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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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    Chris Pearson said:
    Isn't this about Note 2 to Table 41.5? 1667 Ω is the theoretical maximum resistance, but 200 Ω is the practical maximum.

    Thanks for clarifying Andy, I've see 'Z0' confused with what you've said 'Ze' is taken to include in a TT system.

    Regulation 643.7.2 confuses matters by using the term RA for earth electrode resistance alone (which I would term REE) ... I think it assumes the earth electrode is there because it's a TT system, but in fact it might be there for other reasons (e.g. means of earthing for TN-S source or transformer if you don't take your supply from the distributor's LV network, or additional earth electrode in TN system, etc.).

    I think it's generally cleared up by the fact that, usually you are relying on RCDs for ADS so (Ze+RA) >> R1, and in fact in a lot of installations, but most importantly, the condition we are trying to achieve is in 411.5.3, basically trying to achieve touch-voltage maximum 50 V for residual currents (notionally flowing down R2 and REE, i.e. RA)  up-to and including IΔn.

    Chris Pearson said:
    Isn't this about Note 2 to Table 41.5? 1667 Ω is the theoretical maximum resistance, but 200 Ω is the practical maximum.

    Absolutely ... Table 41.5 is actually aimed at ZS being the 'earth electrode resistance' substitute ... (if you read Note 2 to Regulation to 411.5.3).

    So, basically, Note 2 to 411.5.3, along with Table 41.5, is saying that if ZS < 200 Ω then  RA < 200 Ω, simply because ZS = Ze+R1+RA... but it's really RA we are after.

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  • 0
    Chris Pearson said:
    Isn't this about Note 2 to Table 41.5? 1667 Ω is the theoretical maximum resistance, but 200 Ω is the practical maximum.

    Thanks for clarifying Andy, I've see 'Z0' confused with what you've said 'Ze' is taken to include in a TT system.

    Regulation 643.7.2 confuses matters by using the term RA for earth electrode resistance alone (which I would term REE) ... I think it assumes the earth electrode is there because it's a TT system, but in fact it might be there for other reasons (e.g. means of earthing for TN-S source or transformer if you don't take your supply from the distributor's LV network, or additional earth electrode in TN system, etc.).

    I think it's generally cleared up by the fact that, usually you are relying on RCDs for ADS so (Ze+RA) >> R1, and in fact in a lot of installations, but most importantly, the condition we are trying to achieve is in 411.5.3, basically trying to achieve touch-voltage maximum 50 V for residual currents (notionally flowing down R2 and REE, i.e. RA)  up-to and including IΔn.

    Chris Pearson said:
    Isn't this about Note 2 to Table 41.5? 1667 Ω is the theoretical maximum resistance, but 200 Ω is the practical maximum.

    Absolutely ... Table 41.5 is actually aimed at ZS being the 'earth electrode resistance' substitute ... (if you read Note 2 to Regulation to 411.5.3).

    So, basically, Note 2 to 411.5.3, along with Table 41.5, is saying that if ZS < 200 Ω then  RA < 200 Ω, simply because ZS = Ze+R1+RA... but it's really RA we are after.

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