- 27 Jan 2008
- Reaction score
- Llanfair Caereinion, Nr Welshpool
There seems to be a slight change, however I have always wondered what the pass limit is when incoming is under the stated figures? If one measures 0.20 Ω at incomer then should the earth loop impedance for a B32 MCB be 1.22 Ω to allow for the incomer to rise to 0.35 Ω or is this what the 95% is for, so 1.37 Ω is used?IET Wiring matters issue 72 said:: Values of Ze for 230 V single-phase TN and TT supplies not exceeding 100 A according to ENA ER P23/1:1991
TN-C-S (Note 1) 0.35 Ω (Note 2)
TN-S 0.8 Ω (Note 2)
TT 21 Ω (Note 3)
- Note 1: In ENA ER P23/1:1991, this value was quoted for both Protective Multiple Earthing (PME) and Protective Neutral Bonding (PNB) earthing arrangements. Higher values may apply where the consumer was supplied from small capacity pole-mounted transformers and/or long lengths of low voltage overhead line.
- Note 2: The external earth fault-loop impedance for TT systems consists of the resistance of the neutral to earth plus the impedance of the transformer winding and line conductor, but does not include the resistance of the consumer's earth electrode.
We are told in one breath we should not rely on the RCD yet with 21 Ω plus what ever the premises earth rod is, clearly we do rely on the RCD. OK in the main with a B type MCB the volt drop not earth loop impedance is the limiting factor for cable length so in real terms we are not that close anyway.
Most of my work the supply was over 100A so the figures quoted did not apply, with the step down transformer on site, even if we had no access to it, we would clearly know if the supply impedance changed, so we could use actually readings. So this is some thing which did not really matter for me.
So what does the team think?