Main Earthing conductor

[Megaohm7]

About to do a CU change and I notice the main earthing conductor is 10mm. It's a T N C S with a Ze of 0.18 ohms. 230/0.18=1278KA. Done adiabatic equation and comes out at 7mm so technically 10mm would be in limits. To change the main earthing conductor would be difficult but if it has to be then so be it. The main tails are 25mm. Should I change the CU based on that calculation.

 

[NozSpark]

The regs say no less than half the size of the supply neutral.... so IMO 16mm

 

[Sidewinder]

Noz,

Unless calcs can prove OK!

---------- Post Auto-Merged at 22:26 ---------- Previous post was made at 22:25 ----------

However on your head be it.

Sorry clicked post before I finished.

OOh I do like auto merge!

 

[NozSpark]

Yeh, I know:innocent

 

[Megaohm7]

Cheers for your reply Noz/sidewinder. I thought I'd read somewhere if calcs proved ok then it was fine in the case of PME. How would this be best coded in a periodic inspection report.

 

[spinlondon]

The Regs. say 10mm

 

[SPECIAL LOCATION]

Agreed,

complies with reg 544.1.1

and as earthing conductors starts at 542.3..

which references 544.1.1 where PME conditions apply!

( pg 127 / 134 ).

no need to comment or code.

:Salute

 

[steptoe]

right, without getting into the arguement about TNCS vs PME the regs state for TNCS 1/2 neutral size unless calcs prove otherwise, AFAIAA PME states NO less than 1/2 size neutral.

you can make your own mind up as to whether they are the same or why we have 2 different regs for what is supposedly the same system.

for a periodic WITHOUT a PME sticker then I would make this apparent on my report and dictate accordingly.

EDIT,

seems the regs now class as calcs ok then, BTW, I havent looked, just going by prev posts.

 

[springcrocus]

The Regs. say 10mm

 

[Megaohm7]

I've just read that in my regs book and I've dug out my snags and solutions book and again it states that in there. Ironically the Ze value in the book is the same that I measured at the property concerned. the snag in the book was a main earthing conductor of 6mm. which was too small for continued use and a reccomended code would be 2 and to visually check for signs of over heating.. In my situation there is no signs of over heating on the conducter whatsoever. The installation is approx 15yrs old.

 

[spinlondon]

Don't quite get what you're on about there Steptoe?

Not aware that it states anywhere n BS7676 that the earthing conductor should be half the CSA of the neutral conductor for any sytem, irrespective of calculations?

---------- Post Auto-Merged at 23:29 ---------- Previous post was made at 23:26 ----------

Is that not for bonding conductors? The OP mentioned earthing conductor for which table 54.7 or the adiabatic applies, surely.Edit: typing too slowly, Steps beat me to it.

Yes that is for bonding conductors.

No the requirement for earthing conductors where PME conditions apply are the same as for bonding conductors.

See Regulation 542.3.1.

 

[steptoe]

perhaps Im confused by the term main earthing conductor,

that is not referred to in BS7671 either AFAIAA , or is it.?

BTW,

I dont think 7671 makes any difference between TNCS or PME , or does it.?

its only TN systems that it makes a difference in, T systems are different.

I may very possibly be wrong here,

but having a 10mm EC on a 15 YO system is not major, are you sure the tails are 25mm.?

 

[springcrocus]

No the requirement for earthing conductors where PME conditions apply are the same as for bonding conductors.

See Regulation 542.3.1.

Quote "...shall comply with Section 543 and, where PME conditions exist..."

I read this to mean that if Section 543 produces a conductor size that is less than that shown in Regulation 544.1.1, then reg. 544.1.1 shall take precedence. And 543.1.4 points to the adiabatic or table 54.7.

I feel that this is not the case here.

 

[spinlondon]

Section 543 requires that protective conductors be selected either by use of the adiabatic equation (543.1.3), or Table 54.7 (543.1.4).

However if a cable does not meet the requirements of Table 54.7, that does not mean that it will not satisfy the adiabatic equation.

In this particular instance the OP states that the adiabatic equation indicates that a CSA of 7mm

 

[Megaohm7]

Thanks for your replies everyone. The tails are definately 25mm which I was a bit surprised at the earthing conductor being 10mm. Reading some of the regs doesn't always seem that clear cut but It's always best to ask to be sure. I'm sure the customer will be happy anyway.

 

[ADS]

I don't know how you are all interpreting the 'Regs' - but each to there own, I suppose.

Page 28 of the onsite guide - Table 4.1

At the top of this page it states:

'The MINIMUM cross-sectional areas of the 'Earthing Conductor' and 'Main Protective Bonding Conductor' are given in Table 4.1'

Now, in my mind, when something is given as a minimum, it means 'the lowest' - no ifs, no buts, no calculations.............otherwise, what's the bloody point of having the table and stating it in the first place??

If you consult said table, you will find that the minimum CSA of an Earthing Conductor, for TN-S or TN-C-S supplies is, with 25mm Line/Neutral supply, 16mm.

The same info is given in Table 54.7 in the 'Regs' - again...MINIMUM

Also, GN8 states that the 'adiabatic equation' cannot be applied to PME supplies anyway, because the 'Earthing Conductor' also has to satisfy 'bonding' requirements.

And to the OP, it doesn't require a code - it requires changing, because you are doing a CU upgrade.

If you were just reporting on it, then you would code it.

My interpretation - Like it or Lump it:run

 

[spinlondon]

The OSG, as the title suggests is just a guide.

Table 54.7 is there for people who are unable or unwilling to use the adiabatic equation.

The reference to minimum, is not in reference to the minimum CSA required for compliance with the Regulations.

It is the minimum that would ensure compliance in all instances, without calculation.

GN8 is talking rubbish, or you're misquoting it, of course the adiabatic equation can be applied to PME supplies.

Table 54.8 only indicates the CSA for protective bonding, not for fault protection. If the adiabatic equation indicates a larger CSA is required than that in Table 54.8 you would be negligent in not opting for that CSA.

 

[ADS]

The OSG is a guide to the Regs, which is also a 'guide', so we might as well throw them both in the bin, then.

I don't want to get into a big argument on this, it's just that I would rather use Table 54.7(Regs) or Table 4.1 (OSG) as minimums, if the alternative is dodgy calculations based on not-very-accurate measurements of Ze/PFC.

Before anyone jumps down my throat, we measure Ze/Zs/PFC to assess whether they meet certain requirements within the regs - e.g. Zs lower than maximums etc.

The measurements that we take are hardly pinpoint accurate - there are too many factors influencing the results.

For example, in the OPs scenario it would only take a 0.06 ohm adjustment to his measured Ze to have the 'adiabatic equation' pointing to a 16mm Earthing Conductor'

So, instead of his 0.18 ohms Ze, he reads 0.12 ohms - then 16mm required.....come on!! Are we really that accurate???

Bottom line is, if I turned up for a CU change and saw a 10mm Earthing Conductor on a TN system, I wouldn't think "That looks a bit tricky to replace, I wonder if the 'adiabatic equation' will let me leave it there" - I'd go by the tables, price it in the job and upgrade it.

But, that''s just me.

 

[safedepth]

I've got my assessment again tomorrow. I'll ask if there is a definitive answer. I always upgrade to 16mm. Have I've been wrong all this time?

 

[springcrocus]

ADS, I'm with you on this one. As much as I respect Spins interpretations of the regs, and I admit to having learned a fair bit by reading his posts, I think table 54.7 is more appropriate than 54.8 in this instance. It's what I abide by, anyway.