High Ze

[sparkylad]

No T. the ze and zs are certainly not irrelevant.
You still need to verify that you have an acceptable and stable reading at the Earth electrode, For a 30ma rcd you could mathematically have an Ra of 1667 ohms, however a max of between 100 ohms and 200 ohms is deemed the acceptable stable value. you are additionally still required to measure the zs of each circuit to indicate integrity of the Line/cpc fault loop continuity.


sparkylad

 

[sjm]

For TT systems I usually put n/a for max Zs allowable under BS7671 and then record the tested value in the relevant box.
Ze and Zs are still relevant though. sparkylad is spot on.

 

[Risteard]

This is why all TT systems require additional protection with RCDs in order to meet the disconnection times.

Theoretically speaking they don't necessarily require RCD protection to meet disconnection times if disconnection times can be achieved otherwise. However it is unlikely that this will be the case.

Additional protection is a different concept however.

 

[brucelee]

to slightly diverse, here's one that has me concerned. on a TT system, say if your Ze is 19.something the your Zs is going to be Ze+R1+R2 i.e. 19or 20 ohms. How do you equate this with a max. permissable Zs of say 1.16

by protecting with a 30mA rcd which will allow a theoretical value of 1667ohms but may be unstable over 200ohms fot tt

 

[Guest123]

For TT systems I usually put n/a for max Zs allowable under BS7671

IMO that is the incorrect thing to do. Depending on what rating of RCD you have there IS a maximum permitted Zs that must not be exceeded in order to achieve disconnection times.