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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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  • 0

    On a slight tangent, the symbol Ze can be confusing on TT systems. Technically it's the loop impedance external to the installation, and as the consumer's electrode is part of the consumer's installation, Ze does NOT include the resistance of the consumer's electrode. That makes sense when official documents quote a maximum Ze for TT systems as 21Ω for example.

    On the other hand a lot of people who are perhaps more used to TN systems often refer to the loop impedance at the origin as Ze and then try to do things like Zs = Ze + R1 + R2, whereas technically it should be Zs = Ze + RA + R1.  (not Zs = Ze + RA + R1 + R2, since R2 is included in RA).

    Maybe we could do with a symbol for the earth fault loop impedance at the origin - Zo perhaps?

       - Andy.

  • 0 in reply to AJJewsbury

    Whichever you choose, 643.7.1 permits you to either ignore R1, and/or you can ignore REE (the actual consumer earth electrode resistance) and use Ze (not including REE ... but then how do you measure what you termed Z0, terminology which incidentally is also sometimes used in guidance).

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  • 0 in reply to AJJewsbury

    Whichever you choose, 643.7.1 permits you to either ignore R1, and/or you can ignore REE (the actual consumer earth electrode resistance) and use Ze (not including REE ... but then how do you measure what you termed Z0, terminology which incidentally is also sometimes used in guidance).

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  • 0 in reply to gkenyon

    but then how do you measure what you termed Z0

    Exactly as you would for Ze in a TN system - e.g. loop test from L to the disconnected end of the earthing conductor.- just call it Zo (or whatever) rather than Ze to avoid confusion with the 21Ω figure etc.

       - Andy.

  • 0 in reply to AJJewsbury
    AJJewsbury said:
    Exactly as you would for Ze in a TN system - e.g. loop test from L to the disconnected end of the earthing conductor.- just call it Zo (or whatever) rather than Ze to avoid confusion with the 21Ω figure etc.

    I would have thought that Ze of a TT installation was a simple contemplation of Ra and Re in series. The line part of the measurement is irrelevant as it is so small in comparison. 
    In any event, the regs require confirmation that the RCD will operate within the required time so measuring Ra is only half the battle. Using the standard Ze technique will also confirm that all is good at the DNO tx. 
    As I understand it, the 21 ohm value is maximum Re of 20 plus an arbitrary 1 ohm for connections. I do not see the need to introduce anther symbol like Zo to describe something that is already perfectly clear.

  • 0 in reply to lyledunn

    I would have thought that Ze of a TT installation was a simple contemplation of Ra and Re in series.

    The definitions differ though - the consumer's electrode is part of the installation, not External to it, hence its resistance isn't included in Ze. Hence we can have simultaneously Ze ≤ 21Ω and Zo ≤ 200 Ω.

       - Andy.

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