TT systems???

[Grant M]

I recently changed a fuse box and began testing to discover the property has no earth. I put an earth rod down and my Ze is 49ohm, which is an acceptable figure according the the 17th, which requires less than 200ohm. How do my Zs readings work? I am measuring about 12ohm on the socket circuits. All circuits are RCD protected. Do I just accept these high readings compared to TN systems, is this normal? Another question I have is what size earth cable should be used?

This is only the second time I have installed an earth rod, the first being for a temp supply.

Regards,
GrantM.

 

[p11jor]

Couldn't a 100ma rcd be installed in place of the main switch then 2 30ma rcds on the duel split?

 

[Deleted member 26818]

So that rule out a 100mA s type then spin?

It makes it difficult, as you would have to set the delay to 1s, and hope that it's accurate.
With the old disconnection time of 5s you had a little more leeway.

 

[p11jor]

It's a joke really isn't it, the same with
The DNOs clips on the main fuse and meters, there is books and books of regs about saftey we join a part p scheme to prove we work saftley yet the DNO (in most cases) have not provided a safe way of the contractor being able to install a new board.

 

[Deleted member 26818]

Couldn't a 100ma rcd be installed in place of the main switch then 2 30ma rcds on the duel split?

Unless the 100mA is an S type, there's no guarantee it would provide discrimination.
Most earth fault currents are higer than 100mA, so it's anyone's guess as to which will trip.

 

[Octopus]

Another question I have is what size earth cable should be used?

This is only the second time I have installed an earth rod, the first being for a temp supply.

Regards,
GrantM.

With respect to the earth size, the OSG and regs are open to interpretation and you could fit 6mm but I always use 10mm because I've always got that to hand and as you'll probably be upgrading the bonding too its easy to use the same throughout.