adiabatic on earth bonding

[pencilpusher]

You wont find any reference to BS 1361 in the new amendment as it no longer exists. It has been replaced by BS 88-3.:stooge_curly:

Ah ha Thanks Lenny so it has.

 

[i=p/u]

as for your fault current you use to size your earthing conductor

 

[pencilpusher]

Thanks. So TNCS Ze=0.22 ohms. Supply fuse BS 1361 type 2. Fault current is 230/0.22 = 1045 amps. Graph in Appx 3 (BS 88-3) gives about 0.5 seconds disconnection time on a 100amp fuse. So equation = SQ root 1045x 1045 x 0.5/143 (value of K) = 5.16mm2 or 6mm2 earthing. So on TNCS 6mm2 would be OK but minimum for new installs is 16mm2. My Elecsa assessor stated I could leave 10mm2 in place if it was already there if the customer didn't want her house re arranged whilst putting in a new shiny 16mm2 conductor. Noted on EIC of course.

BUT couldn't I use the 5 secs disconnetion time needing 580 amps (from Appx 3) and use that as the fault current which would make the earth size even smaller?

I think that is what Spin was talking about?

Have I got the adiabatic correct?

PP

 

[Deleted member 26818]

No.
You can't use a measured value with a tabulated value.
A 100A BS 88-3 fuse will operate in just over 0.4s with an earth fault current of 1045, as it's only 5A, you might want to use the 0.4s value.
Otherwise, you would have to use the 850A and 1s values.
With cartridge fuses, 10mm² is sufficient for up to 100A, where the value for k is 143.

 

[pencilpusher]

No.
You can't use a measured value with a tabulated value.
A 100A BS 88-3 fuse will operate in just over 0.4s with an earth fault current of 1045, as it's only 5A, you might want to use the 0.4s value.
Otherwise, you would have to use the 850A and 1s values.
With cartridge fuses, 10mm² is sufficient for up to 100A, where the value for k is 143.

Thank you. I am still a bit confused though. If I use 1045 amps as the measured value and OK, 0.4 secs, I still get a minumum earth of 4.62mm. So why is 10mm2 the minimum for that fuse. Is there any point in using the adiabatic then like this if 10mm2 is the minimum size anyway?

Just trying to clear this up in my head!

PP

 

[Deleted member 26818]

10mm² is the minimum where PME conditions apply.
However with TN-S and TT, 2.5mm² is the minimum.
You only really need to use the adiabatic equation, when determining whether existing cables will be adequate, when installing larger supplies or when multicore cables are being considered.

 

[pencilpusher]

Cheers Spin. I wasn't challenging the Regs, just trying to understand why they are written as they are!

 

[brman]

Ok. on this minimum csa point, I to am struggling to cross reference the OSG table 4.4 against the BGB.

So for the earthing conductor......

1) min csa is 2.5mm2 (protected mechanically) or 4mm2 (unprotected)
543.1.1

2) if buried, table 54.1 applies. (2.5mm2, 16mm2, 25mm2 depending on protection against corrosion and damage)

3) then there is reg 543.1.4, table 54.7 to be used as an alternative to the adiabatic which relates it to the line conductor.

4) 544.1 states that the main bonding is 1/2 that of the earthing conductor with a min of 6mm for TNS/TT and 10mm for PME.

Ok, so a small point but why does the table 4.4(i) in the OSG give a earthing conductor size of 6mm2 against a line conductor of 4mm2. My reading (from table 54.7) is that it should be 4mm2 (which still meets the minimum for an unprotected cable in 543.1.1

Have I missed something?

 

[Deleted member 26818]

The OSGs are not neccessarily based on the Regulations.
The first one (The 16th) was intended to provide guidance so that in just about all cases, the requirements of BS7671 would be met.
A sort of one size fits all.
There were even practices recommended that have not been requirements of the Regulations since the 14th edition.
The writers or contributers to the OSG are offering what they consider to be the best methods to achieve compliance with BS7671, and in many instance they are interpreting BS7671 based on their opinion.
I must admit, that I have always thought it a bit odd, that you can have a bonding conductor with a larger CSA than the main earthing conductor.

 

[brman]

I guess that makes sense, thanks. It did seam a bit odd but that isn't the first time I have thought that with the regs

As a co-incidence I started my 2395 today and we had a bit of a discussion with my tutor on whether I should use Ze by enquiry, PFC by enquiry, max allowable Ze, measured, 5s trip current etc for the adiabatic so this thread was very useful!

 

[rfears]

Off topic, sorry, but curiosity requires

The terms "adiabatic" / "adiabatic equation" are used quite frequently, especially w.r.t. earth bond csa calculations.

Oxford English Dictionary defines adiabatic as "relating to or denoting a process or condition in which heat does not enter of leave the system concerned", Wiki states "In thermodynamics, an adiabatic process is a conversion that occurs without input or release of heat within a system."

So how has this term come to be used w.r.t. earth bonding?

Like I say, just curiosity...

 

[telectrix]

it's basically to do with the cable/s withstanding a certain level of current without overheating to the detriment of the cable. it's not exactly a correct use of the word adiabatic, but ......

 

[rfears]

it's basically to do with the cable/s withstanding a certain level of current without overheating to the detriment of the cable. it's not exactly a correct use of the word adiabatic, but ......

Thanks Tel. Do you happen to know if the word is mentioned in BGB, or is just (mis)usage within the industry?

 

[telectrix]

look at 543.1.3 the bs7671 refers to the equation as an adiabatic equation in that reg. however, it's not exactly accurate, as the term adiabatic means " WITHOUT LOSS OR GAIN OF HEAT"

 

[rfears]

look at 543.1.3 the bs7671 refers to the equation as an adiabatic equation in that reg. however, it's not exactly accurate, as the term adiabatic means " WITHOUT LOSS OR GAIN OF HEAT"

Aren't language and physics wonderfully flexible things? Lead to things like "(2) As may be necessary to prevent danger, all systems shall be maintained so as to prevent, sofar as is reasonably practicable, such danger. ". You've probably seen that sentence before somewhere...:28:

Thanks for the information - Rick.