Circuit breaker for 160KW motor
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Dear experts, we have a disagreement regarding equipment selection; several engineers are unable to reach an agreement, so we’ve come here to seek advice from you all. We hope for your guidance and suggestions! ! Thank you in advance! ! The setup is as follows: pumps driven by 3 AC380V/160KW motors, with soft start functionality. The rated current is I=304A. One engineer suggested using a circuit breaker with a rating of I=630A, while another said 400A would be sufficient; there was disagreement on this matter. I think 400A is enough, but it might be a bit risky and not entirely safe. 500A would likely work, but Schneider does not offer circuit breakers with a 500A rating; the highest rating available is 630A. Therefore, I support the design proposed by the first engineer. 400A will definitely be cheaper. I’d like to ask the experts here: is a 400A circuit breaker sufficient? Is there a risk of tripping upon startup? I don’t have any experience in this area; please help to assess the situation! ! Thank you! !
You mentioned only 400 and 630; those are just the frame currents. The rated current includes a value of 500, so that can be chosen. There’s also a setting for the current limit, which can be set lower. It seems like those of you who keep arguing don’t really understand the specifications of circuit breakers at all. If cost is a concern, 400 is also sufficient, as long as your motor current is actual. It seems that you haven’t even considered which type of trip device to choose. For a rated current of 630A, it appears that there are no TMD options available; only electronic trip devices are available. For a rated current of 500A, TMD options are available, and they are cheaper. If the release mechanisms are the same, the price difference between circuit breakers with the same current rating in the same enclosure is very small; the difference is probably due to the amount of usage.
Your system uses a circuit breaker in combination with a soft starter; according to Schneider’s instructions for soft starters, only a Type 1 combination is allowed. Per GB14048, this combination has been verified through tests conducted by the supplier. Conversely, you can consult the coordination table in the soft starter manual to select the corresponding circuit breaker. The starting current during soft start is lower than that at full voltage; therefore, it is not necessary to select a circuit breaker based on full-voltage starting requirements. Moreover, soft start offers more comprehensive protection features, reducing the need for considerations related to protection coordination across various circuits. According to the current instructions for soft starters of joint-venture brands, if type 2 coordination is desired, fast fuses must be used; moreover, for soft starters that employ circuit breakers as protection for the main circuit, thyristors are not covered under warranty by some brands. You can look it up according to the actual situation for the rest.
Thank you to user 2 upstairs for their valuable guidance and insights! ! My mind still thinks of it in terms of straightening it out; I forgot that the current increases gradually during a soft start. It’s really as the expert on the second floor said – for a soft start, there’s no need to use a switch with a very high inrush current rating. Thank you so much! !
400A*capacity discount coefficient》304? If that’s possible, then it’s fine; the tripping curve of the release mechanism in the molded case circuit breaker just needs to match the electrical starting current curve.