Circuit Breakers

Fuses work on the principal of a large current melting the fuse wire. In some cases faults can occur in which the current may not be large enough to melt the fuse but enough to seriously harm the user of the electrical appliance. For protection against faults of this nature a circuit-breaker is used in place of a fuse.

Circuit-breakers offer the following advantages compared to fuses.

  • Circuit-breakers respond quicker than fuses.
  • Circuit-breakers are more reliable.
  • Circuit-breakers are more sensitive.
  • Unlike fuses which only operate once and need to be replaced a circuit-breaker can be reset.

There are two main types of circuit-breakers.

1. Miniature Circuit Breaker (MCB)
2. Residual Current Circuit Breaker (RCCB)

 

1. Miniature Circuit Breaker (MCB)

Many different designs and mechanisms are available for this type of circuit-breaker, below are a few examples.

Magnetic Type

This type of MCB consists of an electromagnet. If the current exceeds the rating of the circuit-breaker the pulling force of the magnet attracts an iron latch which breaks the electrical contacts.

Thermal Type

This type makes use of the heating effect of electricity on a bi-metallic strip. An increase in current causes the bi-metallic strip to bend which breaks contact made via a latch mechanism.

 



2. Residual Current Circuit Breaker (RCCB)

Residual current circuit breakers work by comparing the current entering the appliance via the live wire with the current leaving the appliance through the neutral wire.

The live wire and the neutral wire are wound around iron cores in opposite directions. When the appliance is working correctly all the electrical current entering the appliance via the live wire leaves the appliance through the neutral wire and the magnetic fields generated around the iron cores cancel out. In the event of a fault some of the electric current will flow through the earth wire or in the absence of earthing through the body of the user. This result’s in an imbalance between the current entering the appliance through the live wire and the current exiting through the neutral wire. This difference in electrical current is called the residual current.

This difference means the decreased electrical current in the neutral wire has a weaker magnetic field associated it to it than the live wire. The two magnetic fields do not balance out and the iron pivot is attracted to the live wire and the contacts are disconnected breaking the circuit.

Residual Current Circuit Breakers have the advantage of being highly sensitive and a very quick response time making them safe.