Open Circuits

Open circuits and short circuits can often cause more confusion and difficulty in the analysis of a system than standard series or parallel configurations.

What is an open circuit?

An open circuit is two isolated terminals not connected by an element of any kind, as shown in Fig. 1. Since a path for conduction does not exist, the current associated with an open circuit must always be zero. The voltage across the open circuit, however, can be any value, as determined by the system it is connected to.
In summary, therefore,
an open circuit can have a potential difference (voltage) across its terminals, but the current is always zero amperes.
Fig. 1: Defining an open circuit.
In Fig. 1(b), an open circuit exists between terminals a and b. The voltage across the open-circuit terminals is the supply voltage, but the current is zero due to the absence of a complete circuit.
Example of an open circuit
Fig. 2: Example of an open circuit.
In a practical example provided in Fig. 2, the excessive current demanded by the circuit caused a fuse to fail, creating an open circuit that reduced the current to zero amperes. However, it is important to note that the full applied voltage is now across the open circuit, so you must be careful when changing the fuse. If there is a main breaker ahead of the fuse, throw it first to remove the possibility of getting a shock. This situation clearly reveals the benefit of circuit breakers: You can reset the breaker without having to get near the hot wires.

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