general testing questions

[just add water sparks]

On a PIR test to confirm Zs values i obviously go to thr BRB page 48.But when do i use the rule of thumb i.e 80% Method ?. I understand there are differences between a cold install to one that is in service as resistance changes when cables are warmer.
On previous posts i have noted to qualify for a extraneous part to be bonded a test can be made by using a insulation resistance tester set on 500v connecting one end to the MET and the other to the metal part in question.The reading shall not be greater than 0.022 meg ohms.On a large installation is the MET at the main intake or could it be at a sub board?Other posts and various books relate to a value of 0.05 OHMS to confirm if a bond is adequate does this relate to the main bond or supplementary and does MET mean main intake or sub board,I hope this makes sense

 

[IQ Electrical]

The 80% factor is applied to the maximum values of earth fault loop impedance listed in BS 7671:2008.

Example, a 60898 B32 device is listed with a maximum Zs of 1.44 Ohms. This value is then multiplied by 0.80 to give a value of 1.15 Ohms.

Your measured/calculated value of Zs should then be < 1.15 Ohms.

The extraneous conductive part test described in Guidance Note 8 must be conducted from the Main Earth Terminal of which there is only 1 per installation.

If the value of the test is > 22KOhms then the part does not meet the definition of an extraneous conductive part.

Main protective bonding conductors should have a resistance of 0.05 Ohms or less.

This equates to maximum runs of 15 Metres for 6mm and 25 Metres for 10mm

 

[just add water sparks]

Thank you IQ for your time and answers it was well appreciated. Just to clarify the 80% Applies to both a periodic and a new install.Measuring extraneous conductive part i guess you automatically auto null leads on continuity range and then switch over to the insulation resistance for the 500v test.

 

[IQ Electrical]

Yes, it applies to both.

The test leads are not nulled on IR tests, what's 0.10 or so of an Ohm when you're measuring thousands (KOhms) or millions (MOhms) of Ohms.

 

[just add water sparks]

ironically have just been on a job where there are remote outbuildings situated 170 metres from the intake met (to test adequate bond) so this seems quite a distance even for the purpose made R2 leads which are between 50 metres to 100 metres long,Any sugestions please i know the obvious would be to buy an extra couple of leads and join the two or three together and would you still omit auto-nulling leads as this is a fair distance.

 

[Deleted member 26818]

Use a conductor in the supply cable.
Don't quite understand the reference to auto nulling the leads?

 

[just add water sparks]

just to clarify i was refering to question (1 ) testing if extraneous conductive are adequate by testing with an insulation resistance tester and long leads and does not meet requirements if >22Kohms i was just using the long lead r2 method were you refering to an r1+r2 method which does not apply to this situation?

 

[Chr!s]

Main protective bonding conductors should have a resistance of 0.05 Ohms or less

I believe this is a misinterpretation IQ.

 

[SirKit Breaker]

We only have to correct the tabulated values cos some **** at the IEE didn't produce the OSG till well after the BRB. In the OSG they have been corrected for us.

Cheers.........Howard

 

[pushrod]

I believe this is a misinterpretation IQ.

GN3 p35 talks about test method 2 results on bonding conductors being in the order of 0.05ohms or less.

 

[Chr!s]

GN3 p35 talks about test method 2 results on bonding conductors being in the order of 0.05ohms or less.

Must be a different GN3 to mine then

 

[IQ Electrical]

Must be a different GN3 to mine then

You are right, I should have made the distinction between MPB conductors and SBC between extraneous-conductive parts!

I apologise, hope the OP didn't pay any attention to the post that he thanked me for!

 

[Chr!s]

Had the nic try to tell me bonding should be no more than .05, i asked him to show me where it asks for this, he tried the GN3, so i explained to him that its referring to a test between two points which can not be disconnected for example.

 

[Chr!s]

As far as i am aware the only restriction would be the equation used for SBC, and that would be between two simultaneous parts not the overall length.

The BRB only mentions csa.

 

[pushrod]

Had the nic try to tell me bonding should be no more than .005, i asked him to show me where it asks for this, he tried the GN3, so i explained to him that its referring to a test between two points which can not be disconnected for example.

Hi Chris - very interesting. I'm aware that brb does not specify resistances. I had this very sort of discussion with the tutor who was teaching me the 2391. I was saying so what if a mpb conductor had to be 30metres long , that would put it over 0.05Ω he said you would have to increase the csa.

AFAIK the brb does not ask you to test mpb conductors if you can visually confirm continuity. However, if you can not visually confirm continuity a test would have to be done. What result would you be looking at if you were testing between a gas and water pipe (two extraneous-conductive-parts) - isn't gn3 suggesting it should be this 0.05 value?

edit : reading gn3 again 2nd bottom paragraph P35 it talks about using test method 2 where the bonding clamps have been "built in" then connect across extraneous-conductive-parts and check for the 0.05 value. The extraneous conductors could be gas and water? I don't know why it doesn't say between
extraneous-conductive-part and the MET

Are you and IQ talking of supplementary protective bonding conductors ? Surely they connect between an exposed-conductive-part and an extraneous-conductive-part?

 

[just add water sparks]

i am well and truly confused now.great are 'nt they abbreviations. yesterday i drove my m.p.v to do a p.i.r on an install it was wired in p.v.c and some old v.i.r had some good Zs's tho.Not sure on some cable sizes as g.a.t.s was not taken into calcs da da da lol only joking guys.I know are my few brain cells right in telling me right that m p b is main ? bond and s b c is supplementary bond connection.Has the figure gone from 0.05 to .005 and not sure what figure i am looking for & what i am testing now.genuinely all help is appreciated this game is certainly ambiguous ain't it

 

[pushrod]

i am well and truly confused now.great are 'nt they abbreviations. yesterday i drove my m.p.v to do a p.i.r on an install it was wired in p.v.c and some old v.i.r had some good Zs's tho.Not sure on some cable sizes as g.a.t.s was not taken into calcs da da da lol only joking guys.I know are my few brain cells right in telling me right that m p b is main ? bond and s b c is supplementary bond connection.Has the figure gone from 0.05 to .005 and not sure what figure i am looking for & what i am testing now.genuinely all help is appreciated this game is certainly ambiguous ain't it

know how you feel lol - certainly keeps you on your toes with all the abbreviations especially when people make up their own (not this thread)
MPBC =main protective bonding conductor
SPBC = supplementary protective bonding conductor .

The correct figure is 0.05 ohms it is what it applies to is the problem !!!

 

[Deleted member 26818]

just to clarify i was refering to question (1 ) testing if extraneous conductive are adequate by testing with an insulation resistance tester and long leads and does not meet requirements if >22Kohms i was just using the long lead r2 method were you refering to an r1+r2 method which does not apply to this situation?

No I'm not refering r1+r2.
I was suggesting that rather than obtaining extra leads and joining them together, that you use a conductor out of the supply cable as a long lead.

 

[Chr!s]

Hi Chris - very interesting. I'm aware that brb does not specify resistances. I had this very sort of discussion with the tutor who was teaching me the 2391. I was saying so what if a mpb conductor had to be 30metres long , that would put it over 0.05Ω he said you would have to increase the csa.

It a common misconception, i remember my 2391 trying to tell us this, i explained to him he was incorrect, if you read GN3 carefully you will see that its not placing the 0.05 on the overall length. As i said above the nic also gave the same advice, so i explained and they agreed.

AFAIK the brb does not ask you to test mpb conductors if you can visually confirm continuity. However, if you can not visually confirm continuity a test would have to be done. What result would you be looking at if you were testing between a gas and water pipe (two extraneous-conductive-parts) - isn't gn3 suggesting it should be this 0.05 value?

GN3 is referring to where say the structural steel is used as a bonding conductor for example and can not be removed, testing between two points the resistance should be no more than 0.05.

What your looking to achieve is to limit the touch voltage, so you could use the equation used for effectiveness of supplementary bonding as this is in effect what your trying to achieve. That would need to bee one hell of a length of of cable between two points especially when the minimum is 6mm.

edit : reading gn3 again 2nd bottom paragraph P35 it talks about using test method 2 where the bonding clamps have been "built in" then connect across extraneous-conductive-parts and check for the 0.05 value. The extraneous conductors could be gas and water? I don't know why it doesn't say between
extraneous-conductive-part and the MET

Well its referring to any two points, this could be two of many in one given installation.

Are you and IQ talking of supplementary protective bonding conductors ? Surely they connect between an exposed-conductive-part and an extraneous-conductive-part?

Im talking about Main protective bonding though in principle your trying to achieve the same as supplementary bonding conductor limiting touch voltage.

Oh and yes the figure is 0.05 ohms my typo sorry

 

[Chr!s]

Pushrod, i dont have my GN3 to hand, but does it mention simultaneous?

 

[pushrod]

Thanks for the very detailed reply Chris - must admit i still haven't got my head around it completely - will have to give it a bit of time for it to be assimilated

[

GN3 is referring to where say the structural steel is used as a bonding conductor for example and can not be removed, testing between two points the resistance should be no more than 0.05.

Take your point and can vaguely remember something about this in the brb or a niceic guide, will have to look when i get a chance.

I must admit i interpreted gn3 differently.

I took the "eg where bonding clamps have been 'built in'. " just to mean that access to the clamps on MPBCs was restricted, ie behind panelling. Definitely see that you could be quite correct though.

Those IEE guys should get someone on board from the Plain English Campaign!

 

[pushrod]

Pushrod, i dont have my GN3 to hand, but does it mention simultaneous?

No nothing at all in this section about that, or being able to touch at same time. [That is why i was wondering about distant gas and water pipes where clamps might not be accessible] .

 

[Chr!s]

I took the "eg where bonding clamps have been 'built in'. " just to mean that access to the clamps on MPBCs was restricted, ie behind panelling. Definitely see that you could be quite correct though.

Agreed, GN3 just gives and example, its just saying where the bonding can not be removed the resistance between the two points should be no more than 0.05, this figure is only guidance too.

The BRB just requires a continuity test, 612.2.1, for PMB amd SMB, but SMB has the 50V rule which seeing as your trying to acheive the same thing through PMB makes sense to apply the same principle.

No nothing at all in this section about that, or being able to touch at same time. [That is why i was wondering about distant gas and water pipes where clamps might not be accessible] .

No nothing at all in this section about that, or being able to touch at same time. [That is why i was wondering about distant gas and water pipes where clamps might not be accessible] .

No your correct, looked earlier on, just states between two points

 

[tony mc]

Thanks

Must admit I was of the same opinion the main bonding conductor being 005 ohms or less and have just read it again and can see why.

Its amazing what you learn from the forum.

Thanks again Chr!s.

 

[Chr!s]

Well were in good company, below was ECA take on it, i hasten to add others shared the same view too.

Hi Please could you give some guidance on the maximum length of a Main protective Bonding Conductor. People mention the 0.05, though the only reference is in GN3, though this reads as a test between two points where the bonding connection is say structural and not applied to an overhaul length. The regs only mention the CSA. Now we are trying to limit touch voltage, so is there a fundamental calculation which may be used? Many thanks Chris

ECA technicals response

The length of main bonding conductors is not normally something that needs to be considered in most installations Chris. The maximum touch voltage between extraneous conductive parts for general installations is 50v (some special locations are limited to 25v). You would have to have a very long main bonding conductor to get that kind of voltage drop.

 

[pushrod]

Thanks Chris- i must admit i was always a bit sceptical about that 0.05ohm figure.

 

[pushrod]

Just playing with the numbers (must have too much time on my hands lol)

A figure in the order of 0.05Ω for a 10mm² single with a R/m of 0.00183 means that you can have a max length for your main protective bond of just over 27m.

Just randomly thinking about a circuit with the largest load that could come in contact with pipework in a house i thought "shower circuit with a 40A C type mcb"(suppose could equally could be a cooker circuit). The fault current for operation within stated time is 400A.

to restrict the Vd to 50 V

since Vd = V/A/m x Ib x L

So max length L = Vd/(Ib x V/A/m)
= 50/(400 x 0.0044)
= 28m for the length of the MPB

agreeing with a Resistance in the order of 0.05Ω
Bit of a coincidence or something in it - what do you think?

Seems like if you had a resistance that was considerably greater than 0.05 Ω you would not be guaranteeing that your bonding would be limiting touch voltage to 50V in worst case scenarios. So whether you go with that 0.05 or work it out like you do for limiting volt drop the answers are pretty similar.

(chose C type breakers as the IEE's Electrical Installation Design Guide table quotes max lengths for supplementary bonding conductors for C type MCBs)

 

[Chr!s]

Hence the equation 50/Ia

 

[Chr!s]

I think the other point is that the 50V is additional protection, where additional protection is not required touch voltages may exceed 50V.

 

[pushrod]

Hence the equation 50/Ia

TBH i don't think that does follow. The equation R = 50V/Ia is more commonly used to confirm the effectiveness of supplementary bonding and calc the max length of sup bonding. If that were used to work out the length of 10mm bonding then in the case i gave
R = 50V/400A = 0.125 ohms.
Resistance of 10mm = 0.00183Ω/m so that would correspond to a bond length of 68m.

What i did was work out the length of conductor you could have with a 400A fault current before you got more than a 50V volt drop... 28m

 

[pushrod]

Its not as straight forward as you think.

.

Honestly i don't think you can say that to me - i know it is blooming complicated and i'm just hanging in by my fingernails

was just pointing out a discrepancy.

 

[Chr!s]

was just pointing out a discrepancy.

Well its not a discrepancy, the 50/ia or 50/I delta n, is in effect the only limiting factor. The installation may not require additional protection, so then theres no limiting factor.

If additional protection is required then 415.2.2 will be the limiting factor. Though the limit will only be applicable for that special location.

 

[Chr!s]

Are you in disagreement with whats been said IQ?

Your input would be very welcome.

 

[IQ Electrical]

Have i bored you to sleep LOL

Not at all, one of the best threads in a long while!

 

[pushrod]

Hi Chris, I don't disagree with what you were saying ( one of your posts seems to have disappeared now) i think we may have been at cross purposes a bit because i might have misinterpreted the way you were using the word "hence".

Are you saying that looking at length of main protective bonding from a voltage drop [Vd=mV/A/m x Ib x L] point of view is not really valid?

 

[Des 56]

It was interesting reading through the thread
Here is the not so interesting bit

A very well presented and thorough understanding of the issues has been given
But isn't it ironic that whilst these technical issues can be discussed, presented, and mused over in such detail, that in the big wide world, what is being discussed is of no consequence to the reality of standards now being eroded in the electrical industry
With the countrywide dumbing down of he trade,can you imagine such a discussion taking place in years to come ?

The trade is fast becoming a sum of the lowest common knowlege

It is a shame that what was once regarded as a trade that required technical understanding and practical competence, has now become a occupation that can be grasped " just like that"
Well done to all parties for the discussion and appreciation for the chance to observe

 

[Chr!s]

Hi Chris, I don't disagree with what you were saying ( one of your posts seems to have disappeared now) i think we may have been at cross purposes a bit because i might have misinterpreted the way you were using the word "hence".

Are you saying that looking at length of main protective bonding from a voltage drop [Vd=mV/A/m x Ib x L] point of view is not really valid?

If you comply with ADS then theres no limitation on resistance of main protective boding unless....

You require additional protection then the limiting factor to the main protective bonding will be 415.2.2

Dependant upon supply impedances, especially where you have a reduced cpc, you may find more than 50v is dropped across r2, so squeezing the life out of your cpc will have the down side of increasing your touch voltage.

 

[pushrod]

Well i asked the question of the IET and here is the answer received - just about a full circle i think!

Hi P

Main Earth bonds (gas ,water etc) should have a value of resistance not exceeding .05ohms, in general terms this would negate the use of a 10mm cable after about 25M after which the conductor size must be increased in order to keep the resistance of the cable below .05 ohms. This is a commonly misunderstood arrangement and contractors constantly fit 10mm bonds in excess of 25M due to a misunderstanding of the regulations

Many Thanks

Richard


Richard Townsend
Senior Engineer
Standards and Compliance
The IET

[email protected]

www.------.org

I

 

[Chr!s]

Well i asked the question of the IET and here is the answer received - just about a full circle i think!

Hi P

Main Earth bonds (gas ,water etc) should have a value of resistance not exceeding .05ohms, in general terms this would negate the use of a 10mm cable after about 25M after which the conductor size must be increased in order to keep the resistance of the cable below .05 ohms. This is a commonly misunderstood arrangement and contractors constantly fit 10mm bonds in excess of 25M due to a misunderstanding of the regulations

Many Thanks

Richard


Richard Townsend
Senior Engineer
Standards and Compliance
The IET

[email protected]

www.------.org

I

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.

 

[telectrix]

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.

 

[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