DC Circuits

Open and Short Circuits

A circuit is said to be open when a break exists in a complete conducting pathway. Although an open occurs when a switch is used to deenergize a circuit, an open may also develop accidentally. To restore a circuit to proper operation, the open must be located, its cause determined, and repairs made.

Sometimes an open can be located visually by a close inspection of the circuit components. Defective components, such as burned out resistors, can usually be discovered by this method. Others, such as a break in wire covered by insulation or the melted element of an enclosed fuse, are not visible to the eye. Under such conditions, the understanding of the effect an open has on circuit conditions enables a technician to make use of test equipment to locate the open component.

In the figure below, the series circuit consists of two resistors and a fuse. Notice the effects on circuit conditions when the fuse opens.

Current ceases to flow; therefore, there is no longer a voltage drop across the resistors. Each end of the open conducting path becomes an extension of the voltage source terminals and the voltage felt across the open is equal to the applied voltage (V_A).

An open circuit has infinite resistance. Infinity represents a quantity so large it cannot be measured. The symbol for infinity is ∞.

A short circuit is an accidental path of low resistance which passes an abnormally high amount of current. A short circuit exists whenever the resistance of a circuit or the resistance of a part of a circuit drops in value to almost zero ohms. A short often occurs as a result of improper wiring or broken insulation.

In the figure below, a short is caused by improper wiring. Note the effect on current flow. Since the resistor has in effect been replaced with a piece of wire, practically all the current flows through the short and very little current flows through the resistor. Electrons flow through the short (a path of almost zero resistance) and the remainder of the circuit by passing through the 10-ohm resistor and the voltage source. The amount of current flow increases greatly because its resistive path has decreased from 10,010 ohms to 10 ohms. Due to the excessive current flow the 10-ohm resistor becomes heated. As it attempts to dissipate this heat, the resistor will probably be destroyed.

The following discussion deals with the effects on a circuit when an open or a short occurs.

The major difference between an open in a parallel circuit and an open in a series circuit is that in the parallel circuit the open would not necessarily disable the circuit. If the open condition occurs in a series portion of the circuit, there will be no current because there is no complete path for current flow. If, on the other hand, the open occurs in a parallel path, some current will still flow in the circuit. The parallel branch where the open occurs will be effectively disabled, total resistance of the circuit will increase, and total current will decrease.

To clarify these points, the figure shown below illustrates a series parallel circuit. First the effect of an open in the series portion of this circuit will be examined. The figure below (view A) shows the normal circuit, R_t = 40 ohms and I_t = 3 amps. In the figure below (view B) an open is shown in the series portion of the circuit, there is no complete path for current and the resistance of the circuit is considered to be infinite.

In the figure above (view C) an open is shown in the parallel branch of R₃. There is no path for current through R₃. In the circuit, current flows through R₁ and R₂ only. Since there is only one path for current flow, R₁ and R₂ are effectively in series.

Under these conditions R_t = 120Ω and I_t = 1 A. As you can see, when an open occurs in a parallel branch, total circuit resistance increases and total circuit current decreases.

A short circuit in a parallel network has an effect similar to a short in a series circuit. In general, the short will cause an increase in current and the possibility of component damage regardless of the type of circuit involved. To illustrate this point, the figure below shows a series-parallel network in which shorts are developed. In the figure below (view A) the normal circuit is shown. R_t = 40 Ω and I_t = 3 A.

In the figure above (view B), R₁ has shorted. R₁ now has zero ohms of resistance. The total of the resistance of the circuit is now equal to the resistance of the parallel network of R₂ and R₃, or 20 ohms. Circuit current has increased to 6 amps. All of this current goes through the parallel network (R₂, R₃) and this increase in current would most likely damage the components.

In the figure above (view C), R₃ has shorted. With R₃ shorted there is a short circuit in parallel with R₂. The short circuit routes the current around R₂, effectively removing R₂ from the circuit. Total circuit resistance is now equal to the resistance of R₁, or 20 ohms.

As you know, R₂ and R₃ form a parallel network. Resistance of the network can be calculated as follows:

The total circuit current with R₃ shorted is 6 amps. All of this current flows through R₁ and would most likely damage R₁. Notice that even though only one portion of the parallel network was shorted, the entire paralleled network was disabled.

Opens and shorts alike, if occurring in a circuit, result in an overall change in the equivalent resistance. This can cause undesirable effects in other parts of the circuit due to the corresponding change in the total current flow. A short usually causes components to fail in a circuit which is not properly fused or otherwise protected. The failure may take the form of a burned-out resistor, damaged source, or a fire in the circuit components and wiring.

Fuses and other circuit protection devices are installed in equipment circuits to prevent damage caused by increases in current. These circuit protection devices are designed to open if current increases to a predetermined value. Circuit protection devices are connected in series with the circuit or portion of the circuit that the device is protecting. When the circuit protection device opens, current flow ceases in the circuit.