earth wire , what's it's function?

[lebesset]

with modern 30mA trips replacing fuses , what function does the earth wire fulfill?

[rojash]

Here's an example. The live wire on your cooker works loose and touches the metal casing. If the casing is earthed, the breaker will trip (almost) immediately. If it's not, the breaker will trip when the casing becomes earthed (i.e. when you touch it and current starts to flow to earth through you).

Personally, I prefer the first option.

[brachiopod]

The 'trips' are RCD's - Residual Current Devices.

A residual current is what occurs when there is a leakage to earth, the device detects it and 'trips'.

As rojash rightly says, this can be through you or through an earthing wire.

[RAYMAN]

as every electrician knows is that the earth wire is a life saver.

[lebesset]

so if I am using a power tool , touch the live wire by accident so that it earths through me , the trip won't save me?

[rojash]

The trip may well save your life, but it may not, and even if it does, you may still experience a nasty shock. The trip will reduce the chance that the shock is fatal. Think of the trip as a safety belt. It's still better not to crash the car at all!