adiabatic equation

[Welchyboy]

Anybody got their books handy, and feel like giving the old grey matter a work out?

Just finished a EICR on a school

200A TPN TNS system

Main conductors 95mm
Earth conductor 50mm
Bonding conductors 16mm

ZE-0.13
PEFC-1.78KA
PFC-3.56KA

is the bonding up to spec?, looks like its ok, but its been a long day

 

[telectrix]

who nees a book, just use a calculator.

 

[rattlehead85]

The adiabatic equation is not used to determine sizing of bonding conductors. This equation is used to determine if the earthing conductor is of suitable size to handle current under fault conditions. If you refer to 543.1.1 this confirms. To determine the sizing of bonding you need to review table 54.8. From what you say the bonding is a size to small as they need to be 25mm if your neutral is 95mm. However as they are only one size below the required this would only warrant a C3 departure on an EICR as long as they show no signs of deterioration or damage.

 

[Welchyboy]

The adiabatic equation is not used to determine sizing of bonding conductors. This equation is used to determine if the earthing conductor is of suitable size to handle current under fault conditions. If you refer to 543.1.1 this confirms. To determine the sizing of bonding you need to review table 54.8. From what you say the bonding is a size to small as they need to be 25mm if your neutral is 95mm. However as they are only one size below the required this would only warrant a C3 departure on an EICR as long as they show no signs of deterioration or damage.

TNS earthing system

the minimum main earthing conductor is obtained by the adiabatic equation, THEN the main bonding conductor MUST be no less than HALF the size of the MINIMUM (calculated)size earthing conductor required, as long its over 6mm2

selection table 54.8 refers to the option of selection OR calculation for TT/TNS systems and is absolute only for PME systems

544.1.1
543.1.1

 

[rattlehead85]

So what is half of 50 then......? You stated the bonding was 16mm and asked if it was correct. You just answered yourself in the last post!! Re-read the regulation you stated. This does not permit you to install any size earth as long as it is not less than 6mm. You need 25mm bonding cables mate!

 

[Welchyboy]

Just because a 50mmearth is installed doesn't mean it's the minimum required!

it's a big job to reinstall the bonding and so much as I stick to installing from the table when new, on an eicr calculating the main earthing size usually brings up smaller sizes so it needs to be justified that they definatley are too small, when this could not be the case,
upon calculating the minimum earth size it could be say 25mm thus 16 mm for bonding would be acceptable,
what I'm getting at is the table is very much belt and braces for tns and tt systems and smaller sizes for earth (and therefore bonding conductors) will still comply

with a pme system then yes it's undersized, but you do have some room for manoeuvre with a tns

 

[rattlehead85]

Yeah not disputing any of that. But your original post asked if 16 was o.k. You then answered your own question.
You said the size of the earthing was 50mm which is correctly sized. So in relation to the fact that you have 50mm earth the bonds should be sized to 25mm. Now say you found the earth to be 25 then firstly as you correctly pointed out you would use the adiabatic to determine if it was suitable to carry fault current. If the result reveals it is satisfactory then we can say that 16mm bonding would be acceptable. You would have related the size of the bond in accordance with the earthing conductor present by applying the correct regulations.Fact is you got 50mm earth so the regs require your bonding to be half of that.
Anyways you don't need to redo the bonding anyway as i said as this only warrants a C3 departure which does not constitute a safety issue.
I take it you confirmed the bonding was continuos?

 

[Welchyboy]

Sorry to confuse i think you are misunderstanding what im getting at

it may well be too small, by the table it is!

but it may just scrape in by calculation of the earth conductor

and I can't be arsed to go into it tonight, just thought there may be some eager beaver on tonight to sve me a job

 

[rattlehead85]

Mate word of advice and please don't take this the wrong way but you need to be clear in your head about earthing and bonding and how to interpret sizing if you are carrying out EICR's. It is essential you confirm if the bonding is continuos by measurement not just by visually checking a bonding clamp! Bonding irregularities are one of the most important things to highlight during condition reporting!

 

[rattlehead85]

You don't need to confirm the suitability of the earthing conductor!! You have correct 50mm,relate the bonding size to what you have present! Sounds to me as though you already know it is undersized and you are trying to justify not upgrading it by trying to use the adiabatic equation against the existing earth where in this case it isn't applicable as you have the correct earth in place to size the bonding from!

 

[Guest55]

using the figures you gave , adiabatic says you only need a 7.8mm main earth :-D
which means you can get away withjust 4mm for the bonds :-D

 

[rattlehead85]

The adiabatic equation is not used to determine sizing of bonding conductors. This equation is used to determine if the earthing conductor is of suitable size to handle current under fault conditions. If you refer to 543.1.1 this confirms. To determine the sizing of bonding you need to review table 54.8. From what you say the bonding is a size to small as they need to be 25mm if your neutral is 95mm. However as they are only one size below the required this would only warrant a C3 departure on an EICR as long as they show no signs of deterioration or damage.

TNS earthing system

the minimum main earthing conductor is obtained by the adiabatic equation, THEN the main bonding conductor MUST be no less than HALF the size of the MINIMUM (calculated)size earthing conductor required, as long its over 6mm2

selection table 54.8 refers to the option of selection OR calculation for TT/TNS systems and is absolute only for PME systems

544.1.1
543.1.1

The regulation states the following.
"Except where PME conditions apply, a main protective bonding conductor shall have a c.s.a. not less than half the c.s.a required for the earthing conductor of the installation and not less than 6mm2"
Where does the word MINUM appear from in that sentence in the regs?
In your situation the earth needs to be 50mm2 therefore the bonding needs to be 25mm2. If you have an earth less than 50mm present and you have checked that it complies by using the adiabatic then your bonding has to be half the size of the earth you have!

 

[Guest55]

The regulation states the following.
"Except where PME conditions apply, a main protective bonding conductor shall have a c.s.a. not less than half the c.s.a required for the earthing conductor of the installation and not less than 6mm2"
Where does the word MINUM appear from in that sentence in the regs?
In your situation the earth needs to be 50mm2 therefore the bonding needs to be 25mm2. If you have an earth less than 50mm present and you have checked that it complies by using the adiabatic then your bonding has to be half the size of the earth you have!

so if he changes his main earth from 50mm to 25mm his 16mm bonds will comply lol
;-)

 

[Geoffsd]

Replying to ratttlehead -

But if the earthing conductor is oversized, i.e. larger than necessary (50 instead of 25 and fitted because that is all someone had) then the bonding conductor will not have to be half of this.

 

[telectrix]

a common mis conception here, rattlehead. it does not matter what size the main earething conductor actually is, it's trhe size it needs to be thast you apply the 50% reule to gor the bonding. this means that the main eareth can be 95mm, but if it only needs to be 25mm to satisfy the adiabatic, then it's 50% of 25mm