Views: 13 Author: Site Editor Publish Time: 2022-10-27 Origin: zhihu
The main reason why the air circuit breaker is used as the main switch in the low-voltage primary distribution system is that the air circuit breaker has a large rated current and complete protection.
For the switch selected for the low-voltage main incoming circuit, the following points are mainly considered:
1. Rated Current
The rated current is closely related to the temperature rise.
The temperature rise referred to here is the stable temperature rise in the continuous operation state, that is, the long-term working system. The temperature rise will affect the contacts, conductive bars and insulation performance of the switch, and is a very important indicator.
In addition, we can calculate that the stable temperature rise of the contacts is only a fraction of the stable temperature rise of the circuit breaker current drawn and introduced into the copper bar.
Therefore, three values of temperature rise are specified in the national standard: 60K, 65K and 70K, respectively. Corresponds to bare copper, tinned copper, and silver-plated copper.
Including operating short-circuit breaking capacity Ics, ultimate short-circuit breaking capacity Icu and so on.
3. Kinetic And Thermal Stability
Corresponding to the short-circuit making capacity Icm and the short-time withstand current Icw, respectively. The ratio of the two is the crest factor n.
Clearances and creepage distances are particularly important for switchgear when circuit breakers are installed in switchgear.
But be careful: do not confuse creepage distances with clearances.
Creepage distance parameters depend on the properties of insulating materials, surface shape and installation conditions, and clearance parameters depend on the distance between conductive materials, which are related to air humidity, air temperature, altitude, etc. It is a comprehensive parameter.
When the capacity of the power transformer is small, the molded case circuit breaker can be used as the main switch.
For example, the capacity of the power transformer is 400kVA, let's calculate its specific parameters:
The rated current of the transformer: 400000/(1.732x400)=577A;
The short-circuit current of the transformer: 577/(1000x0.06)=9.6kA;
The peak value of the impulse short-circuit current of the transformer: 2.0X9.6=19.2kA.
It can be seen that we can completely use the molded case circuit breaker MCCB as the main switch.
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