After some advise please- Tn-s earth arrangement

[Jimmer84]

Evening all,

Just after some info on the photo attached. It’s a tn-s system with an Earth going to the cu met from under the floor boards. After lifting the boards I found the earth attached as so... I was expecting to see a clamp around the supply cable, not a choc block with legs of unknown origin attached. Can anyone enlighten me on what’s going on here? Thanks

 

[Deleted member 26818]

Guidance notes are written by the EIT to help you interpret the Regs correctly and the fact that people do a Regs course, usually at a college that simply prepares you to pass, doesn't mean that you are not responsible for studying it further.

How would you have managed 2391,2,4,5 or 6 without GN 3? Do you ever read it or quote it here, or just use the OSG like any domestic install, r?

Just because people can't be asked to study doesn't mean they can substitute opinion for fact.

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.

 

[oracle]

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.

OK, point of order. All the books published by the IET, including BS7671, are written by the contributions from the members.

That said, the IET seal of approval is on these guidance books.

My understanding of main protective bonding is that it must be >=10mm, resistance not >0.05 Ohms, that it is run from the MET or Earthing bar in a CCU to extraneous and/or exposed conductive parts.

Where there is a connection between the MET and earthing bar it is normally provided from the MET to the Earthing bar it is usually to minimise the number of 10 mm conductors into the CCU.

The Earthing conductor is from the Earth electrode or conductor to the MET. This can be seen in Fig 2.1.

In your example, these are interconnections not joins in the length of the conductor so comply.

 

[oracle]

You don't mind perusing the Best Practice Guides though do you?

Not at all, I value the guidance that the best practice guides and other reputable books give. The declaration in the model forms uses "to the best of my knowledge and belief".

The reason I contribute to the debates is that my knowledge may be lacking in certain things and wrong in certain beliefs and I would rather be proven wrong here than in court.

 

[oracle]

What do you think ?

 

[telectrix]

My understanding of main protective bonding is that it must be >=10mm, resistance not >0.05 Ohms,

that is wrong.the 0.05ohm relates to the resistance between a bonding cable and the pipe to which it is attached.nothing to do with the resistance of the cable itself.

 

[oracle]

By way of an example... consider an SWA supply from the meter to a consumer unit some way away. There are various joins in that connection, this is acceptable. I find it hard to believe a correctly made join (say a crimp or a service terminal block) would not comply.

Have a look at Fig 2.1 .The local earthing arrangements may include interconnections between components like Meters, CCU's and MET's, but should not be joined in their length.

 

[oracle]

My understanding of main protective bonding is that it must be >=10mm, resistance not >0.05 Ohms,

that is wrong.the 0.05ohm relates to the resistance between a bonding cable and the pipe to which it is attached.nothing to do with the resistance of the cable itself.

The Regs don't give a maximum length of the main protective bond. One way in which the maximum length can be determined is to use the figure of 0.05Ω given in IET GN 3, 2.6.5. This gives the maximum resistance of a main protective bonding conductor measured from end-to-end when carrying out test method R2 on the bond.

 

[westward10]

Not at all, I value the guidance that the best practice guides and other reputable books give. The declaration in the model forms uses "to the best of my knowledge and belief".

The reason I contribute to the debates is that my knowledge may be lacking in certain things and wrong in certain beliefs and I would rather be proven wrong here than in court.

I wasn't aiming that at you. It was for someone who disregards the Guidance Notes in this yet happily makes reference to the Best Practice Guides in another thread.

 

[Octopus]

I wasn't aiming that at you. It was for someone who disregards the Guidance Notes in this yet happily makes reference to the Best Practice Guides in another thread.

Having a pop at me matey .................

The Best Practice Guides are simply that ............. in fact No 4 which I referred to is the only publication I'm aware of that gives some reasonably decent examples of the codes ...

Did I actually say I disregard the Guidance notes ......... no ............ I stated that they are not compulsory reading or owning and thus an NICEIC can't use them to give you a "black" mark on an assessment ..

 

[oracle]

If the Regs were statutory, written in English instead of Committee, we would be able to follow them properly without needing supportive documents.

Other countries, even in the so called 3rd World, get this right. Well, it's been interesting, but I think I'll go and have a nice lie down now.

 

[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.