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AC vs DC Breakers

Solar 101/

Most of us are familiar with AC breakers found in the typical residential service panel. Every once in a while we need to work on the wiring in the house, so we make a trip out to the side of the house to manually open (trip) the breaker to kill the power to that circuit, allowing us to safely perform our work. Though AC and DC breakers appear similar in form and function, they are very different internally.

The function of a breaker (AC or DC) is to detect when too much current (amps) is flowing through the circuit, then disconnect the circuit from the main power source to protect the wiring from overheating. During the act of disconnecting, the internal contacts separate. As they pull apart from each other, an arc will form as the current jumps across the air gap. (You have experienced this on a smaller scale with a static electric shock.) If this arc continues to jump the air gap, the current will continue to flow through the circuit, defeating the purpose of the breaker. This arc must be extinguished. The AC and DC breakers extinguish this arc differently. This design difference is why AC and DC breakers are not interchangeable.

AC Breakers

In North America, the grid alternates at 60 Hz, or 60 cycles per second; hence the name “Alternating Current”. The voltage alternates between +V and –V, 60 times a second. That means there is a point at which the voltage is 0v, 60 times a second. It is at this 0v point that the AC breaker will “break” the connection, extinguish the arc, and protect the wiring from too much current.

DC Breakers

In contrast, a DC circuit does NOT alternate. It stays at a constant voltage. Since there is no 0v point, the AC breaker design will NOT work in a DC circuit. The DC breaker uses a magnet to attract the arc, pulling it from the air gap, and extinguishes it. The AC breaker is NOT equiped with a magnet, and cannot extinguish a DC arc.

Moral of the strory, use AC-rated breakers for AC circuits, and DC-rated breakers for DC circuits.

Only breakers that are labeled as DC-rated should be used for DC applications. NEVER attempt to use an AC-rated breaker in a DC circuit! Why? It will fail to extinguish the arc, the wires will overheat and cause a fire. If a breaker is DC rated, it will state so. NEVER assume an AC breaker is DC rated just because the amps and volts match what you need. Conversely, don’t use a DC rated breaker in an AC circuit. NOTE: It is OK to use a breaker that has a dual AC and DC rating (stated on the manufacture’s label). It will state clearly on the breaker if it is rated for both. WARNING: Most dual rated breakers have their DC voltage rating DIFFERENT from the AC voltage rating, for the same amps. (such as 60A 150VDC, 120VAC) Be sure to double-check these values.

Ryan @ Midnite Solar does a good job of describing their DC breakers in this video:

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