Circuit Breaker
In most homes today you will not find a 'fuse box'. When you have a modern wiring system iinstalled you will have a circuit breaker box installed. Instead of 'changing the fuse' when the lights go out you have to reset the switch for the circuit instead.
Circuit-breakers offer the following advantages compared to fuses.
- they respond more rapidly to current surges than fuses do.
- they are more reliable.
- they are more sensitive.
- Unlike fuses which only operate once and need to be replaced a circuit-breaker can be reset at the flick of a switch.
Miniature Circuit Breaker
These circuit breakers allow examiners to link questions to other topics you have studied - they are therefore excellent material for examination questions!
Magnetic MCB
This uses an electromagnet. If the current exceeds the rating of the circuit-breaker the pulling force of the electro-magnet attracts an iron latch which breaks the electrical contacts.
Remeber to revise electromagnets - make sure you KNOW why the core should be made of a soft magnetic material!
The 'springy piece of metal' acts as a catch to reset the switch. The force of attraction between the iron rocker and the electromagnet has to be large engough to overcome the spring catch. Once the iron rocker has been moved down on the left hand side of the pivot the side on the right is pushed past the catch.
When this happens the electrical contacts are broken and the electromagnet loses its magnetic field. The catch prevents the rocker from moving back and re-making the circuit before someone manually resets it.
This is a link to a Flash animation to illustrate the idea.
Thermal MCB
This uses a bi-metallic strip. The bi-metallic strip is made up of two metals bound together. When heated or cooled the one metal expands (or contracts) more than the other - therefore the strip 'bends'. An increase in current causes an increase in the temperature of the bi-metallic strip. This causes it to bend away from an electrical contact if the current is too large. As soon as the current stops flowing it cools rapidly and is then ready to be reset.
The above vid-clip shows a bimetallics strip operating as a heat sensitive switch - same principle - just used in reverse!
This is a link to a Flash animation to illustrate the idea.
Residual Current Circuit Breakers (RCCB)
AC 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 each other out.
When there is 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 a decreased electrical current in the neutral wire compared to that in the live wire. That results in a weaker magnetic field around the neutral wire than the live wire has. The two magnetic fields do not cancel each other out and the iron pivot is attracted to the live wire breaking the circuit.
Residual Current Circuit Breakers have the advantage of being highly sensitive and have a very quick response time.
The diagram below is of a DC arrangement
The power tool, coil A and coil B are all in the same series circuit. Coils A and B are wound in opposite directions around the soft iron core. Therefore when there is no current leakage from either side of the power tool the magnetic fields produced by the coils cancel each other out.
If the wire was accidentally cut through at point X this balance would be disturbed. There would therefore be a net magnetic field. The steel armature would be attracted to the core. It is fixed at the hinge points - therefore it will 'bend' at a different angle at the joint as the armature moves to the left. This would result in a break in the electrical contacts.