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I salvaged this breaker unit from the scrape yard. Compared to every other breakers i have seen before, this is very different. It has 5 terminals.

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The labels are very clear but there is no information about the usage of its terminals. The terminals are labelled A, B and C. Its not even labelled clearly which side is load and which side is line.

The greatest mystery is the terminal C. What is it's purpose and how is it used?

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It's an interesting type of circuit breaker, a Hydraulic-Magnetic. See the catalog here.

Hydraulic-Magnetic circuit breaker

Anyway, the third pin is actually for a trip coil. Which is a coil when energized, will trip the circuit breaker. Ther are most often found as clip-on units for rail circuit breakers. But this one has it integrated.

shunt/trap enter image description here

Different catalog.

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See the diagram on the lower-left of page 19 of this document. Your breaker would appear to be a "dual rating" type, in which an overload on either of the B or C ("load") terminals will open the circuit to both.

Terminal A would be the "line" connection.

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