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Maximum Ze, by earthing system1.09 ohms on a TN-C-S. Now what?

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These are the numbers you check your measured Ze against. They are the typical maximum figures the DNO quotes, given in Electrician's Guide to the Building Regulations · p.53 — a reading above them means the supply earth needs investigating.

SystemMaximum Ze
TN-C-S (PEN conductor)0.35 Ω
TN-S0.8 Ω
TTSee the electrode section — up to 200 Ω
Maximum Zₑ, by earthing system what you check your measured reading against — and you want margin, not just compliance TN-C-S 0.35 Ω a PEN conductor — earth and neutral combined in the supply TN-S 0.8 Ω separate earth back to the supply TT up to 200 Ω your own electrode — see the electrode section 1.09 Ω measured on a TN-C-S board — against a 0.35 Ω maximum. Three times over: investigate.
The bar lengths are not to scale across systems — TT is in a different order of magnitude, because it depends on an electrode in the ground rather than a conductor back to the supply. What matters is the comparison against your own reading: a TN-S result of 0.42 Ω against a 0.8 Ω maximum is comfortable, while 1.09 Ω on a TN-C-S system is three times the typical maximum.
TN-C-S means a PEN conductor — a combined protective and neutral conductor in the supply, with the earth and neutral joined.
You want margin, not just compliance. A TN-S reading of 0.42 Ω against a 0.8 Ω maximum is comfortable. A 1.09 Ω reading on a TN-C-S supply is well above 0.35 Ω — exactly the sort of thing a measurement catches and an assumption does not.

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