The determination of the electrical clearance of electrical products must be based on the insulation coordination of the low-voltage system, which is established when the instantaneous overvoltage is limited to the specified impulse withstand voltage, and the instantaneous overvoltage generated by the electrical appliances or equipment in the system must also be lower than the impulse voltage specified by the power system. Therefore:

1. The rated insulation voltage of electrical appliances should be ≥ the rated voltage of the power supply system

2. The rated impulse withstand voltage of electrical appliances should be ≥ the rated impulse withstand voltage of the power supply system

3. The transient overvoltage generated by electrical appliances should be ≤ the rated impulse withstand voltage of the power supply system.

Based on the above three principles, the rated impulse withstand voltage (priority value) Uimp of electrical appliances is closely related to the relative ground voltage of the power supply system and the installation category of the appliances: the larger the relative ground voltage value, the higher the installation category [divided into I (signal level), II (load level), III (distribution level), IV (power level)], and the higher the rated impulse withstand voltage is required. For example, when the phase to ground voltage is 220V and the installation category is III, Uimp is 4.0KV. If the installation category is IV, Uimp is 6.0KV. The Uimp of a typical molded case circuit breaker is 6.0KV pollution level 3 or 4, and its minimum electrical clearance is 5.5mm. The actual electrical clearance of the product is greater than 5.5mm.


Regarding the creepage distance, GB/T14048.1 "General Principles for Low Voltage Switchgear and Control Equipment" stipulates that the minimum creepage distance of electrical appliances (products) is related to the rated insulation voltage (or actual working voltage), the pollution level of the place where the electrical products are used, and? For example, the rated insulation voltage is 660 (690) V, the pollution level is 3, the insulation material group used in the product is IIIa (175 ≤ cti ＜ 400, CTI is the leakage trace index of the insulation material), and the minimum creepage distance is 10mm.

The creepage distance of general molded case circuit breakers greatly exceeds the specified value. In summary, if the electrical clearance and creepage distance of the electrical product meet the insulation coordination requirements, it will not cause dielectric breakdown of the equipment due to external overvoltage or overvoltage caused by the operation of the line equipment itself. GB7251.1-1997 "Low voltage switchgear and control equipment - Part 1: Type testing and partial type testing complete equipment" (equivalent to IEC439-1-1992), the requirements for insulation coordination are exactly the same as GB/T14048.1.

Some complete electrical equipment manufacturers propose using copper bars for circuit breaker wiring, with a (air) distance between phases greater than 12mm, and some even propose that the electrical clearance of circuit breakers should be greater than 20mm. This requirement is unreasonable as it exceeds the requirements for insulation coordination. For high current specifications, in order to avoid the generation of electric repulsion in the event of short circuit currents or the heating of conductors during high currents, and to increase heat dissipation space, it is also possible to appropriately widen the spatial distance between phases.

At this point, whether it reaches 12mm or 20mm, it can be solved by the complete electrical equipment manufacturer themselves, or by requesting the electrical component factory to provide wiring terminals or connecting plates (pieces) with elbows to achieve this. When general circuit breakers leave the factory, they are equipped with arc barriers between the power supply terminals to prevent phase to phase short circuits caused by arc spraying. The circuit breaker with zero flashover is also equipped with this type of arc isolation plate to prevent ionized molecules from escaping during the interruption of short-circuit current. If there is no arc barrier, insulation tape can be wrapped around the bare copper busbar, and the distance should not be less than 100mm. Kailong Electric Appliances