general testing questions

[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.