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TT acceptable earth fault loop impedance. OSG 7.2.6 (B)

Alan B

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

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    Surely the answer to why it's presented like this in the OSG is in BS 7671 itself? Regulation 643.7.1 b) tells you what is to be measured to verify ADS where RCDs are used for this in TT systems:

    (1) measurement of resistance of earth electrode (with option to take Ze reading ... not Zs ... instead of measuring RA): limit therefore 200 Ω (but this is Ze not Zs) ... provided that for downstream of the RCD, R2 is confirmed (but this is a required Dead Test in any case).

    (2) verification of effectiveness of protective device. With an RCD visual inspection (or characteristic type) and RCD testing (1x test: 300 ms max for non-delay, 130 ms min and 500 ms max for S type time-delay).

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  • 0

    Surely the answer to why it's presented like this in the OSG is in BS 7671 itself? Regulation 643.7.1 b) tells you what is to be measured to verify ADS where RCDs are used for this in TT systems:

    (1) measurement of resistance of earth electrode (with option to take Ze reading ... not Zs ... instead of measuring RA): limit therefore 200 Ω (but this is Ze not Zs) ... provided that for downstream of the RCD, R2 is confirmed (but this is a required Dead Test in any case).

    (2) verification of effectiveness of protective device. With an RCD visual inspection (or characteristic type) and RCD testing (1x test: 300 ms max for non-delay, 130 ms min and 500 ms max for S type time-delay).

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