adiabatic on earth bonding

[telectrix]

min size for bonding conductors on TNC-S is 10mm.

 

[malcolmsanford]

i=p/u read regulation 544.1.1 it will reveal the lesson

 

[Engineer54]

Shut up you lol ....................All this talk of Sunday Roasts, I have a chicken Swarma for my tea

And very nice they are and cheap too!! lol!!

I see your saving the pennies up for your next trip home then...lol!!

 

[brman]

As Tel said by using the 115 value your not going wrong as your erring on the side of caution, all your going to do is get a higher CSA value which in anything under 16mm is not going to be that expensive. Once your starting to go over that and it's running 60-70 metres then it can become an expensive mistake

In your scenario I would not consider a single earth conductor taped to a single SWA as "bunched". if there were 6/7 SWA and 6/7 Earth conductors all taped together and run together, then to me that would be "bunched", and I would use start to use table 54.3 and the lower value for safety.

Ok. thanks, makes sense.
Actually I started all this to decide whether an installation I've looked at is ok. 10mm2 main earth and 4mm2 main bonding.
I think I can now safely say (pending a proper check of the Ze and fuse type) the 10mm2 main earth is fine but the main bonding could do with replacing, particularly as it magically turns from 4mm2 at the MET to 10mm2 at the water pipe! :-O

Thanks to all the other comments too, I am struggling to keep up with you all but I think it makes sense to me now.

 

[i=p/u]

so 6mm for TN-S and TT as minimum and 10mm for TN-C-S.. thats me lesson.. i know this , think i need a holiday, info overload

 

[telectrix]

TT can be as low as 2.5mm if protected. i need a holiday as well. was thinking of ireland this year, never been.

 

[i=p/u]

just uproot and move here, im sure my scottish lady would take your house/home, swap

 

[i=p/u]

and see when you say a TT can be as low as 2.5mm (protected against mechanical damage)

is on the surface then going under the floorboards of a house covering mechanical damage in your mind..

the only tt instalation ive seen this is how it goes and i reckon is in 2.5mm

 

[pencilpusher]

In a way, you do use the fuse trip current.
If you had a PEFC of 500A, with a BS1361 fuse, you would either have to use the values in the table (460A and 5s), or determine from the graph at what the actual disconnection time would be (about 1.5s).
Where people often go wrong, is they use the time value from the table, along with the measured value of PEFC. This results in a larger CSA than is necessary.

Spin - I was thinking about this myself but cannot find a graph in Appx 3 for a BS 1361 fuse. Am I going nuts?

For example, on my TNCS I have a Ze of 0.22 ohms. 230/0.22 = 1045 amps. Where am I checking my fault current against in the tables?

PP

 

[Guest123]

Spin - I was thinking about this myself but cannot find a graph in Appx 3 for a BS 1361 fuse. Am I going nuts?

PP

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:

 

[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!