I suspect it relates to UK Public Utility Networks and not domestic installs, which specifically states 5s disconnection for fixed lighting equipment.

The values 0.35 ohms and 0.80 ohms are for run-of-the-mill metered supplies from approx 40 A to 100 A. The values were quoted in Engineering Recommendation P23/1, which was updated back in 2018 and P23/2 says something different.

This article provides a little more information regarding the replacement of the old version of P23/1, from which the rule of thumb values came, with P23/2: electrical.theiet.org/.../

Unmetered supplies ( larger metered single-phase supplies) are (and to be honest always were) "out of scope" for the rule of thumb.

The 0.35 and 0.8 figures have has more to do with the longest typical cable length used to feed a house in a street supply, and on a TNC-S feed, the resistance is partly related to the maximum voltage drop at full load as well, as the neutral and external 'CPC' resistance are the same.

As such these figures are really for the house at the wrong end of the street, regardless of 100A service or 30.

(and you might like to extrapolate that table you quote to a 100 A single phase supply like the largest you may get at a house, and see that it is in the same ballpark more or less, about half an ohm.)

Mike.

Graham, thank you. I thought that Table 2 was particularly interesting. Does it matter if Ze is say, 1.0 Ω? Well yes if R1 + R2 > 0.37 Ω and your sockets are on a 32 A type B MCB. (Which is how I discovered my "excessive" Ze at home.) If I understand the article correctly, there is no hard and fast limit, just a distribution of values. But of course, I may have misunderstood!

So where did the .35 and 0.8 come from

There's a few factors involved. For TN-C-S where the L-PE loop is also the L-N loop, there's voltage drop in the supplier's lines to consider - with a maximum permitted drop from 253V (230V+10%) to 216.2V (230V-6%) they must loose less than 36.8V - and with a 100A supply - then by Ohm's Law in the simplest case the L-N impedance must be less than 0.368Ω. Rounding to a nicer number with a just a small margin for safety would give you 0.35Ω It also meets the Zs requirement for a 100A fuse for 5s disconnection times.

0.8Ω for TN-S seems curious since it doesn't match at all the Zs required for a 100A (or even 60A) fuse for 5s disconnection time. But go back a few editions of the regulations - before we had 5s and 0.4s - and disconnection times were deemed to be met if the earth fault current was 3x the fuse rating -  and in 240V days 0.8Ω gives exactly 300A. Perhaps one to think about if you're installing a new metal CU on an existing TN-S supply...

   - Andy.

So are there any Ze limits over which the DNO is required (or at least is likely) to come out and inspect and/or fix? SSE recently came out for me on a 3.0Ω ohm TN-S domestic system and replaced the head. What if it was 0.85Ω? Etc.

Thanks Andy, 

makes a lot of sense along with other info from Graham. Wallywombat does ask a good question though. Many times in our more remote areas in the SW it does go a fair bit over .35. Mind you WPD are getting reticent to even give an earth terminal in a lot of cases. It's T.T. mate sort it out yourself. Also not that easy on Dartmoor = Granite

Many thanks

Marc

Agreed it's not easy. An earth electrode doesn't have to be one (or two or three) driven rods. Sometimes, buried bare copper wire/tape, or an earth mat, is much easier, provided you can get it buried 600 mm (roughly two spades' depth), and can often (but not always) help.

wallywombat said:

What if it was 0.85Ω? Etc.

Well, it doesn't have to be TT even in that case ... and RCDs can be used for ADS, provided cpc is selected for the OCPD just in case extraneous-conductive-parts pull the real loop impedance down once everything is connected up, so provided the issue isn't on L or N of the supply, and voltage drop is OK ... is there really an issue today?

Granted, if you're relying on existing protective devices, it's a lot more explaining to the customer ...