TNS DNO earth Braid

[davesparks]

Draaat, I typed out the title of the table and a detailed response.
then when going to post I was logged out. :ack2:

I edited it, the title does to say what I though it did. Read the regulation carefully again then come back to us

 

[newtothis]

Re-read, I maintain It supports upgrading.
What have I missed ?

 

[Simonslimline]

What size main earthing conductor does the installation have/require?

 

[davesparks]

Re-read, I maintain It supports upgrading.
What have I missed ?

With the exception of PME supplies the main bonding need not be bigger than half the required size of the earthing conductor, subject to a minimum of 6mm[SUP]2[/SUP] and a maximum of 25mm[SUP]2[/SUP]. Where the supply is PME the bonding size is based on the supply neutral conductor (the one in the service cable, NOT the tails, many people miss this point) and you need to refer to the table you mention above.

You supply is clearly TNS so to assess the required size of main bond you need to calculate the required size of earthing conductor. For this we need to know the prospective fault current Ipf, which can be calculated from Ze and U (for a worst case scenario i'll use 250V) using I=V/R
So
Ipf = 250/0.29
Ipf = 879A
I'll round it up to 900A for simplicity
We also need the disconnection time from appendix 3, which gives us 0.6seconds for 900A
And a value of K of 143 from table 54.2

So for the adiabatic equation we have values of
I = 900A
t = 0.6S
k = 143

Which gives a minimum CSA of 4.88mm[SUP]2[/SUP], the nearest standard size conductor above this is 6mm[SUP]2[/SUP] so you require an earthing conductor of 6mm[SUP]2 [/SUP]
So the size of your main bonds will be governed by the minimum requirement of 6mm[SUP]2

[/SUP]Hopefully this will make sense!

 

[newtothis]

Thanks for the detailed reply. I’m still confused.
I’m re-posting the pic labelled up to show what my current mis / understanding is.


You use the adiabatic equation to calculate the size of the earthing conductor.
From the result of that I use a table to see what CSA to use for the main bonding

What size main earthing conductor does the installation have/require?
​

I think the braid is my earthing conductor and installed by the DNO and as it’s a braid I Haven’t a clue what the CCC is or how to go about finding out. I know if the Ze was above 0.80 I’d have a problem.

 

[Richard Burns]

If this table comes out this may give you an idea of the cross sectional area of the braid.
This is a Cabletec table but will be similar for most standard flat braids.
Product Details: Standard Flat Braid

Part Number	Cross-sectional area (mm[SUP]2[/SUP])	Width x depth (mm)	Current rating (amps)
F B-02.5-X-15	2.5	6.0 x 0.8	34
FB-04.0-X-15	4.0	8.0x 1.0	53
F B-06.0-X-15	6.0	10.0 x 1.0	69
FB-10.0-X-15	10.0	13.0 x 1.3	97
FB.16.0-X.15	16.0	19.0 x 1.5	132
F8-25.0-X-15	25.0	23.0 x 2.0	178
F B-35.0-X-15	35.0	25.0 x 3.0	223
F13-50.0-X-15	50.0	20.0 x 5.8	282

Note: Current ratings are based on temperature rise of 50°C above ambient.

 

[Simonslimline]

If your Zs values comply then nothing to worry about on that side of things regarding your comment on the Ze.

Is that GY out the top of the MET 10mm2 if so then your well within the 6.0mm2 required for the installation. The braid to the sheath is the DNO's property so not yours to worry about mate.

The green and yellow is your main earthing conductor IMO, some may not agree with this though.

 

[newtothis]

Thanks mate,
From that table looks like the braid is 6mm csa.
I'm used to seeing 16mm in tn-s and t-n-cs, then again I haven't got much experience.

 

[newtothis]

Going to set a new world record for testing an installation slowly - a week on I haven't got past the.
Earthing and bonding :93:

 

[davesparks]

Thanks for the detailed reply. I’m still confused.
I’m re-posting the pic labelled up to show what my current mis / understanding is.
View attachment 29889

You use the adiabatic equation to calculate the size of the earthing conductor.
From the result of that I use a table to see what CSA to use for the main bonding

I think the braid is my earthing conductor and installed by the DNO and as it’s a braid I Haven’t a clue what the CCC is or how to go about finding out. I know if the Ze was above 0.80 I’d have a problem.

You don't have an earthing conductor there technically, the braid is the DNOs means of earthing which terminates directly into your MET.
The bonds and cpc to the CU then connect to that MET.

In TNS you don't use a table to get your bonding. Your main bonds need to be half the size required for the earthing conductor but no smaller than 6mm and no larger than 25mm.

 

[Simonslimline]

You don't have an earthing conductor there technically, the braid is the DNOs means of earthing which terminates directly into your MET.
The bonds and cpc to the CU then connect to that MET.

In TNS you don't use a table to get your bonding. Your main bonds need to be half the size required for the earthing conductor but no smaller than 6mm and no larger than 25mm.

Would it not be the case that the GY entering the top of the MET is the MEC and is connecting all the exposed conductive and extraneous conductive parts to the means of earthing, ie the DNO braid in this instance.

 

[davesparks]

Would it not be the case that the GY entering the top of the MET is the MEC and is connecting all the exposed conductive and extraneous conductive parts to the means of earthing, ie the DNO braid in this instance.

MEC?

The earthing conductor is the consumers conductor which connects the MET to the means of earthing. In this case the distributors means of earthing connects directly to the MET.
Any installation can only have one earthing conductor, just as there can be only one MET

 

[Simonslimline]

Ok, which would you put down on an EIC as the earthing conductor, the GY or the braid?

I meant main earthing conductor. Got my terminology slightly mixed for a moment there!

 

[davesparks]

Ok, which would you put down on an EIC as the earthing conductor, the GY or the braid?

I meant main earthing conductor. Got my terminology slightly mixed for a moment there!

Neither, I'd record it as N/A with an explanatory note.

I know what you meant, it was just a polite nudge that that term doesn't exist in 7671, it's just the earthing conductor.

 

[Simonslimline]

Neither, I'd record it as N/A with an explanatory note.

I know what you meant, it was just a polite nudge that that term doesn't exist in 7671, it's just the earthing conductor.

Cool thanks for pointing that out. I was getting mixed up with Main protective bonding and earthing for a minute there.