Open circuits

An open circuit is the exact opposite of a short: the loop is broken, so no current can flow at all. An open switch, a snapped wire, a leg that popped out of the breadboard, or a blown fuse all create an open circuit.

In a simple DC series circuit with one open gap and no alternative paths, the gap can have nearly the full supply voltage while steady current is zero. Other circuit arrangements, leakage and capacitors can change the voltage across an open connection.

In practice, most "dead" projects are simply open somewhere innocent: a loose jumper, an un-pushed button, a battery clip not fully seated, or a wire that wriggled out of its hole. The first instinct should be "where is the loop broken?" not "what component failed?".