This is exactly what the title of the question from the reader Ellina sounded like.
I continue to publish the questions that readers asked me. For new dear readers who came to my channel for the first time, I inform you that this is already 30 such publication. You can familiarize yourself with some other questions and answers to them by clicking on the links at the bottom of this article.
The text of the question itself was as follows:
There is a main switchboard of a multi-storey residential building. We carry out the documentation of the RD stage. The short-circuit current on the input busbars is 32 kA. According to the data of stage P, the input automatic devices were selected with a breaking capacity of 35 kA, and the automatic devices of the outgoing lines on the panels are all modular with a breaking capacity of 10 kA. Is such a solution permissible? Will the outgoing machines burn out at a short-circuit current on the buses or immediately behind them? Would it be more correct to make these machines with a breaking capacity of 25 kA or does it not matter?
I gave the following answer:
The question is too voluminous to answer specifically for each item, it is necessary to see both the main switchboard connection diagram itself, and parameters of the electrical installation of a multi-storey residential building with an indication of the section of the wiring and the allocated power for consumers. Otherwise, I want to note that there is a significant difference between the breaking capacity of the circuit breaker and its rated current.
So, under the rated current is meant such an electric current that the circuit breaker able to carry out in a continuous mode at a certain control ambient temperature air.
If we consider the issue of the breaking capacity of the circuit breaker, then this parameter indicates what maximum permissible current it can disconnect without harm to the contacts. Therefore, considering your example, with a calculated short-circuit current at the input to the main switchboard of 32 kA, the breaking capacity of the circuit breaker should be greater. This is what the design documentation provides - 35kA, so the selection principle is applied correctly.
If you are worried that a circuit breaker with a 35 kA breaking capacity is not enough, you can install a circuit breaker with a larger value. But the need for such a device can be determined by knowing the design parameters of the building's electrical installation. As for the outgoing lines, then for them the breaking capacity is also determined based on the cross-section of the conductors and the allocated power of the consumers. In general, 10 kA is the normal value for the average household consumer.
Thanks for reading to the end! And I would be grateful if you shared the article with friends on social networks. Special thanks for like and subscription - stay on the "ASUTPP" channel further!
P.S. Link to some past parts - Part 29, Part 28, Part 27, Part 26, Part 25.