Bad ZS readings

[rob1967]

Got a Ze reading of 33.1 OHMS on a TT system and a PFC of 1.8KA....All the R1+R2 readings are very good but all the ZS readings are between 16.4 and 17.8 OHMS.....

I know on a TT system you can have a reading upto 1667 ohms but only on circuits upto 32 Amps.....Trouble is I have 16 circuits and 6 of them are over 32 Amps.......

They are all proceted by 30ma RCDS....please can anyone advise? Thanks..


Also, the garage supply has lost the earthing so cannot do a R1 + R2 test on that circuit, fuse board in garage is fine and there is a seperate Earth rod for it.........is this accecptable????

 

[Sintra]

As long as they are on 30mA RCD then 1667 ohms for all circuits.

 

[Sintra]

Yes outbuildings on seperate TT is fine although the supply cable should be earthed from main cu but not connected to earth at garage end.

 

[DurhamSparky]

garage has no eart but has a rod installed...!! Yes thats acceptable provoding the rod is earthing the DB in teh garage and not just for decoration

 

[Des 56]

Got a Ze reading of 33.1 OHMS on a TT system and a PFC of 1.8KA....All the R1+R2 readings are very good but all the ZS readings are between 16.4 and 17.8 OHMS.....


Also, the garage supply has lost the earthing so cannot do a R1 + R2 test on that circuit, fuse board in garage is fine and there is a seperate Earth rod for it.........is this accecptable????

Is this supply cable double insulated tails ,if not, how is it protected from earth fault ?

 

[rob1967]

Is this supply cable double insulated tails ,if not, how is it protected from earth fault ?


From the board in the house it's A 6mm pvc/pvc then at the garage end it's SWA into a metal conduit but can only see the caables and not the Armour........Cable lookes to be Alu and not copper.....Have done a Insulation resistance test and it reads at >299........Done a R1 + Rn instead of a R1 + R2 and it reads 0.20 ohms .........Circuit that supplies the Garage is protected by a 30ma RCD..............

 

[rob1967]

As long as they are on 30mA RCD then 1667 ohms for all circuits.

If this is so then why does it say in Table 41.5 that the Zs readings are for Final circuits not Exceeding 32Amps

 

[Sintra]

If this is so then why does it say in Table 41.5 that the Zs readings are for Final circuits not Exceeding 32Amps

Yes have to agree it does say that but when the first ammendment to 7671 (2008) is issued the words Final circuits not Exceeding 32Amps will be omitted

And a new column created for final circuits exceeding 32A and distribution circuits this value is still 1667 ohms.


This to me shows it was mis wording in the first instance