general testing questions

[Chr!s]

My final point is if this is the case, why is it not mentioned in the regs?

Its not in GN8 Earthing and Bonding

Its not in GN5 protection against electric shock.

Its not in BS 7430 Code of practice for earthing.

So what and where is the 0.05 based on.

The only place i know of is GN3, and it reads quite clearly to me of its extent.

 

[Chr!s]

the last 2 posts bring me to ask a question. what are the criteria involved if, say, when doing a CU change and the existing main bonding is 6mm , regarding upgrading to 10mm? various posts have given conflicting views on this. is it acceptable to leave if resistance readings are below a certain figure, or should it be upgraded regardless of anything.

This is my point, take a TT fault current, it may be quite low due to Ra, so the voltage drop will mainly be at the electrode, so the voltage dropped across R2 will be very low, hence the touch voltage will be low.

Take a TN with a low supply impedance, you could quite easily drop more than 50 V across R2, and have a high touch voltage, hence the 0.4 disconnection time at 230 v.

So if im dropping 80v across R2, assuming little current flow through the bonding conductor what is the purpose a resistance of 0.05.

Do you design your installations to ensure a touch voltage below 50V?

The 50 v rule is additional protection, where at 230v 0.4 disconnection requires supplementing.

 

[pushrod]

Strange that Pushrod, i spoke to some of his colleagues and they disagreed with th 0.05 ohms and so do i. I can not see how restricting the resistance to 0.05 will achieve anything. I believe its his job to update GN3 so may be he can clarify for the new GN3.

Also, the ECA NICEIC and SELECT all agreed, someones wrong

The other point is, if this was the case then why in 701.415.2 refer you to 415.2.2 for the effectiveness of main bonding?

Why does it not say 0.05?

Theres no requirement to keep the touch voltage below 50v in general installations, due to supply impedance, in most cases little current will flow in the bonding(tn tt).

I fail to see what it will achieve.

I must agree that it seems strange to have this "important value" but only have reference to it in a guidance note and not BS 7671.
Some on here will know that in the past I have argued long and hard based on info in GN1 and the OSG only to eventually find that those references were flawed. So i must admit to now being a little sceptical about them. Mind i am getting used to the two answer scenario, especially from part P organisations as i have already experienced one answer from niceic and a different one from napit . I think it depends on how close it is to their tea break as to how much consideration they give!

Anyway i have also asked a follow up question about the significance and derivation of the magic 0.05Ω and will post it up here if i get one that is at all interesting

 

[pushrod]

the last 2 posts bring me to ask a question. what are the criteria involved if, say, when doing a CU change and the existing main bonding is 6mm , regarding upgrading to 10mm? various posts have given conflicting views on this. is it acceptable to leave if resistance readings are below a certain figure, or should it be upgraded regardless of anything.

Regardless of readings on it brb says pme must be a min of 10mm. With the others its based on more than half the csa of the earth with a min of 6mm - so you would have to be operating with a 10mm main earth which you would have to justify with the adiabatic. Then you would have a max main protective bond length of 16m for a 6mm csa
( i think... it has been along day selling chickens at the local market lol!)

edit : oops just seen you have said something on this as well chris, but not got time to read it properly

 

[telectrix]

This is my point, take a TT fault current, it may be quite low due to Ra, so the voltage drop will mainly be at the electrode, so the voltage dropped across R2 will be very low, hence the touch voltage will be low.

Take a TN with a low supply impedance, you could quite easily drop more than 50 V across R2, and have a high touch voltage, hence the 0.4 disconnection time at 230 v.

So if im dropping 80v across R2, assuming little current flow through the bonding conductor what is the purpose a resistance of 0.05.

Do you design your installations to ensure a touch voltage below 50V?

The 50 v rule is additional protection, where at 230v 0.4 disconnection requires supplementing.

but that does not really answer the q. my take is that if the bonding conductor is capable of withstanding the likely fault current for an excess of 0.4 secs. then it is adequate. but would you replace 6mm with 10mm regardless?

 

[Chr!s]

Yes you get a lot of differing of opinions between organisations, i think sometimes the help desks are unsure when it comes to design,not all, if they dont have a full understanding you tend to get conflicting opinions.

 

[Chr!s]

but that does not really answer the q. my take is that if the bonding conductor is capable of withstanding the likely fault current for an excess of 0.4 secs. then it is adequate. but would you replace 6mm with 10mm regardless?

Well you would need to take into account the supply earth impedance and the impedance of any extraneous conductive parts in parallel.

Normally in TT and TN systems little current flows in the bonding under fault condition (unless a broken Pen).

 

[pushrod]

The other point is, if this was the case then why in 701.415.2 refer you to 415.2.2 for the effectiveness of main bonding?

.

Mind the whole of this section (701.415.2) is entitled "supplementary equipotential bonding" and (iii) at the bottom of it says "all extraneous-conductive-parts of the location are effectively connected ... then there is the note referring to 415.2.2 for the assessment of whether parts (rather than a part) are effectively connected to the MET so it could be meaning that they are supplementary bonded and not actually be meaning the mpb, hence the reference... ?


I think the colleges should be saying to their prospective students you need 4 good GCSEs grades A to C, one of them must be science, it would be useful to for one to be maths, oh and of course you must have a degree in English to understand the regs

 

[Chr!s]

Mind the whole of this section (701.415.2) is entitled "supplementary equipotential bonding" and (iii) at the bottom of it says "all extraneous-conductive-parts of the location are effectively connected ... then there is the note referring to 415.2.2 for the assessment of whether parts (rather than a part) are effectively connected to the MET so it could be meaning that they are supplementary bonded and not actually be meaning the mpb, hence the reference... ?

Well the final section refers to omitted supplementary bonding, where the connection from the extraneous to the met meets 415.2.2.

So if you have a extraneous pipe that enters the location then you would bond it back to the met, its effectiveness it met by 415.2.2.

415.2.2 actually says between the exposed and extraneous parts, which would take into account R2. But seeing how we have ommitted SMB we are in fact testing MPB using the method in 415.2.2

 

[Chr!s]

If the 0.05 is there for volt drop in reference to touch voltage, which reg?

And all these circuits with reduced cpc are in trouble where the supply impedance is low

 

[pushrod]

Who can tell, maybe the next edition of the regs will make it clearer or an updated GN 8, think that one is still in the 16th.
One of the best things about this thread, that maybe a few other posters could learn from, is that there has been people taking opposing views and arguing their case but never once has there been a derogatory comment...




- i'm waiting for one now

 

[telectrix]

no comment. still licking wounds after being undercut on what would have been a nice job

 

[Chr!s]

Spoke to IET the other day, now Richards colleague has a differing opinion. He agrees with what ive stated and agrees some clarification in the new GN3 would be useful.

Ive also sent an email, but no reply as yet.

 

[pushrod]

Spoke to IET the other day, now Richards colleague has a differing opinion. He agrees with what ive stated and agrees some clarification in the new GN3 would be useful.

Ive also sent an email, but no reply as yet.

Yep the old 2 answers thing again lol at least it is a fairly minor point i suppose. I still have not had an answer to my 2nd question either. Be very interested to see what they reply to you. Maybe they are all going to go down the pub and have a chat about it

 

[Chr!s]

Yep the old 2 answers thing again lol at least it is a fairly minor point i suppose. I still have not had an answer to my 2nd question either. Be very interested to see what they reply to you. Maybe they are all going to go down the pub and have a chat about it

You wont be far wrong there, as for the 0.05 Paul could not give a definitive answer other than its a test between two points and shows a good sound connection.

 

[pushrod]

Hi Chris,
Well at least he didn't contradict himself. Sounds a bit like they have still not decided but pretty obvious that your points are all valid so well done to you in seeing the inconsistency of the arguements. Be interesting to see what they come up with in a new GN3, mind i don't think they have even published a GN8 to the 17th, so no idea when a review of a 17th updated one will be lol

BTW I never even received a reply to my email asking about the origins of the 0.05Ω value and how it was calculated

regards Pushrod