DC circuit breakers

DC circuit breakers protect the DC circuits in a solar installation: panel strings, battery lines and regulator outputs. They are not the same devices as AC circuit breakers, even though they may look similar.

Why do DC circuit breakers have their polarity marked?

This is because the arc that forms when the contacts open must be actively extinguished. In DC mode, the current never passes through zero, so the device incorporates permanent magnets that direct the arc towards the extinguishing chambers.

That magnetic pull it only works one way. Fitting the circuit breaker the wrong way round (with the ‘in’ and ‘out’ markings reversed) means the arc is not extinguished: the contacts become worn and the device may eventually fail to trip. This is a fault that goes unnoticed until an actual fault occurs.

What DC voltage do I need?

The circuit’s maximum voltage, which in a string of panels is the open-circuit voltage (Voc) corrected for the minimum site temperature, not the operating voltage. On cold, clear days, the Voc rises above the catalogue value.

Enhance your protection with string fuses and check the AC circuit breakers for the rest of the table.