Just a Theory !!!

[Invisible-Man]

Just a theory, but why do you need a circuit to be protected by an earth if it is protected via a RCD, if all your circuits are protected via RCD say for instance on a TT system, would you need the earth rod?

Many thanks, Martin

 

[miajanjua]

You need an earth rod and the CPC so that if an appliance develops a fault and the chassis of the appliance will becomes live, the CPC carries the fault current to earth via the rod and hence trips the RCD.

If you had no rod, then no earth path and the RCD would only trip when someone touched the appliance and completed the earth path.

 

[securespark]

Also, in the worst case scenario (poor earth connection), an RCD will only operate effectively with a Zs of max 1666 Ohms.

If you have NO earth connection, there will be no RCD protection either.

 

[JohnD]

And it is bad practice to rely on an RCD to save your life.

Sometimes RCDs fail.

 

[crystal ball]

Also, in the worst case scenario (poor earth connection), an RCD will only operate effectively with a Zs of max 1666 Ohms.

If you have NO earth connection, there will be no RCD protection either.

So if you are cutting the hedge with a class 2 trimmer and cut the flex, grab the live end, the RCD will not trip, waste of time having them in that case

 

[JohnD]

The RCD will trip if there is 30mA or more difference between the current in the P and N conductors.

It is possible, if you are wearing rubber boots, in the example you give, that you could grab the two cut conductors in one hand and have no current flowing to earth.

That would give you a painful burn to the hand, and the RCD would not trip.

"Luckily" the current would not be flowing through the rest of your body and might not stop your heart, I don't know.

If you only grabbed the live conductor, not the neutral, and there was no current flowing to earth, you would not get a shock and the RCd would not trip. No problem

However, if you were standing on damp grass in bare feet, some of the current would flow through your body to earth, and the RCD would trip.

 

[crystal ball]

Exactly my point, it will trip without an earth connection (apart from through bare feet)

 

[JohnD]

But of the combinations possible, it will trip in the most dangerous, though not in the most safe condition, so is still worth having.