Open circuits and short circuits can often cause more confusion and difficulty
in the analysis of a system than standard series or parallel configurations.
An open circuit is two isolated terminals not connected by an element
of any kind, as shown in Fig. no.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,
**Fig. no.1: **Defining an open circuit.
In Fig. no.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.
**Fig.no.2: **Example of an open circuit.
In a practical example provided in Fig. no.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.
#### What is short circuit?

A short circuit is a very low resistance, direct connection between
two terminals of a network, as shown in Fig. no.3. The current through
the short circuit can be any value, as determined by the system it is
connected to, but the voltage across the short circuit is always zero
volts because the resistance of the short circuit is assumed to be essentially
zero ohms and $V = IR = I(0 Ω) = 0 V$.
**Fig.no.3: **Defining a short circuit.
In summary, therefore,
In Fig. no.3(a), the current through the 2 Ω resistor is 5 A. If a short
circuit should develop across the 2 Ω resistor, the total resistance of the
parallel combination of the 2 Ω resistor and the short (of essentially zero
ohms) will be
$$2 Ω || 0 Ω = {(2 Ω)(0 Ω) \over
2 Ω + 0 Ω} = 0 Ω$$
as indicated in Fig. no.3(b), and the current will rise to very high levels,
as determined by Ohm's law:
$$I ={E \over R}= {10V \over 0Ω} = \infty A$$
The effect of the 2 Ω resistor has effectively been "shorted out" by
the low-resistance connection. The maximum current is now limited
only by the circuit breaker or fuse in series with the source.
**Fig.no.4: **Demonstrating the effect of a short circuit on current levels.
For the layperson, the terminology short circuit or open circuit is
usually associated with dire situations such as power loss, smoke, or
fire. However, in network analysis, both can play an integral role in
determining specific parameters of a system. Most often, however, if a
short-circuit condition is to be established, it is accomplished with a
jumper-a lead of negligible resistance to be connected between the
points of interest. Establishing an open circuit just requires making sure
that the terminals of interest are isolated from each other.