What should we be recording for max Zs on TT systems
On-site guide 7.2.6 TT systems
For TT systems the figures for TNC's systems, with RCD's, may be used providing that:
(b) the total earth fault loop impedance is verified as being less than 200 ohms;and
(c)....................................
In the past I have worked on the basis that an earth rod should give me an impedance of less than 200 ohms, and I would expect Zs results to be very close to whatever I had for Ze and the circuit impedance would be small compared to the rod.
But I have always recorded max Zs as 1667 for a 30mA RCD, following table 41.5 in BS7671.
To me at least it feels like the OSG and and BS7671 are sending out different messages, admittedly in slight;y different contexts.
Interestingly I use CertSuite for certificates and it now seems to default to 200 ohms with RCD's
What are your thoughts