Determine the clearance of electrical products, must be based on the insulation of the low pressure system, and insulation coordination is based on instantaneous overvoltage is limited within the prescribed impulse withstand voltage, and the transient overvoltage generated by the electrical appliance or equipment in the system must also be lower than the impact of the power system voltage. Therefore:
1. The rated insulation voltage of the electrical appliances shall be the rated voltage of the power system
2. The rated impact voltage of the electrical appliances shall be greater than the rated impact voltage of the power system
3. The transient overvoltage generated by electrical appliances shall be less than the rated impact voltage of the power supply system.
Based on the above three principles, the rated impulse withstand voltage of the electrical appliances (priority) Uimp relatively voltage and power supply system and the installation of electrical appliances category has a lot to do, such as: relative to the voltage value, the greater the higher installation category I grade (signal level), [into Ⅱ (load), Ⅲ distribution level (level), Ⅳ (power level)], the greater the rated impulse withstand voltage. Relatively voltage of 220 v, for example, installing a category for Ⅲ, Uimp 4.0 KV, if installation category as Ⅳ Uimp is 6.0 KV. The Uimp of the normal molded case circuit breaker is 6.0KV pollution level 3 or 4, and its smallest electric clearance is 5.5 mm. The actual electrical clearance of the product is greater than 5.5 mm.
About the creepage distance, GB/T14048.1 general rules “low-voltage switchgear and control equipment” : electric equipment (products), the minimum creepage distance and the rated insulation voltage (or actual working voltage), the pollution levels of electrical products use place and? For example: the rated insulation voltage is 660 V (690), the pollution level of 3, product use insulation material group for Ⅲ a (cti or less 175 < 400, cti for insulation material tracking index , minimum creepage distance of 10 mm.
The creepage distance of the ordinary molded case circuit breaker is much higher than the specified value. Above all, if the electrical products of the clearance and creepage distance, meet the requirements of insulation coordination, won’t because foreign overvoltage or lines device itself operating overvoltage caused by medium electric breakdown of the equipment. Standard GB7251.1-1997 “low-voltage complete switch equipment and control equipment in the first part: type testing and partial type testing equipment” (jiao in it such as “92), the requirement of insulation coordination with GB/T14048.1 is exactly the same.
There are some complete set of electrical apparatus manufacturers to put forward the wiring of the circuit breaker wire, the distance between the phase and the phase should be greater than 12mm, some even suggest that the electric clearance of the circuit breaker should be greater than 20mm. This requirement is unreasonable and it is beyond the requirements of insulation. For large current specifications, in order to avoid in case of short circuit current generated when electric repulsion, or large current conductor heating, in order to increase the heat dissipation space, so appropriate and widen the space distance is also possible.
At this time, either the 12mm or 20mm can be solved by the complete set of electrical equipment manufacturer, or the electrical component factory can provide the curved terminal or connecting plate (piece). Generally, when the circuit breaker is out of the factory, it is provided with an alternate arc plate to prevent the short circuit when the arc is ejected. The circuit breaker of zero – flight arc has been escaped by ionizing molecules to prevent the short circuit current. If there is no arc plate, the insulation tape of the bare copper platoon can be wrapped, and the distance shall be 100mm.