# What are the Effects of an Open Circuit, a Closed Circuit and a Short-Circuit

The article discusses the effects of different circuit conditions: open circuit, closed circuit, and short-circuit. It explains how each condition affects the flow of current and highlights the potential dangers and benefits associated with short-circuits.

Electric circuits can usually be turned on or off, although they can, at times, have a fault that changes them entirely. In electrical circles, the term ‘circuit’ may often be abbreviated to ‘cct’.

## What is an Open Circuit?

If the current path is not continuous between two points of potential difference, the circuit is referred to as an ‘open circuit’ (see Figure 1(a)). The circuit has a break in it, and the switch is said to be ‘open.’ No current can flow, and the lamp will not light. This condition occurs in a normal circuit when it is switched off but may also occur when a wire is broken.

## What is a Closed Circuit?

If the circuit is complete, as in Figure 1(b), current can flow, and the lamp lights up (assuming it is a lamp meant for 12 V and the battery is not flat). This condition occurs in a normal circuit when it is switched on and the switch is ‘closed’.

A closed circuit is an essential condition for current flow. For a continuous current flow, a continuous source of electrical energy must be provided.

In Figure 1, a car battery is shown as the source of energy, but other devices, such as generators, can be used when greater quantities of energy are needed. Static electric charges generated by friction are not usually able to carry out this function. The circuit in Figure 1(b) is known as a ‘simple circuit’.

Figure 1. Open-circuit, Closed Circuit, Short-Circuit Diagram

## What is a Short-Circuit?

This type of circuit is one that has a ‘fault’, a condition that is to be avoided. In Figure 1(c), the lamp is bypassed by a conductor connected directly from one terminal of the battery to the other. The circuit is now shorter than it was before, and the lamp is said to be ‘shorted’ out of the circuit; or the circuit is said to have a ‘dead short’ in it.

‘Shorted’ means that the current now has a shorter path. Because of this, the current flow through the circuit’s component or components is either limited or entirely prevented and so the circuit does not function as expected.

Dead short’ refers to an almost-zero-resistance short as compared with a partial short-circuit, where the resistance is low but not zero. A dead short is a dangerous situation where large amounts of current can flow. Usually, a protection device will operate to open the circuit and prevent further current flow.

In a short-circuit, no current flows through the lamp but excessive current flows directly from one terminal of the energy source to the other. The current flow is not limited by the load, as it is in Figure 1(b). In this particular case, unless the energy source (such as a battery) is protected against excessive current flow internally, the battery and the conductors can easily be damaged and may melt before the current stops. The battery may even explode.

Short-circuits are not always bad. Sometimes they are introduced deliberately to cause a particular effect. When a load is short-circuited, the high current generated by the low-resistance current path is what activates the circuit breaker or fuse. For example, if a wire were to break off from the circuitry in an appliance and touch the exterior metal frame, it would be dangerous for someone to touch it. However, a low-resistance circuit path would be created by connecting an earth wire to the frame; as soon as the wire touched the frame, a high current would be produced, activating the circuit breaker and protecting the circuit.

### Which of the following is generally a fault condition: an open circuit, a closed circuit, or a short circuit?

A short-circuit is normally a fault condition.

### When can the use of a short-circuit be beneficial?

To ensure the circuit protection device operates, for example, if a live wire touches the earthed frame of an appliance, high current flows, and the circuit protection operates to automatically disconnect the supply and thus provide personal protection.