adiabatic on earth bonding

[brman]

I'm trying to get my head around using the adiabatic to calculate minimum size earth conductors and realised I am getting myself confused over which fuses they relate to and how they relate to the tables in the OSG.
This is a theoretical situation, I am just trying to make sure I understand things properly.....

So..... (TN-S system)

1) Main earth conductor => supplier fuse.
So a BS88-2 @ 80A needs 400A for 5s => sqrt(400^2 * 5) / 115 = 7.7 so min csa = 10mm2?
Or should I use k = 143 (table 54.2) and get 6.25, ok still 10mm2.....

2) Main earth bonding => supplier fuse again so same as above.

So how come the OSG (table 4.4, P40) says this depends on the line/neutral conductors? They aren't included in the sums above.

And how come those tables require a larger main earth compared to the earth bonding? What am I missing?

Thinking further, if I have a max Ze of 0.8 my PEFC is only 290A so my BS88 fuse will take more like 20s to go. Should I be worrying about that?

Hmmm... thinking further. if I use my PEFC in the adiabatic I get sqrt(290^2 * 20) / 115 = 11.3, so CSA = 16mm2. Is this what I should actually be using?

Sounds sensible to use the PEFC but it still doesn't explain why the tables relate things to the live conductor size and the tables don't define the supplier fuse type which affects things.

So, I am confused :dizzy2:

 

[malcolmsanford]

First off brman it's the Ze your looking to find as the formula concerning I so if you had a Ze of 0.21 say you would have 230/0.21 = 1095.24amps on the graph then it would line up at 0.2 seconds .

Therefore so your formula would √1095 X 1095 x 0.2 / 143 = 3.4mm and so 4mm would be ample

You would then size your bonding conductor as per reg 544.1.1 for a TN-S and that is half the size of your main earth conductor, but minimum of 6mm. As you would have a 4mm main earth your bonding would need to be 6mm

 

[brman]

Thanks Malcolm! Ok, so I should be using Ze. I think I sort of realised this when typing my post but thanks for confirming it.

Actually, I am feeling slightly embarrassed now - I was using the adiabatic to check final cct cable ratings last night so why I decided this morning to use the fuse trip current instead of the PEFC I don't know.

Ref 544.1.1. Again, I don't know why I missed that. I think the lesson is to not do sums with a hangover

 

[telectrix]

and the values for k are 143 for a single conductor, 115 if bunched or part of a multicore cable.

 

[Engineer54]

As to the thread title, the adiabatic equation doesn't apply to Bonding Cables, only to the main earthing conductor and CPC conductors...

 

[brman]

and the values for k are 143 for a single conductor, 115 if bunched or part of a multicore cable.

Got it. So I guess if the main earth is taped to the neutral tail (as I have seen a few times) I really should be using 115.

 

[brman]

As to the thread title, the adiabatic equation doesn't apply to Bonding Cables, only to the main earthing conductor and CPC conductors...

I think that was one of the things confusing me having not read 544.1.1 properly. I think all is clear now

 

[malcolmsanford]

Got it. So I guess if the main earth is taped to the neutral tail (as I have seen a few times) I really should be using 115.

No mate it would still be classed as not incorporated in a cable ie it's still a single conductor that is bunched with others.

See table 54.2

 

[telectrix]

As to the thread title, the adiabatic equation doesn't apply to Bonding Cables, only to the main earthing conductor and CPC conductors...

n
think the idea on this thread is to calculate S for the main earthing conductor, using adiabatic, then, select the bonding conductor/s as 50% of that as a min.

 

[telectrix]

Got it. So I guess if the main earth is taped to the neutral tail (as I have seen a few times) I really should be using 115.

still a single cable in free air. however, using 115 ain't bad as it would give a greater value for S, this erring on the side of caution .

 

[Engineer54]

n
think the idea on this thread is to calculate S for the main earthing conductor, using adiabatic, then, select the bonding conductor/s as 50% of that as a min.

OK, i'm with you, ...lol!! Must be a little tired tonight.

Surprising how many are not aware of the limitations of using the adiabatic equation though, especially when it comes to Bonding conductors...

 

[telectrix]

of course, in china it's tonight already. here it's not even beer o'clock yet. LOL.:toetap05:

 

[Engineer54]

Sunday Roast, and a glass of wine time there surely? Beer time comes later, after the Sunday afternoon nap time!!! lol!!:drool5:

 

[i=p/u]

you cant selecvt bonding conductors as 50% as a min telectrix????

 

[malcolmsanford]

Sunday Roast, and a glass of wine time there surely? Beer time comes later, after the Sunday afternoon nap time!!! lol!!:drool5:

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

 

[malcolmsanford]

you cant selecvt bonding conductors as 50% as a min telectrix????

Why can't you on a non TNC-S system ??

 

[Deleted member 26818]

Thanks Malcolm! Ok, so I should be using Ze. I think I sort of realised this when typing my post but thanks for confirming it.

Actually, I am feeling slightly embarrassed now - I was using the adiabatic to check final cct cable ratings last night so why I decided this morning to use the fuse trip current instead of the PEFC I don't know.

Ref 544.1.1. Again, I don't know why I missed that. I think the lesson is to not do sums with a hangover

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.

 

[brman]

Ok, still a little confused then.
Table 54.3 "...incorporated in a cable or bunched with cables..."
table 54.2 "...not incorporated in a cable and not bunched with cables"

So isn't a single cable taped to another single cable (tail) counted as "bunched with cables"? and so table 54.3 applies?

There again, on 54.2 it also says "or for a separate bare protective conductor in contact with cable covering but not bunched with cables"
I am struggling to see how it can be in contact with a cable covering but not bunched with a cable.

I assumed it must be 54.3 with the logic that it is affected by the heating of the tail.

Or is my hangover still stopping me thinking straight......

 

[i=p/u]

i cant swnser that with out stuttering and starting again, so i dont know, but i would be greatfull if you teach me a lesson

Why can't you on a non TNC-S system ??

 

[malcolmsanford]

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.