zs results on tt system

[nursa]

my supervisor told me, recently, that zs readings on a tt system do not have to comply with the maximum zs figures in the regs. i have always worked to max zs figures no mater what system. whose right?

 

[acat]

Your maximum permitted is 1667 ohms over 200 ohms it may be unstable NICEIC recommend 100 ohms. Remember that BS 7671 is guidance so not legally enforcable despite the fact that they will quote BS 7671 in a prosecution against you, so I treat BS7671 as a legal requirement.




Chris

 

[nursa]

ive recently tested a tt system and got 0.25 ohms ze. when testing zs on all circuits my readings are in the region of 260 ohms. this fails the test?

 

[Mr Mark Sparks]

when you test the ze the best way to get a pure reading is to remove the main earth from the cu
then test onto the end of the cable
as chris said aim to get 100-200 ohms
then replace and test zs readings these may be lower due to better earth paths through bonds etc

your .25 reading is very low and there maybe something else causing this

 

[nursa]

yup it was disconnected for test. also tested gas and earth bonds getting 0.3.and 0.29 respective. cant figure out why zs readings so high. new ccu, connections all good. havent checked every termination in property but ring, lights, cooker and single spured socket all too similar to think this is the problem.

 

[Had8Lives]

Seems very strange that Zs readings are so high. You might want to check the busbar has gone into the bottom of the RCDs properly. I've been caught out with thinking it was screwed in tight and got no power. The busbar had gone in front of the screw instead.

 

[scotsparky]

As its a TT system the Max Zs must be covered by 411.5.3 disconnection times on rcds must be acheeved as well as Ra x In < 50 so no more than 50v touch votlage.

 

[Guest123]

Ze on TT at 0.25....not a chance.

The lowest I've ever heard of was on a job I did a while back in a quarry, we ran 120mm earth on to 9 sets of 2x14mm copper rods on a grassy bank with bentonite in the hole and we only got 4.22ohms then!

Surely your mistaken??