However, the requirements of the clearance do not consider the instantaneous voltage of the power supply in extreme cases, but consider the rated voltage plus the voltage drift value, if there is a surge voltage in the use of the appliance, then the designer should increase the rated voltage value of the appliance.
Nina She