Fuses and Circuit breakers → used to prevent excessive current.
Earthed metal case or double insulation → prevent contact with live wires.
(b) is with safety features and (a) no safety features → both are AC, not DC.
Fuse
-
safety device added to electric circuit → prevent excessive current flow.
-
consists of strip of fusible wire that will melt when current flowing through it exceeds certain value.
-
limits amount of current, value of which depends on its rating.
when fuse limit is reached → electric circuit is broken → until fuse is replaced with a new one. Fuses are found in the plugs of electrical appliances.
Circuit Breaker
- safety device → prevent excessive current flow.
- circuit breaker not added to plug of electric appliances → unlike fuse.
- circuit breakers are part of the main electrical wiring to an apartment or room
Important
need to state that circuit breakers work on the principle of magnetism, which we will dive into later
circuit breakers → also have current ratings.
if current exceeds certain rating → circuit breaker will trip → causing open circuit.
they are like on-off switches. when circuit breaker trips → faulty appliance causing trip has to be identified, removed, from circuit then circuit breaker may be turned on again.
Earth Metal Casing
- metal casing that encloses the electrical parts of an appliance, keeping user safe in event of electrical fault.
- live current passing through earth metal casing flows to the ground → electricity takes path of lowest resistance
- since earth wire has almost no resistance → large current flows → causes circuit breaker to trip or fuse to blow.
- prevents further current flow.
Without Earthing:
- due to mechanical vibration → live wires can become loose → come into contact with the metal casing
- metal casing → would be at high electrical potential
- closed path may be formed when person touches metal casing. Large current flows through the person → the person gets electric shock.
With Earthing
- if live wire comes into contact with metal casing → current is diverted to the ground. If this current is larger and exceeds the rated value of the fuse → fuse will melt and circuit will be opened.
- Person touching washing machine → is protected from electric shock → because earthing provides lower resistance path for current to flow. If fuse is blown, no current will flow at all.
Double Insulation
- not all appliances use earth metal casing
- earth metal casing → can increase cost of appliances.
- some appliances use double insulation as a safety feature to keep user in event of fault.
- these appliances → enclosed in plastic casing → which is not earthed
- 2 layers of insulation to enclose all exposed metalwork and “live” parts
3 pin plugs, or fuse plugs → commonly used in SG.
earth wires do not have electric current running through them most of the time
- if there is sudden electric surge → that causes electric current to flow through them → it can still be dangerous to touch them.
- hence when unsure about whether or not wire is live → always treat wire as if current is flowing through it.
switches and safety features → such as fuses and circuit breakers → need connected to the live wire → n order to break or complete a circuit
If switch is connected to neutral wire → appliance becomes “live” and can cause electric shock when person touches it even when the switch is open
- live wire may become loose and come into contact with the casing
- appliance’s casing would be at high electric potential
- closed path may form when person touches the metal casing, even when switch is open.
on other hand → if switch is connected to live wire → situation above can be prevented. appliance is disconnected from the mains.
- switch disconnects live wire from metal casing
- no closed loop path is formed, keeping user safe
similarly, having safety features → fuse / circuit breaker → connected to live wire → instead of neutral wire → allow these devices to cut off supply from the mains.
Summary
| safety features | how it prevents electrical hazard |
|---|---|
| fuses | used in socket plugs to prevent excessive current |
| circuit breaker | used in distribution box to prevent excessive current |
| earthed metal casing | prevent live wire from causing metal casing to be at high electrical potential |
| double insulation | prevent contact with live wire |
| earth wire | to earth metal casing of appliance |