80A, 30mA RCCB on a TT Installation

[Jaymack]

Hello All,
I've recently returned to this country from South Africa and still learning.
I have come across this RCCB, used as a main switch on a consumer unit, for a TT installation on two recent occasions. There is no other RCD downstream and the Zs at socket outlets has been around 0.4 ohms. When did the regs change to the current requirement of a 100mA main switch and 30mA downstream (for sockets etc), as the OSG PAGES 21/22. What would the code be for this, on a PIR?

 

[Adam_151]

4 IMHO

Socket outlets have 30ma protection and the whole installation is protected by an rcd rated at 100ma or less, the only issue is discrimination, or 2 if you are particulary worried about issues arrising from loss of the lighting circuits.

But then thats just my opinion, and I'm not qualified or anything, so might be best to wait for some other answers

 

[Spark123]

You are doing well to get a Zs of 0.4ohms in a TT system. Are you sure it is TT?

 

[Qedelec]

No way is that TT
OSG says 21 Ohms but your more likely to get 100+
NICEIC quote RaIa < 50V which in pratice with an RCD could be 1.66K

How I love them calcs

 

[Adam_151]

He read his tester wrong and its 40 ohms IIRC

/guess who lurks over on the screwfix forums

 

[Big_Spark]

No way is that TT
OSG says 21 Ohms but your more likely to get 100+
NICEIC quote RaIa < 50V which in pratice with an RCD could be 1.66K

How I love them calcs

I hate to say this, but recently I had a TT installation with a resistance of 0.38 Ohms.

OK so the stacks we drove in went down 6.8m into a clay layer, but it is possible to get very low TT readings...

 

[Steve]

No way is that TT
OSG says 21 Ohms but your more likely to get 100+
NICEIC quote RaIa < 50V which in pratice with an RCD could be 1.66K

How I love them calcs

I hate to say this, but recently I had a TT installation with a resistance of 0.38 Ohms.

OK so the stacks we drove in went down 6.8m into a clay layer, but it is possible to get very low TT readings...

Why isn't this the norm then??? Shouldn't all TT installs have an earth rod this deep instead of the normal 4 foot?

 

[andy]

I hate to say this, but recently I had a TT installation with a resistance of 0.38 Ohms.

OK so the stacks we drove in went down 6.8m into a clay layer, but it is possible to get very low TT readings...

6.8M earth rod... you must have some roof rack!