Electrical Safety - Short Circuit Protection

Electrical Safety – Short Circuit Protection

A short-circuit condition means a circuit allows the current to flow through an unintended path with very low electrical impedance. It is a direct contact between two points of different electric potential.

The short circuit protection system is broken down into the following systems −

Alternating Current System

  • Phase to Ground contact
  • Phase to Neutral contact
  • Phase to Phase contact
  • Contact between windings of an electrical machine in a phase

Direct Current System

  • Pole to Ground contact
  • Contact between two poles

There can be numerous causes resulting in the above type of contacts including damage to the insulation of conductors, loose, broken or stripped wires and cables, and deposition of conducting materials such as dust, moisture, etc.

Major Causes of Short Circuit

A sudden surge of current equals to hundred times of working current flows through the circuit. This leads to the damage of electrical equipment. The following two phenomena are responsible for the devastating effects of short circuits −

Thermal Phenomenon

This phenomenon refers to the energy released into the electrical circuit when short-circuiting current flows through the circuit. This thermal effect results in the causes of a short circuit −

  • Melting of the conductor contacts
  • Damage to insulation
  • Generation of electrical arcs
  • Destruction of the thermal elements in the bimetallic relay

Electro-dynamic Phenomenon

This phenomenon refers to the production of intensive mechanical stress when the current crosses and results in the following conditions −

  • Breakage of the conductors
  • Repulsion of contacts inside the contactors
  • Distortion of conductors in windings

Short Circuit Protection Devices

To protect the devices and people from short circuit hazards, protecting devices are used in electrical circuits. These devices can detect the faults and trip the circuit immediately before the surge current reaches to the maximum.

There are two popular protecting devices used frequently in every electrical circuit.

Fuse

Fuse is operated once in the circuit and then must be replaced after the trip occurs. It is helpful for phase by phase (single pole) protection. It offers a high breaking capacity at low volume, which limits electro-dynamic stress.

Following images show different types of fuse −

Different Fuses

Circuit Breaker

Circuit breakers can be reset either manually or automatically. It automathy breaks the circuit within a short cutoff time and separates the load from the power supply that protects the circuit from any damage. The magnetic triggers of CB open the poles. CBs limit both the thermal and thermodynamic effects. It works faster than a fuse. For example, Molded Case Circuit Breaker (MCCB), Molded Case Switch (MCS), Air/Oil/SF6/Vacuum Circuit Breaker (ACB/OCB/SCB/VCB).

The following images show different types of circuit breakers:

Different Circuit Breakers

Characteristics of Short Circuit Protection Devices

We will now learn the different characteristics of short circuit protection devices. The characteristics are shown below −

Breaking Capacity

The maximum value of the estimated short circuit current that can enable the device to break the circuit at a given voltage is called the breaking capacity.

Closing Capacity

The maximum short circuit current that can enable the device to reach its rated voltage at specific condition is called the closing capacity. It is the rational multiple of breaking capacity.