Main Earthing conductor

[steptoe]

I have had a look in the regs today. (shock!)

and 10mm is given as the minimum for <35mm tails,

but if you are on PME then it is half the neutral,

you could always downsize your tails to 16mm,

or go on the basis that its TNCS and there is NO PME label present,

personally I would use the latter arguement if you really dont want to upgrade it,

BTW,

I assume you realise why there is this larger minimum requirement for an EARTHING CONDUCTOR under PME regs.?

and its not really something to take lightly.

 

[spinlondon]

These particular Regulations have been in force now for over 30 years.

I don't understand why it is that recently they have become so misinterpreted.

I imagine Part P is the cause, not many electricians had copies of the Regs. before it was introduced. They just knew what was what. Now everyone and their Uncle has a copy, there are as many interpretations as there are copies.

Here's an excerpt from Geoff Cronshaw's article in Wiring Matters, relating to this matter:

"BS 7671 provides two methods for sizing protective conductors including earthing conductors (see also Table 54A). The easier method is to determine the protective conductor size from Table 54G but this may produce a larger size than is strictly necessary, since it employs a simple relationship to the cross-sectional area of the phase conductor(s).

The second method involves a formula calculation.

The formula is commonly referred to as the

 

[Larnacaman]

Are there people here really questioning the values of 'adiabatic equation' ???? The values obtained by these calculations are far nearer to the actual cable size requirements than the simple play it safe then add a little more cable sizes found in the various tables in the Reg's. ...Far too many people treat the Reg's as some form of bible, when in fact it's a guide, ...but even the guide points you in the direction of 'adiabatic equation' in many areas..... The Reg's change from edition to edition while the adiabatic remains a constant!!!

 

[spinlondon]

This all earthing conductors must be sized as per Table 54.7 is becoming quite prevelant.

Not as bad as the exporting a TN-C-S earth myth, but getting there.

 

[Larnacaman]

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

Well, if you were a mager contractors Design Engineer and used this philosophy, you would soon find yourself out the door. The difference between 100s and possibly 1000s of metres of 10mm or 16mm cable, works out to substantial cost savings.

 

[ADS]

Are there people here really questioning the values of 'adiabatic equation' ???? The values obtained by these calculations are far nearer to the actual cable size requirements than the simple play it safe then add a little more cable sizes found in the various tables in the Reg's. ...Far too many people treat the Reg's as some form of bible, when in fact it's a guide, ...but even the guide points you in the direction of 'adiabatic equation' in many areas..... The Reg's change from edition to edition while the adiabatic remains a constant!!!

The 'adiabatic equation' may well be an accurate form of calculation BUT, like anything, it depends on two very important things:

1) The person applying the equation and their ability to understand what they are doing and why.

and most importantly.....

2) The accuracy of the values input into the formula in order to perform the calculation.

Take the example mentioned by the OP, found in the NICEIC 'Snags and Solutions' book:

Here they use the following info to perform the calculation:

To calculate I

Uo / Ze = 230 / 0.18

Uo is an estimated amount (nominal) - the voltage is probably higher.

Ze is a measured amount - How accurate.....only the tester knows.

So we get I at 1278 A

This is the calculated PEFC.

This is then squared and multiplied by t

t is taken as 0.4 s.......hardly accurate, disconnection only has to be within that time.

Then the 'square root' is found of this number and divided by k - from the regs.........probably the only accurate part of the whole formula (and it's a correction factor)

Some of these inaccuracies will increase 'conductor' size and some will decrease it, but, like I said, hardly an accurate calculation

And finally:

"It has to be acknowledged that the use of the 'Adiabatic Equation' to determine the CSA of an Earthing Conductor is by far more problematic than to select the CSA by referring to the Tables in BS 7671"-

Guidance Note 8, Page 23

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''So, instead of his 0.18 Ze, he reads 0.12 - then 16mm required.....come on!! Are we really that accurate???''Well, if you were a mager contractors Design Engineer and used this philosophy, you would soon find yourself out the door. The difference between 100s and possibly 1000s of metres of 10mm or 16mm cable, works out to substantial cost savings.

If you were designing you would be dealing with lengths, CSA and resistivity - not measurement..........so the calculation would probably be more accurate - within your installation design.

But were talking about a one or two meter length of Earthing Conductor here, that people think you should be applying the 'adiabatic equation' to.

 

[safedepth]

From my NICEIC assessment this morning.

Use whichever method suits but bear in mind the variables can change, supply voltage, Ze etc. Factor that in. He said if i had used 6mm as the earth conductor and could justify it using the adiabatic equation he would be satisfied. It's in the regs and therefore allowable.

Now before people jump down my throat, I am aware he is an assessor and not god but his comments must count for something.

P.S. I passed again.

 

[ADS]

From my NICEIC assessment this morning.Use whichever method suits but bear in mind the variables can change, supply voltage, Ze etc. Factor that in. He said if i had used 6mm as the earth conductor and could justify it using the adiabatic equation he would be satisfied. It's in the regs and therefore allowable.

Now before people jump down my throat, I am aware he is an assessor and not god but his comments must count for something.

P.S. I passed again.

I take it the system was TN-S, as it definitely wouldn't be allowed on a PME supply - the 'earthing conductor' has to satisfy bonding requirements as well for a PME supply i.e. 10mm on 25mm tails

 

[Megaohm7]

From my NICEIC assessment this morning.Use whichever method suits but bear in mind the variables can change, supply voltage, Ze etc. Factor that in. He said if i had used 6mm as the earth conductor and could justify it using the adiabatic equation he would be satisfied. It's in the regs and therefore allowable.

Now before people jump down my throat, I am aware he is an assessor and not god but his comments must count for something.

P.S. I passed again.

Well done to you !! At least that's done for another year..

---------- Post Auto-Merged at 13:21 ---------- Previous post was made at 13:03 ----------

How many properties do any of you come across whilst doing periodic inspections that have smaller earthing conductors.As an inspector you have to make a decision and a justifiable one at that. You decide when the next inspection is due. The regs are a very important guide to us and it does allow us to have a sensible approach on our decisions to a degree.I understand to err on the side of caution but this install has been there for long enough to show any signs of overheating to the conductor of which there is none because if there had of been, then the conductor size would have been upgraded without question but there are safe options here.

 

[spinlondon]

I take it the system was TN-S, as it definitely wouldn't be allowed on a PME supply - the 'earthing conductor' has to satisfy bonding requirements as well for a PME supply i.e. 10mm on 25mm tails

Isn't that what we have been stating all along?

Use Table 54.8 not 54.7.

I do though understand why you consider the adiabatic equation to be dodgy, it's because you are performing it in a dodgy manner.

Measured Ze/PEFC will not use a nominal voltage, it will use the measured voltage.

From the values you have used in your example, I obtain a required CSA of 3.5mm

 

[Larnacaman]

ADS,

There is nothing dodgy or inaccurate about conducting an ''Adiabatic Equation'' it's been accepted and used by every electrical design engineer/office around the world, but you seem to think it's a pile of poo!! Ever considered that it is you, that is wrong???

Let's look at your reference notes above....

To calculate I

Uo / Ze = 230 / 0.18

Uo is an estimated amount (nominal) - the voltage is probably higher.

Nothing to stop you measuring it, your calculation will then be that more accurate!!

Ze is a measured amount - How accurate.....only the tester knows.

So now, even calibrated test equipment isn't good enough for you now, ...wow!!!

So we get I at 1278 A

This is the calculated PEFC.

Not if it's been measured it isn't!!!

This is then squared and multiplied by t

t is taken as 0.4 s.......hardly accurate, disconnection only has to be within that time.

My god, just how accurate do you want this to be?? If you want too, you can pick a time of your choosing if you want it spot on!!!

Then the 'square root' is found of this number and divided by k - from the regs.........probably the only accurate part of the whole formula (and it's a correction factor)

K figures are again, pretty much accurate, and again have been an industry accepted standard for many a year...

Nothing to stop you using the tables in the Reg's, but don't try and rubbish, cable sizes that have been chosen by calculation, just because you can't get your mind around the fact that many of the stated cable sizes in various sections of the Reg's, are not always correct. Or that god forgive, .....a substantially smaller cable csa meets ALL the criteria required....

 

[ADS]

Measured Ze/PEFC will not use a nominal voltage, it will use the measured voltage.It does, actually - but that's not what I meant.

The example given by the OP (from the 'Snags and Solutions' book) uses the nominal voltage to calculate the PFC i.e. Uo / Ze

Your mistake of determining that a CSA of 16mm

 

[spinlondon]

The operating time is not an arbitary value.

It is determined by the measured or calculated fault current.

No it does not point to a 16mm

 

[ADS]

Spin, like I said, this is definitely an 'each to their own' - For an 'earthing conductor', I would go by the tables and change it - others wouldn't.

An error reading Ze/PFC wouldn't necessarily lead to an oversized conductor, though.

If you read a higher Ze than it actually was (for whatever reason), then this could cause you to choose a smaller CSA than is actually required - this is obviously more dangerous than choosing one that is to large.

As for Larnacamans statement about accuracy of calibrated meters, we had a Fluke, a Megger Multi and a Megger Loop (all calibrated), and measured PFC at the same point - each one gave a different reading - not miles out...but different....I rest my case.

 

[Sidewinder]

OK,

Let's get a few things straight.

Accuracy & repeatability of measuring equpment are 2 completely different things.

I am NOT going into this.

A meter can be accurate to 0.00001, but repeat to within 1.

Any use?

The Adiabatic equation is used in MANY branches of engineering.

It is almost a fundemental principle, and as Scotty would say "Ye cannae deny the laws of physics Captain"!

It is the application of the adiabatic that is under question and the input data.

What we are all concerend with here is IF the "earthing conductor" can be damaged.

This is related DIRECTLY to the energy let through.

I have explained this in the past in great depth & am NOT going to again.

There is a search function, look for it.

Larnacaman is righ, in that there is an economic trade off.

This IS allowable under EAWR89.

Forget the regs they are NOT LAW.

My last comment is that if you don't understand that it is the energy let through that causes the possible damage and that this is related to many parameters, then give up now and find another job because you should not be working as a spark as you are likely to burn something down or even worse kill someone.

Any further snide remarks or nasty comments, name calling or rude remarks from anyone & I'll be reporting the posts to the Mods just in case they don't come in here because they are busy.

Grow up and learn some real engineering.

 

[ADS]

OK, Sidewinder - good post.

So, a simple question - you turn up to a house to do a CU swap, it's a PME supply with a 100 A 1361, 25mm tails, a Ze of 0.18 and a 10mm earthing conductor.

Do you change it or not?

I would, like I said, going by the tables.

Others say they would start prattling about with the 'adiabatic equation' to see if they could leave it!

C'mon, where's the economic trade off - it's probably 2 meters long at most.

 

[spinlondon]

ADS the OP has already pratted about with the adiabatic equation to determine whether he had to upgrade it. So your point is moot.

Then there is the OP's second question, as to what code should be applied when conducting a PIR.

Now it appears that your stance has gone from change it, it doesn't comply; to change it, what's the point in leaving it.

I try to answer questions based on what BS7671 actually contains, not what I think it should contain.

Perhaps you should consider doing the same.

 

[Adrian]

If it was easy changed I think most would, but if not would you charge the customer on something that doesn't really need doing.I'm sure you wouldn't do it free of charge just to make yourself feel good and have peace of mind

 

[ADS]

ADS the OP has already pratted about with the adiabatic equation to determine whether he had to upgrade it. So your point is moot.Then there is the OP's second question, as to what code should be applied when conducting a PIR.

Now it appears that your stance has gone from change it, it doesn't comply; to change it, what's the point in leaving it.

I try to answer questions based on what BS7671 actually contains, not what I think it should contain.

Perhaps you should consider doing the same.

I don't recall stating 'it doesn't comply, change it' - perhaps you can give me the post number.

For clarity - I would change it because I am doing a CU change and, in doing so, need to ensure that 'earthing and bonding' are up to standard.

Being up to standard, for me, is complying with the minimum CSA values in the 'tables' in the current regs, I'm not going to dick about with the adiabatic equation for the sake of 2 meters of 16mm.

If it was a PIR and I had to pass judgement on this scenario, I would probably use the adiabatic to confirm that it complies - if it didn't, then I'd code it.

My whole point is, if you're doing a CU swap, why on earth wouldn't you change it?? Why would you bother even trying to see if it complies via the 'adiabatic equation'

 

[spinlondon]

Whatever.