Home Electrical - Safety

Practical Electric Circuits Safety

Mains Supply - a.c and d.c a.c. (alternating current) - current changes direction (50Hz)  mains electricity is a.c  In Singapore the a.c is at 230 V  d.c. - (direct current) - current in one direction always  from dry cell, solar cell, car battery 

Mains Supply - a.c circuits 2 wires are always required  Live – at 230 V (dangerous)  Neutral – at 0 V L N

Switch must be on live wire When off appliance will be at 0 V When someone touches the live wire at the appliance, no electric shock (electrocution) No sweat!

L N

If switch is on neutral wire When off appliance will be at 230 V When someone touches the live wire at the appliance, he will get an electric shock (electrocution)

L N

Fuses and circuit breaker Fuses - melts when the current is too high  Made of metals with low melting points  Fuse rating 

 in

A (ampere)  maximum current 

Use P=VxI to calculate the current for suitable fuse to be used

Fuses

So is a fuse Must be on live wire (fuse comes first before the switch) fuse

L N

Fuses and circuit breaker Circuit breakers will break the circuit when the current is too high (30 A)  It can be reset easily 

Fuses and circuit breaker  ELCB

(Earth leakage circuit breaker)  will break the circuit when a fault causes a current flow to the earth wire  It can be reset easily

3-pin plug Earth

Greenishyellow Live Brown

Neutral Blue

Earthing 

N

Earth wire is connected to the outer metal casing of an appliance

L E Metal casing

Earthing When a fault occurs and the live wire touches the casing, current can flow to the earth wire  This current is big (no resistance) and so the fuse at the live wire will blow  Current cannot reach the faulty appliance so preventing an electrocution 

Earthing

Without earthing

Live wire breaks and touches metal casing Metal casing becomes “live” Person touching metal casing will get electric shock

Earthing

Live wire breaks and touches metal casing

Current flows to the earth wire connected to the metal casing The big current blows the fuse

Double insulation Earthing not needed for appliances with no outer metal casing  These appliance have double insulation  Even when there is a fault the current cannot reach the casing because it is an insulator 

Electrical Hazards Damp Conditions Damaged insulation Overheating

Electrical Hazards

Damp Conditions Water is a good conductor of electricity It will cause electric shock

Electrical Hazards Damaged insulation Insulating material is worn off Wires below are exposed –Cause electric shock when touched –Cause short circuit when live wire touches neutral wire

Electrical Hazards Overheating Too many plugs used on the same power point using multiadaptor Causes a big current in the cable/wires that produces heat Can lead to fire

Fuses and circuit breaker

When current is too big due to a fault, iron catch is attracted by electromagnet. Contact is broken current stops flowing

The End Thank you for paying attention