I need a single-pole circuit breaker for a 200A, 48V DC battery bank powering a solar inverter. The batteries are LiFePO4 and capable of large short-circuit currents on the order of 10 to 15 kA. Large molded-case (MCCB) breakers are available with 200A rating and typically 20kA Icu (breaking) rating, but I am having difficulty finding a suitable enclosure. Can I use a DIN mounted 4-pole 50A interlocked breaker with the 4 poles in parallel? Even though this seems like a common case, I am unable to find any trustworthy answers.
The 4P breaker has an Icu rating of 6kA, is non-polarized, and is typically used in high voltage solar PV applications where the DC interrupting voltage needs to be up to 1000 VDC, so the breakers are arranged in series. This means that the breaker has 4x the I2R loss is series. Parallel power loss through 4 breakers would be 4*1/(4^2) = 25% of the series power loss, so that seems fine from a power dissipation (e.g. thermal) standpoint. The question likely comes down to the 6 kA Icu rating and how that works in parallel with interlocked breakers.
Best Answer
No, you shouldn't do this. The problem is that the breakers won't share current equally because they're not totally identical and their contact resistance might vary[1]. This means that one of them will trip much faster than the others, causing their total current-carrying capability to be much lower than expected.
Additionally, the four breakers won't open at the exact same time, which means that one of them will momentarily carry all of the short-circuit current and therefore exceed its rating.
[1] The contact resistance is somewhat random every time the breaker is operated (due to arcing and contaminants being "smeared" across the contact surface when operating) and it will also change with temperature, vibrations, oxidation or just on its own. Here's a bulletin by Schneider Electric about the topic: https://download.schneider-electric.com/files?p_enDocType=Data+Bulletin&p_File_Name=0600DB1901.pdf&p_Doc_Ref=0600DB1901