6mm Earth and Main protective bonding conductors

[Michaelwgroves]

Ian1981, sorry read and forgot your first post, got tied up with bedtime.......

I don't get how you get your 2300 amps, my Ze is 0.22, what calculation do I need to do?

 

[Ian1981]

Ian1981, sorry read and forgot your first post, got tied up with bedtime.......

I don't get how you get your 2300 amps, my Ze is 0.22, what calculation do I need to do?

That was just my example of a ze of 0.1 ohms
A ze of 0.22 is by calculation 230/0.22 =1045 amps
So times 1045x 1045=
Then times that by your fuse disconnect time which I'll take your word as 0.03 without me trying to look it up
If it's a bs1361 fuse 60 amp then your disconnection time will be below 0.1 seconds
Square root the answer then divide by either 115 or 143.
If the answer is below 6mm then your 6mm which is already in place is adequate for the earthing conductor.
Tho why not change it anyway if it's not a chew on?

 

[KEV 1 N]

......Could of had those 10mm2 earths pulled in by now!

 

[Michaelwgroves]

Sorry, lots of hand holding required. It's big job to replace so hoping the figures here work out.

my "t" was wrong, I used RCD being 30ms.

Cut out is BS (EN) 1361 IIa, Appendix 3 does not list this?

 

[KEV 1 N]

It's ok, I realise the situation, was just having a joke!

 

[Ian1981]

Sorry, lots of hand holding required. It's big job to replace so hoping the figures here work out.

my "t" was wrong, I used RCD being 30ms.

Cut out is BS (EN) 1361 IIa, Appendix 3 does not list this?

It will have a 60,80 or 100 amp rated fuse inside.
Thought you said it was a 60 amp to be honest

 

[Michaelwgroves]

Yes it's 60A, but how do I calculate fuse disconnect time?

 

[Ian1981]

Bs 1361 fuses are not listed in bs 7671 17 th edition as they've been superseded by bs 88 fuses but the red version of the 17 th has 1361 fuses still listed in appendix 3.
The disconnection time will be below 0.1 seconds on a 60 amp bs1361 fuse so it's extremely likely that your 6mm earthing conductor is satisfactory

 

[Michaelwgroves]

I don't have the red version. Just checked latest version, it says replaced by BS 88-3
So I'm now on the right table, but don't know how to read it?

 

[Ian1981]

At 0.1 seconds a bs 1361 fuse will open at 880 amps so your fault current of 1045 amps will make the fuse operate below 0.1 seconds
If you used 0.1 seconds as t then I get 2.31 mm required size of earthing conductor so 6mm is fine.
Obviously the higher the disconnection time then the larger the CSA of the earthing conductor will need to be.
But with all this said and done if it can be upgraded then I'd do it anyway tho not easy if it's a ball ache of a job such as an external meter with the consumer unit inside

 

[Michaelwgroves]

So my equation looks like this;

I=230/0.22(Ze) = 1,045
I2=1,045*1,045 = 1,092,025
I2*0.1 (t) = 109,202
Square Rooted = 330
330/143 (70 thermoplastic not bunched) = 2.3

So I only need 2.3 mm earth, something must be wrong?

 

[davesparks]

Ian1981, sorry read and forgot your first post, got tied up with bedtime.......

I don't get how you get your 2300 amps, my Ze is 0.22, what calculation do I need to do?

Yes it's 60A, but how do I calculate fuse disconnect time?

I don't have the red version. Just checked latest version, it says replaced by BS 88-3
So I'm now on the right table, but don't know how to read it?

This is ridiculous!

You don't know how to calculate current, you don't know how to establish fuse disconnection times, you can't read the table once you've found it.
Seriously, stop attempting to do electrical work and hire a competent electrician.

 

[westward10]

Back to #1, I don't think you can ever prove the main protective bonding was adequate at the time and hence that it can be considered safe for continued use. The 15th Edition 1981 never gave a minimum size for main equipotential bonds if PME was employed just that you should consult the supply company, without digging out the book that is from memory so I stand to be corrected.

 

[davesparks]

So my equation looks like this;

I=230/0.22(Ze) = 1,045
I2=1,045*1,045 = 1,092,025
I2*0.1 (t) = 109,202
Square Rooted = 330
330/143 (70 thermoplastic not bunched) = 2.3

So I only need 2.3 mm earth, something must be wrong?

Why must something be wrong? I haven't checked the figures but that is within the ballpark of what you would normally expect for a small supply like this.

 

[Ian1981]

So my equation looks like this;

I=230/0.22(Ze) = 1,045
I2=1,045*1,045 = 1,092,025
I2*0.1 (t) = 109,202
Square Rooted = 330
330/143 (70 thermoplastic not bunched) = 2.3

So I only need 2.3 mm earth, something must be wrong?

No that is correct.

 

[Michaelwgroves]

Why must something be wrong? I haven't checked the figures but that is within the ballpark of what you would normally expect for a small supply like this.

Just seems so small when new installations require 16mm

 

[Ian1981]

Just seems so small when new installations require 16mm

Because everyone used the tables in bs 7671 rather than the calculation
Plus the DNO may stipulate the CSA

 

[Michaelwgroves]

Just so this sinks in, I calculated fault current 1,045A using measured Ze. But I could have equally used the fault current calculated by my meter of 1,080 A ?

 

[davesparks]

Just seems so small when new installations require 16mm

Instead of blindly following a book try actually thinking about what determines the size and what the conductor is required to do.

It is not required to carry the current for any appreciable length of time, it only has to carry it for maybe one tenth of a second.
To take a very simplistic view look at a piece of 60A fuse wire, as long as your earthing conductor is bigger than that it will do its job.

 

[Michaelwgroves]

I like you analogy.