6mm Earth and Main protective bonding conductors

[Michaelwgroves]

Just been to a 1981 TN-C-S installation to fit new CU. Both Earth and Main protective bonding conductors are 6mm, which I understand was typical back then. 16mm tails with 60A DNO fuse.
Upgrading earth and bonding is gonna be tough as run up cavity and round the corner, I ain't never gonna pull a new cable through without some demolition.
There is no sign of over heating and all my Zs are well within tolerance.

Is there anyway of proving this is okay, or am I wasting my breath?

 

[buzzlightyear]

Just been to a 1981 TN-C-S installation to fit new CU. Both Earth and Main protective bonding conductors are 6mm, which I understand was typical back then. 16mm tails with 60A DNO fuse.

25mm tails
16 mm main earth terminal
10mm gas &water
..................................
16mm tails
10mm main earth terminal
10mm gas &water
.....................................
and so one or calculate it .

 

[Andy78]

I'd upgrade the earthing conductor to 10 or 16mm and leave the bonding conductors as they are having proven continuity and a suitable method of connection.

 

[Ian1981]

The size of the main earthing conductor can be calculated using S=I2t square root/ K
Were I = the fault current
t= time for disconnection of protective device i.e. The cut out fuse in relation to the fault current
K= copper conductor either part or bunched with a cable (typically a value of 115) or separate (value of 143 for copper conductors max operating temperature of 70 degrees)

Bonding conductors can't be calculated in that respect and must be selected accordingly as per tables in chapter 5 in relation with the DNO supply neutral, not the CSA of the neutral that connects to the meter say.
If the bonding conductors show no signs of thermal damage and have been in place since they where installed with continuity then they maybe deemed adequate.
I would always try to upgrade but I appreciate it's not always that easy.

 

[Wilko]

Check Reg 542.3.1 which supports Ian's view, but also refers us onto 544.1.1 for PME and onto Table 54.8 where 10mm is minimum.

 

[Andy78]

The current best practice guides suggest a minimum of 6mm bonding conductors, if already installed and showing no signs of thermal damage, to be deemed as adequate and fulfilling the requirements of 132.16, regardless of earthing arrangement.
That'll do for me too.

 

[westward10]

Why would they show signs of thermal damage they probably haven't been put to the test yet, never say never I would change them to 10.0 if that proved to be the required size.

 

[Ian1981]

How do you know they haven't been put to the test as you say it?
Diverted neutral currents can still flow in the bonding conductors all be it rather low

 

[Devonchris]

This may be off interest to you?

The Electrical Safety First best practice guide no 1. Changing a Consumer Unit.

States that 6mm main earth and bonding conductors don't need to be changed when "they have been in place for a significant time and show no sign of thermal damage".

A pdf version is downloadable from their website.

 

[Ian1981]

Oh god you've done it now!!!

 

[Wilko]

Always happy to learn
My read is PME min earthing conductor is 10mm. Of course I may have misunderstood / misread BYB (?)

 

[buzzlightyear]

Oh god you've done it now

now the flood gates will open!

 

[Ian1981]

Older versions of bs7671 may not comply to today's standard but doesn't mean they are unsafe for continued use.

That is written near the front of the current edition of bs 7671

 

[Andy78]

Older versions of bs7671 may not comply to today's standard but doesn't mean they are unsafe for continued use.

That is written near the front of the current edition of bs 7671

Exactly.
The requirements of BS7671 are applied to new installations. For alterations and additions to existing installations see 132.16 regarding earthing and bonding.

 

[Risteard]

Older versions of bs7671 may not comply to today's standard but doesn't mean they are unsafe for continued use.

That is written near the front of the current edition of bs 7671

Not necessarily unsafe. In other words, non-compliances which at one time complied may or may not be unsafe and require proper consideration.

 

[westward10]

This may be off interest to you?

The Electrical Safety First best practice guide no 1. Changing a Consumer Unit.

States that 6mm main earth and bonding conductors don't need to be changed when "they have been in place for a significant time and show no sign of thermal damage".

A pdf version is downloadable from their website.

End of discussion, we have it it black and white from the idiots guide to electrical work.

 

[Ian1981]

An idiots guide put together by members of the iet, niceic, napit, beama, city and guilds,EAL, select,BSI
Yep all idiots with no electrical knowledge or background

 

[Andy78]

End of discussion, we have it it black and white from the idiots guide to electrical work.

The particular idiots guide you mention is endorsed by the same organisations involved with writing the regulations that you said you would blindly follow regardless of new or existing installation.
Is the regs an idiots guide of higher calibre ?

 

[buzzlightyear]

http://electrical.----------/wiring-matters/16/earthing-questions.cfm?type=pdf
may be this will help.

 

[Octopus]

The BYB and OSG are there to confuse the masses

 

[westward10]

An idiots guide put together by members of the iet, niceic, napit, beama, city and guilds,EAL, select,BSI
Yep all idiots with no electrical knowledge or background

It is not relevant who put it together as it obviously not done by idiots, my views on it don't change.

 

[Ian1981]

It is not relevant who put it together as it obviously not done by idiots, my views on it don't change.

That's cool.
I meant no offence

 

[westward10]

That's cool.
I meant no offence

No offence taken Ian

 

[Gavin John Hyde]

Given BS7671 is not a statutory document or law but guidance can't we use our 'competent' knowledge and skill to make a informed decision to list all the departures and do as we see appropriate in the circumstances... after all there was a case recently where some office of officialdom took a case to court where a cowboy spark had been doing notifiable work and it went a bit wrong... the judge refused to allow the big yellow book to be used in the case as he said it has no legal basis its not a statutory document that must be complied with.

 

[westward10]

What did the judge allow.

 

[Gavin John Hyde]

@westward10 sorry case was actually a few years back now not so recent, but principle applies it seems..... British Standards/IEE Regulations and the Judicial System - Letter to the Editor - https://www.voltimum.co.uk/articles/british-standardsiee-regulations-and

 

[Michaelwgroves]

This may be off interest to you?

The Electrical Safety First best practice guide no 1. Changing a Consumer Unit.

States that 6mm main earth and bonding conductors don't need to be changed when "they have been in place for a significant time and show no sign of thermal damage".

A pdf version is downloadable from their website.

Good find, so it says;
A 6mm or 10mm earthing conductor used with an associated line conductor of 25mm could be considered adequate if the requirements of regulation 543.1.3 are met.

This is the good old adiabatic equation. I've tried to do this before but get stuck, anyone know how to do this?

 

[Ian1981]

Good find, so it says;
A 6mm or 10mm earthing conductor used with an associated line conductor of 25mm could be considered adequate if the requirements of regulation 543.1.3 are met.

This is the good old adiabatic equation. I've tried to do this before but get stuck, anyone know how to do this?

I've given you the formula from my first post or have you just ignored it?

 

[Michaelwgroves]

These are my figures (I think);
S=6 (6mm earth cable)
k=143 (70 degree thermoplastic)
t=0.03 (30ms)
I2=1.08 (Prospective fault current)

As a starting pint does this look correct?

 

[Ian1981]

These are my figures (I think);
S=6 (6mm earth cable)
k=143 (70 degree thermoplastic)
t=0.03 (30ms)
I2=1.08 (Prospective fault current)

As a starting pint does this look correct?

S is what you are trying to find.
It's not the 6mm already in place.
First find the fault current which by measuring ze , can be calculated.
So a ze of 0.10 gives a fault current of 2300 amps
I2 so 2300 x2300=
Then multiply by the disconnection times given for the OCPD in the cut out using appendix 3 tables.
If below 0.1 seconds the let through energy figure is to be used given by the manufacturer.
Square root the answer and divide by say 143 if the earthing conductor is a separate cable and not bunched with the live and neutral 'tails'
This will give you S which is your required CSA of the earthing conductor for compliance

In this example scenario ,Using a disconnect time of 0-1 seconds and a fault current of 2.3ka will give a CSA of 5mm rounded off so 6mm is adequate

 

[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