After some advise please- Tn-s earth arrangement

[Pete999]

No the GNs are not written by the EIT, or even the IET.
They are written by members, sometimes with input from other members, and are the opinion or interpretation of that member.
Yes, I have supplied input in the past for a GN3 (think it was the last time it was re-written).
I rarely read any of the GNs, and only read the OSG when looking for specific examples of errors.

According to your interpretation, connecting bonding conductors to an earth bar in a CU, then running a main earth to the main earth terminal in a PME cut-out would not comply.

When you say GNs are"written by Members" is that members of the IET? and then published by the IET?

 

[telectrix]

this is from IET. clearly says that the 0.05 advised is considered to be a negligible resistance between 2 service pipes ( e.g. gas and water ). nothing to do with the bonding conductors, although their resistance would have an effect on the value measured pipe to pipe ( as they are connected via the MET ).

My understanding is that in general with a TN installation at 230 Volts we need a disconnection time of 0.4 seconds to implement effective ADS, therefore any bonding needs to be sized as required by Regulation group 544, and as such there is no restriction upon length, this is due to the fact we have no limit on touch voltage assuming we meet the prerequisite of ADS.
The only limit I can see on main protective bonding is that of 415.2.2, this is for additional protection and is used in locations of increased electric shock. This is shown in Regulation 701.415.2, where we check the effectiveness of the main protective bonding utilizing 415.2.2.
There is a passage in GN3 related to Continuity of Protective Conductors including main and supplementary bonding Test Method 2, in my opinion the 0.05 ohms is clearly a “ball –park” value for measuring between two extraneous conductive parts to confirm a valid bonding connection, and not to be applied to limit the overall length of the bonding conductor.
I’ve checked in GN8, GN5 and BS7430 and I can see no limitation other than CSA or when additional protection is required.
I’ve also spoken to ECA and they are of a similar mind, that in general no limit is placed on the length of main protective bonding.

 

[Wilko]

I'm also of the view that protective bonding conductors end to end resistance is not restricted to 0.05 Ohms. I can't find a direct reg and I don't think it's needed to keep touch voltages down.

For fun, imagine an earth fault current (in the 400ms max before ADS has triggered) travelling back to MET and then transformer. The MET rises in voltage due to the external R2 which might be 0.3 Ohm (say). Our bonding conductor connects MET to the incoming water pipe to keep its voltage about the same as the MET. The water pipe is also connected to Earth at some point and so will take some fault current. But it's path is likely to be much more resistive, maybe a "TT like" 30 Ohms (say). So as long as the bonding conductor resistance is small compared with this value then it will still work to minimise the touch voltages.

Happy for feedback - as I may be barking

 

[johnduffell]

Earthing and Bonding (to the CCU) must be continuous throughout its length. No joints or connections.

I gave you an optimistic because i don't think the nest of bare cables on a chock block is really a connection!
Not to mention that even if it were, a chock block is not a reliable connection method floating around unrestrained under the floor. Need to prevent strain on the termination in all cases anyway.
I guess it's all bad would be a good summary here!

 

[Deleted member 26818]

When you say GNs are"written by Members" is that members of the IET? and then published by the IET?

Yes.
Some staff as well.

 

[Pete999]

Yes.
Some staff as well.

Whay Forum Staff or IET staff?

 

[Pete999]

Yes.
Some staff as well.

What Forum Staff or IET staff? if it's the later then surely GNs are, if not written by the IET at least the GNs are sanctioned by the IET, so in effect they might as well have written them in the first place, don't you think?

 

[Deleted member 26818]

Whay Forum Staff or IET staff?

IET staff.
Geoff Croneshaw springs to mind.
He’s The Chief Engineer in the IET, think he wrote GN6 and 7?

 

[Silly Sausage]

What Forum Staff or IET staff? if it's the later then surely GNs are, if not written by the IET at least the GNs are sanctioned by the IET, so in effect they might as well have written them in the first place, don't you think?

They're published by the IET, so you would have thought so.
I've always taken them as Electrical Gospel.

 

[Pete999]

IET staff.
Geoff Croneshaw springs to mind.
He’s The Chief Engineer in the IET, think he wrote GN6 and 7?

So in Effect, written by the IET?

 

[Deleted member 26818]

What Forum Staff or IET staff? if it's the later then surely GNs are, if not written by the IET at least the GNs are sanctioned by the IET, so in effect they might as well have written them in the first place, don't you think?

Nope, just the author’s opinion.
Not actually had any disagreements with Geoff, but have with some other staff members over their interpretations of the Regs.

 

[DefyG]

Its not really clear from the photo if its an attempt to use an incoming iron water pipe as a main earth or an attempt at main bonding.
Whatever it is and arguments over continuous earthing/bonding conductors aside it needs correcting!

 

[Jimmer84]

Its not really clear from the photo if its an attempt to use an incoming iron water pipe as a main earth or an attempt at main bonding.
Whatever it is and arguments over continuous earthing/bonding conductors aside it needs correcting!

Cheers mate, I used a 1200 rod and TTd it. Got a nice reading of 37.8ohms. New 10mm ran to gas for bond, water already in place