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.