Re: Earthing

[Dambo]

I'm in the process of learning to become an electrician and I was hoping that someone may be able to answer this question:

If leaky or faulty current flows down the earth wire, why doesn't normal current flow down the earth wire?

 

[Apache]

Think what the earth does and remember than electricity takes the path of least resistance and see if you can tell us!

Welcome

 

[Dambo]

Well I believe there's least resistance in the earth wire so fault current would flow down it. But why doesn't it always flow down it then if it's got the least resistance?

 

[Apache]

Because the earth doesn't make up part of the circuit - under normal circumstances it has no electrical connection.

Now take a stainless steel toaster as an example, it's dropped or the workings corrode and the phase is able to come into contact with the metal case - rendering it live. The earth is connected to the case and takes away the fault current. The resistance from phase to earth is less than phase through your body so you don't get a shock and it's taken away.

Also the RCD would trip but that's another story!

 

[Dambo]

Now that makes sense! If you have a plastic light switch though and the earth is connected to that in a connection of its own how will the fault current flow down it?

 

[Apache]

It won't. It's double insulated (class II). It's just a handy place to 'park' the earth wire should you want it later (ie change switch for chrome).

 

[Dambo]

Ah, I see. So if there's a fault in the circuit will the fault current / leakage current then trip the RCD or fuse? (Sorry to bombard you with questions!)

 

[Apache]

Ah, I see. So if there's a fault in the circuit will the fault current / leakage current then trip the RCD or fuse? (Sorry to bombard you with questions!)

Any difference between the phase and neutral and the RCD will trip. Also if any current present on the earth.

A fuse (or MCB) is a very different beastie and designed to protect the circuit from overload. You could wire your house phase to earth and things would work and the fuses wouldn't melt/trip.

 

[green-hornet]

Now that makes sense! If you have a plastic light switch though and the earth is connected to that in a connection of its own how will the fault current flow down it?

Although the earth cable is not required at the switch, unless it is mettalic, the earth still needs to run with the live or line cables to offer an effective means of earthing, to any part of the cable along its installed length.

 

[Dambo]

Thanks to you both!

One final question - Will the RCD trip if any current flows along the earth wire?

 

[green-hornet]

The basics of how a RCD works is like this, current flows through both the line and neutral winding forming electro-magnets.

Because the two coils are wound in opposite directions they make opposite magnetic fields. As the same current is flowing in each coil they both create the same amount of magnetism, so they cancel each other out, thus there is, in effect, no magnet.

When a fault occurs most of the electricity flows back to where it came from via the earth wire instead of the neutral. This causes an imbalance in the magnetic fields.

Now the Line coil is stronger than the neutral, so it is not cancelled and can thus pull the contact apart; breaking the circuit and protecting you from the fault that would otherwise attempt to kill you.

So basically the earth is an escape route for any fault and any current running down it will cause an imbalance and disconnect from the supply.

The amount of current required to "trip" a RCD is down to the sensitivity of the RCD.

A 5mA rcd will trip with lower current faults than a 30mA rcd.

 

[Dambo]

Thanks green-hornet, much appreciated!