TNCS 6mm main earth - EICR

I think that the D.N.O. has a say in minimum main protective conductor sizes. Does Table 54.7 apply? A common size is 16.0mm2 for main protective conductors at the intake position in homes and small offices/shops/flats etc.


If 543.1.3 (applicable for disconnection times up to 5 seconds) is used, will the result still be approved by the D.N.O?


Z.

6 sqmm may satisfy 543.1.3, but note 542.3.1 and 544.1.1  As I read it, the minimum size of the earthing conductor is 10 sqmm because that is the minimum size of the main protective bonding conductor. That is perfectly sensible where there is a MPBC, but if none is required (because of plastic utility pipes), it does seem to be a bit silly. Such is BS 7671!

Just wondering what peoples first thoughts are on a 6mm main earth. 0.27ze, TNCS. 80A 1361 main fuse.

A rough calculation (0.3s disconnection time with an earth fault current of around 850A) suggests it'll pass the adiabatic with ease.


So that only really leaves PME diverted N current considerations. If there are no installation level extraneous-conductive-parts (or a 2nd DNO supply) then it's not really going to be a problem, if there are then if there's no sign of any thermal damage to the 6mm² then it's probably acceptable (but below current standards).

Iv'e tried doing a search on here but it did not come up with any results. (Maybe user error!). I'm sure this subject would have come up before.

Ah, it's probably in the old Forum... ?


  - Andy.

Thanks for the replies. I need to brush up on my adiabatic equations. Every now and again the need comes to do a calculation. I don't do them often enough to remember of the top of my head. 


Just out of interest in what edition of BS7671 was a 6mm main earth the thing to do? I guess the property was built in the early 70's.

Andy,

                where did you get your 0.3S from please? From a Fig 10 of the 15th edition for an 80 Amp B.S. 1361 fuse and 850 Amps, I got about 0.7 second.


Z.

Andy,

                where did you get your 0.3S from please? From a Fig 10 of the 15th edition for an 80 Amp B.S. 1361 fuse and 850 Amps, I got about 0.7 second.

BS 7671:2018 appendix 3 Fig 3A1 (admittedly that's to BS 88-3 rather than BS 1361 but as it's a direct replacement standard it shouldn't be too far different) - double checked by the inset table - 850A falling between the 960A for 0.2s and 800A of 0.4s.


I admit I really should have looked up a proper BS 1361 table - I know the BS 88-3 values aren't exactly the same (indeed a few of the ratings aren't), but I'd be surprised if wasn't in the same ball park.


   - Andy.

850A is very quick if you take the right 1361 curves. In general at a ballpark rule of thumb for anything simple and hot wire based, at 10 times the normal 'no break' rating,

you pass the 0.4 secs required for TN isolation. we are well over 600A, and speed goes as the square of current in this region. even with 3036  the rule of thumb is only a factor of 2 away.



conclusion, if it was only blowing the fuse we worried about, 6mm will be fine.

Thanks for that Andy.


I have a copy of the 14th Edition of the "Regulations for the Electrical Equipment of Buildings" 1966 published by the Institution of Electrical Engineers, now the I.E.T. My edition contains amendments up to 1976.


Table D.2M shows the sizes of earthing leads etc. required, related to the nominal cross-sectional area of the associated copper circuit conductor. The earthing lead is now called the main protective conductor.


If the main tails are 10.0mm2 or 16.0mm2 then the earthing lead is required to be 6.0mm2.


Earth continuity conductors, (circuit protective conductors) should be 6.00mm2 minimum.


Bonding leads should be 2.5mm2 minimum.


Of course the 850 Amp high fault current will only occur at the main terminals, down stream with higher Zs values the fault currents will be lower, but then final circuit protective devices like M.C.B.s will disconnect those lower earth fault currents.


Out of interest, back then, a maximum  earth fault loop impedance of 1.35 Ohms was allowed for a circuit protected by a 60 Amp semi-enclosed or cartridge fuse having a fusing factor exceeding 1.5,or 0.8 Ohms for a 100 Amp fused circuit. (Table D.1).


Z.

Mike, the main fuse is an 80 Amp. B.S. 1361 cartridge fuse. I reckon that it will open in about 0.7 seconds for an earth fault of negligible impedance, with a fault current of about 850 Amps. Does that sound about right?


Z.

Ah sorry, I highlighted for 60A - we need to look at more like a 600-650A fault current on a 60A curve, to see roughly what 850A would do on an 80.

I reckon with a squint it could be nearer half a second, though I don't have an 80 A curve.

One of the things with fuses, as opposed to MCBs, is that the degradation in breaking time with falling fault current is much smoother variation- a breaker will either do a near instant trip, or you are relying on the much slower thermal part, so there is a hard limit, and at fault currents below that  'it's all off in a heartbeat' is not true. (and for ADS for safety of life its getting it off in a fraction of a heartbeat time that matters.)


Also in any case from the adiabatic point of view there is no problem with the single insulated 6mm. 


The thing that has increased the recommended size of earth conductors, and main bonding, is concerns about diverted neutral current, for which you the company fuse will not operate, and nothing will prevent a long term overheating.