Short circuit vs overload: What’s the Difference?

Various faults occur during the operation of power grids and machines. Short circuit and overload are the most dangerous faults in the power system. They change the circuit structure, cause changes in power distribution, will bring energy loss, damage the stability of the power system, and affect the normal operation of electrical equipment. Although these two phenomena have similarities, they are different. Below, we will learn more about short circuit and overload, and compare their differences.

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The difference between short circuit and overload

Below you can see the difference between short circuit and overload.

Definition

The main difference between short circuit and overload is the definition. Overload refers to the situation where the current of the electric load exceeds its rated value for a long time. A short circuit refers to an accidental or intentional conductive path between two or more conductive components, forcing the potential difference between these conductive components to be equal to or close to zero.

Other definitions of short circuit:

  • An unwanted low-resistance connection between two points in the circuit.
  • The circuit is abnormal.
  • This happens when a circuit path is created between the positive and negative poles of a battery, power supply, or circuit.
  • What happens when the hot wire and the neutral wire touch each other.
  • An accidental connection between two points in a circuit, such as a tree branch or an animal connecting the gap between two conductors.

Other definitions of overloading:

Electrical overload is a kind of overcurrent, the current through the wire or circuit exceeds its capacity, which leads to overheating, and there is a risk of fire or damage to the equipment.

Voltage level

For any finite current flowing through a short circuit, the voltage across the short circuit is zero. On the circuit diagram, a short circuit is represented by an ideal wire with zero resistance. In the case of overload, the voltage can be very low, but not zero.

Current level

The maximum value of the short-circuit current is directly related to the size and capacity of the power supply, and has nothing to do with the circuit load current protected by the protection device. The greater the capacity of the power supply, the greater the short-circuit current. The overload current is directly related to the capacity of the load. This is why the short-circuit current level is much higher than the overload current. The short circuit is a multiple of the rated current. The overload is close to the rated current.

Danger

Short circuit is more dangerous than overload. Because the current level is higher. In high voltage applications, short circuits are extremely dangerous. The energy transfer is fast in a short circuit, and energy transfer is slow in an overload.

Reason

Through careful system and equipment design, as well as proper installation and maintenance, the power system should avoid short circuits and overloads as much as possible. However, even if these precautions are taken, short circuits and overloads still occur.

The reasons for the short circuit are:

  • There are pests and rodents in the equipment.
  • Loose connection.
  • Voltage surge.
  • Deterioration of insulation.
  • Accumulation of moisture, dust, concrete sap and contaminants.
  • Intrusion of metal or conductive objects, such as fish belts, tools, jackhammers or loaders.

Some reasons for overloading are:

1) Excessive consumer load.

2)Malfunctioning electrical appliances.

3)Poor wiring and grounding.

4)Misuse.

Protective device

Short circuits and overloads must be quickly eliminated from the power system. This is the job of the circuit protection device. In order to do this, the protection device must have the ability to interrupt the maximum current that can flow at the equipment location.

Fuses and circuit breakers can protect the system from overload and short circuit. Thermal overload relays can only prevent overload. Magnetic circuit breakers can only protect short circuits.

As a result of

When a short circuit occurs in the power system, several things happen-they are all bad:

  • In the short-circuit position, arcing and burning may occur.
  • The short-circuit current flows from different power sources to the short-circuit location.
  • All components that carry short-circuit current are affected by thermal and mechanical stress.
  • The system voltage drop is proportional to the magnitude of the short-circuit current.

The consequences of overloading are:

  • Residual current caused by line isolation problems increases power consumption.
  • Direct contact with damaged wires may cause personal accidents.
  • Continuous overload may cause a short circuit.

Calculate

In industrial and commercial power systems, the calculation of short-circuit current is essential for selecting appropriate rated protection devices and equipment. Nowadays, the power system carries a larger power block and has higher requirements for safety and reliability. There are many parameters to consider when calculating short circuits. Therefore, the short-circuit calculation is not easy and must be performed carefully.

The response of the protective device

The protection device reacts very quickly to a short circuit. However, the opening time of the overload protection device will be delayed. Fast tripping is essential in short-circuit protection.

An overload will cause a thermal trip, and a short circuit will cause an electromagnetic trip of the protection device.

It should be turned off only when the heat generated by the overload exceeds a predetermined limit. Therefore, the trip time of a smaller overload is correspondingly longer than that of a larger overload. The short-circuit current should

be disconnected immediately.

Source

A short-circuit current can come from four different sources:

  • Dynamo
  • Synchronous motor
  • Induction motor
  • Power Systems

All of these can feed short-circuit current into the short-circuit.

The source of the overload may be any electrical load.

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