Why does a fuse blow or circuit breaker trip if a power tool for example had its live mains wire come in contact with earth ?

 

Are you kidding? Why does a fuse flow when the power wire connects to ground?

Because too much current flows.

If I am missing your point, please explain further.

 

In the US, at your electrical distribution panel, power neutral and earth ground are tied together. Shorting to ground is like shorting to neutral.

 

"Don't cross the streams"

curry, was there no arcing or sparking during the contact?

and

Why are your power-tool supply wires not insulated?

 

Why does a fuse blow or circuit breaker trip if a power tool for example had its live mains wire come in contact with earth ?

Actually, they rarely will blow, and that is why bonding is required. Earth returns are typcially very high impedance, unable to pass enough current to trip breakers/blow fuses. A GFI on the other hand will recognize leakage and trip

 

I dont know about the US but in the UK all earthing regulations are designed to limmit the potential on the earth conductor to 50V, whilst ensuring that disconnection devices, fuses or breakers, operate within a specified time.
In practive this means that the earth loop impedance needs to be low enough to ensure that a PSC (Prostecive Short circuit Current) will be enough to open the circuit protective device within 0.5 or 5 seconds depending on the type of circuit.

The utility providers have to ensure that the supply is below a fixed impeedance, I cant remenber the figure but it is sub 3 Ohms, and the circuit designer, within the building, is responcible for the rest.

The general premis is this .....
Any supply must be protected from fault currents to avoid dammage to conductors and transformers.
Isolating a circuit from ground would eliminate ground fault currents and voltages, but simply isnt practical.
Grounding one side of a supply eliminates any ground ambiguity, making ground faults predictable and managable. In this state they can be hanbled by curent or voltage based disconnection devices.

In some cases grounding is not easy to manage, in the UK this is clasified as being outside the equipatentally bonded zone.
When a supply is placed in such a location, a garage or outbuilging for example or where a low localised impedance ground may exist, say in a shower, then a residual current device must be employed to disconnect the supply if a fault occurs
In th UK this could be an RCD (residual current device) and in the US a GFI (Ground Fault Interupter)
In both cases two systems could be used to detect a fault ...
The net difference between the curent in the two circuit legs, which, without any fault would always be equal, and a change in the potential to ground or its resultant current.

Whichever way you do it the circuit is disconnected if significant current flows anywhre but whre it is supposed to be going IE live to nutral.

Hope that helps
Al

 

How does a live in contact with earth allow any large current to flow at all if earth isn't connected to the return wire in the circuit neutral ?

Basically whats the use of a earth wire if earth is not connected to anything other than "earth" at the other end ?

 

If you have a power tool with external metal parts which can come into contact with the internal active mains power conductor, then the external parts will be live. If the external metal isn't bonded to an earth conductor then whoever is touching the now live metal part becomes a possible conductive path to ground. Generally it is mandatory for mains powered equipment with exposed metal parts to have a bonded earth connection. Only certified double insulated devices would be passed as safe without an earth connection. The proliferation of switch mode power supplies in mains powered electronic equipment has somewhat muddied the waters.

 

How does a live in contact with earth allow any large current to flow at all if earth isn't connected to the return wire in the circuit neutral ?

Basically whats the use of a earth wire if earth is not connected to anything other than "earth" at the other end ?

The point here is that the neutral wire IS bonded to earth at some point (or points) in any system using this sort of protection.

Wiring codes in different areas may differ in the detail of how it is done, but the principle requires that the earth and neutral are deliberately connected somewhere, so that any contact between live and earth constitutes a short-circuit.

 

In a house where are the earth and neutral bonded at the meter ?