RCCB 100a type s on a TN-S

[Alan coster-mills]

i come across this today and it does not sit well with me. so im after a little advice.
its a TN-S system Ze 0.12 and Zsdb 0.16 there a type S 100A rccb as a main switch with no RCD's up stream, This does not sound right to me and all the MCB's are C type. all my I.R were greater than 2k except 1 but that was in the 100's so im not worried. it feels like either an old earth fault leakeg problem that was remedied this way! or someone had this ---- on the back of there van and got a sale!
can anyone please shed any light on this for me.

 

[SparkyChick]

I suspect the chaps will want a little more information... domestic, commercial? Type of loads? What's the rated trip current of the RCD?

And clarity on the IR readings... 2k? Did you mean 2M? And 100's of what? Mohms? kohms?

Personally I wouldn't fit type C to a domestic unless there was a good reason (say a 6A Type C on a lighting circuit with a lot of fluorescent or LEDs).

 

[Alan coster-mills]

sorry, my heads in a million places at the moment.
this installation is in a domestic house, it only has 2 fluorescent tubes.
and my IR readings are > 2000 Mohms the lowest IR reading i got was 207 Mohms

 

[westward10]

What is the problem. Type C may not be common in domestic installs but there is nothing to say they must be Type B, I always fitted Type C on lighting circuits in the days of tungsten lighting to prevent tripping with a blown lamp.

 

[Ian1981]

What do you mean zs db 0.16?
You've measured ze at the db I presume and got 0.12 ohms or is there a submain feeding the db?

 

[Alan coster-mills]

Ze is the external loop not including the earth of the property so you disconnect the main earth at the board! and measure.
Zs at DB is what they are now calling it when you have the main earth reconnect! at the board and take reading of the whole system. And your Zs readings are at the furthest point on a particular circuit.

 

[Ian1981]

Ze with the earthing conductor disconnected is required to verify the effectiveness of the earth.
Reconnecting it and doing the same test is not and serves little purpose.
When doing a pefc then yes the earth is connected to the earth bar along with everything else as you want the highest fault current to flow which includes your bonding conductors in the test.

 

[westward10]

So why doesn't it sit well with you.

 

[Richard Burns]

Possibly it used to be a TT system but got changed to TNS (slightly odd as they are usually to TNCS, because TNS would be a lot of work) at a later date without changing the internal electrical system.
100mA non time delayed RCDs were used fairly often on TNS systems but the time delay seems pointless in this case unless there were nuisance tripping issues. Still at least there is some limitation of current on earth faults.

 

[westward10]

Maybe just the board they had to hand no more to it than that. I agree TT to TN-S not likely.