Open Circuit, Short Circuit and Overload fault

A fault is an unattractive or unsatisfactory feature, especially in a piece of work or in a person's character.
An electrical fault, however, can be as a result of  - 
                                                         Open Circuit Fault
                                                         Overload Fault 
                                                         Short Circuit Fault

OPEN CIRCUIT FAULT - 
An open circuit fault is a type of fault that occurs as a result of an opening in a circuit. It is a type of circuit in which there is no current flow which can occur as a result of wire breakage. 

In the open circuit condition, we say that maximum resistance is present between two contacts.
We can also have cases of the partial opening in the circuit, which might lead to contacts of a conducting surface not contacting/touching properly known as Partial contact. 

The result is that there will be some voltage drop as a result of this partial contact which will affect the output terminal voltage. The magnitude of this voltage drop depends on the severity of the partial contact.

For instance, the above diagram indicates partial contact (which can be experienced as a result of improper cable termination or bad contacts of a terminal block ) that result in 0.8V drop resulting 

in 19.2V output.

 OVERLOAD FAULT  
Overload faults are as a result of a device doing work more than its rated capacity. When this happens, the device/machine will be drawing current more than its rated capacity which will result to a higher temperature in the device/machine and over time might cause it to burn and get damaged.

Example  - 

- An electric motor that is designed to drive a maximum load of 100kg at a full load current (nameplate current) of say 5A but it's now subjected to drive a load of say 200kg (which means the motor will be drawing 10A (more than its rated capacity). That means it is doing more than its rated power. This can be termed of the Motor being overloaded. 

This 10A will result to the motor generating more heating than require (the higher the current the higher the temperature) therefore the motor will be getting hot which is not a good one because it will reduce its performance and live span and overtime might burn.  

- A wall socket that is rated 13A (which means that the maximum current is capable of delivering). If a load of greater amperage like a 3500W heater is connected to the socket such that 15.9A now flows from the socket the load. This is will act as an overload on the socket and will cause more temperature rise on it which can melt the socket.

- A 13A wall socket that is connected to like 6 appliances at a time such that each appliance is rated as thus: 320W, 200W, 1200W, 420W, 98W, and 580W totaling a power of 2,818W which result to a current of 12.81A will result to the socket been overloaded










Overload Protection

Overload is also a type of overcurrent. Overload protection typically operates on an inverse time curve where the tripping time becomes less as the current increases. 
Overload relays and “slow blow” fuses are commonly used to protect against overload

In other to protect our circuit against the risk and problem of overload the following device can be used
- Fuse
- Overload relay
- Solid state power switches etc

 SHORT CIRCUIT FAULT  

A short circuit is a type of current that result in over-current flowing in a circuit. In a short-circuited system the resistance of the circuit reduced drastically giving way for excessive current to flow in the circuit.
A short circuit is usually disastrous because of the amount of current flowing which might damage other components parts of the system and causes the temperature of the circuit to increase which can also lead to the formation of electrical arc flash.
Generally, as the current flow in circuit increases, the temperature increases as well (The higher the current the higher the temperature) because of this, if there is a short (live and neutral coming in contact together) there is usually a spark or arc.

 Causes of Short Circuit Fault  
1. Live and Neutral conductor coming in contact together
2. Positive and Negative terminal of a battery coming in contact together which can result in an explosion.
3. If wire insulation breakdown allowing the conduct to come in contact physically

In mains circuits, short circuits may occur between 

- Two phases, (Live to Live fault or L-L)
- Between a phase and neutral (Live to neutral or L-N) which is the most common type or 
- Between a phase and earth (ground) (Live to Ground or L-G). Such short circuits are likely to result in very high current and therefore quickly trigger an overcurrent protection device (e.g fuse or circuit breaker). However, it is possible for short circuits to arise between neutral and earth conductors, and between two conductors of the same phase. Such short circuits can be dangerous, particularly as they may not immediately result in a large current and are therefore less likely to be detected. Possible effects include unexpected energization of a circuit presumed to be isolated. To help reduce the negative effects of short circuits, power distribution transformers are deliberately designed to have a certain amount of leakage reactance. The leakage reactance (usually about 5 to 10% of the full load impedance) helps limit both the magnitude and rate of rising of the fault current.

 Effect of Short-Circuit  

1. Arc Flash - Due to excess temperature rise around the shorted conductor which can ionize the air around the vicinity thereby giving off light.
The heavy current is due to short-circuit causes excessive heating which may result in fire

Short circuit leading to a fire outbreak as a result of melted insulator

or explosion. Sometimes short-circuit takes the form of an arc and causes considerable damage to the system.

2. Insulation Breakdown - Due to the excessive temperature rise of the system when this type of fault occurs. It can result in insulation breakdown of the live and neutral conductor, an electric motor winding coil which can burn off the system.

3. Fire and explosion

Short-Circuit Protection
Due to the above detrimental effects of short-circuit fault, it is advisable to use a form of protection should this fault arise in the circuit.
The following are the device/component that can help to protect against short-circuit fault: 

Circuit Breaker

1. Fuse
2. Circuit-breaker
3. GCFI (Ground fault circuit interrupter) for earth fault protection
The fuse or circuit breaker will be selected according to the current in the circuit.

For example -  
If a lamp that is rated 80W is to be connected to a supply voltage of 220V. Such that the amount of current flow in it is 0.36A (I = P/V). 
So, a fuse of 0.5A can be selected so that if there is a short circuit fault between live and neutral conductor causing excess current of up to say 20A flowing the fuse will trip/cut and will help to protect the circuit against such an heavy current which might cause damage to the lamp and the wire.


NOTE: 
Some devices provide both overcurrent and overload protection. A Thermal-Magnetic Circuit Breaker has both thermal (overload) and magnetic (overcurrent) elements.
Also, the dual element fuse has both instantaneous and inverse time characteristics in the same fuse providing both overcurrent and overload protection.