general testing questions

[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?