Ra greater than 200 ohms on TT system - what classification code (also main bond incorrect)

[Walter Leach]

I have done an EICR today on a small office. Its a single phase install across two Consumer Units/Db's, TT system to a single earth rod. It was rewired in 2005 and I have the original NICEIC install certificate to reference.

It was signed off with an Ra of 297ohms. I'm getting 277 ohms and that's with the ground saturated after all this rain. I reckon it should not have been signed off and plan a C2.

Further it has been signed off as having 10mm main bond to the incoming water and gas services. Actually there is just one 10mm bond with the connection made at to the water and gas pipes by the combi boiler which is at the opposite end of the premises to the incoming services and there is no earth connection at the entry point to either service. Another C2 I reckon. I do get 0.34ohms if I use a wander lead to measure from the DB to the point of entry on the incoming services.

Does anybody strongly disagree? Why ask? Because I have a sense that when I submit my report the client will involve the original contractor so I want to be sure of my ground.

As a PS: The Final circuit Zs readings for the install cert range from 1.5ohms to 6.5 ohms where all my readings are in the order of 170 ohms and there are other anomalies that I wont bore you with which is a shame because apart from the earthing above the workmanship and general install on the final circuit side is very good.

 

[SPECIAL LOCATION]

As a PS: The Final circuit Zs readings for the install cert range from 1.5ohms to 6.5 ohms where all my readings are in the order of 170 ohms and there are other anomalies that I wont bore you with which is a shame because apart from the earthing above the workmanship and general install on the final circuit side is very good.

As an observation on this point...

Zs is taken with all bonding in place...

it may be that some incoming water or gas services have been changed form metal to plastic since 2005?

thus the Zs has less of a good parallel metal water pipe??

:|

 

[sparkytim]

And 200 ohms is only a recomendation not a requirement.

 

[Walter Leach]

And 200 ohms is only a recomendation not a requirement.

agreed but the recommendation is there for the reason that the connection can be unstable if over 200 AND as the conditions are excellent at the moment then I can only see it getting higher not lower.

---------- Post Auto-Merged at 17:46 ---------- Previous post was made at 17:39 ----------

As an observation on this point...Zs is taken with all bonding in place...

it may be that some incoming water or gas services have been changed form metal to plastic since 2005?

thus the Zs has less of a good parallel metal water pipe??

:|

That also affects why the main bond should be at the incoming point and not a remote location as it would be ineffective if plastic is between the connection and the incoming location. I'm not aware of any plastic but did an earth reading from the DB to the incoming points (which are adjacent so I get virtually the same reading). 0.17ohms with the bond connected, 0.34 with the bond disconnected.

If you were the original contractor would that be your best defence?

That's not intended as hostile response - I appreciate the input..

 

[SPECIAL LOCATION]

agreed but the recommendation is there for the reason that the connection can be unstable if over 200 AND as the conditions are excellent at the moment then I can only see it getting higher not lower.---------- Post Auto-Merged at 17:46 ---------- Previous post was made at 17:39 ----------

That also affects why the main bond should be at the incoming point and not a remote location as it would be ineffective if plastic is between the connection and the incoming location. I'm not aware of any plastic but did an earth reading from the DB to the incoming points (which are adjacent so I get virtually the same reading). 0.17ohms with the bond connected, 0.34 with the bond disconnected.

If you were the original contractor would that be your best defence?

That's not intended as hostile response - I appreciate the input..

Zs's Just trying to think of possible causes for the difference in readings..?

Ra:

It was signed off with an Ra of 297ohms. I'm getting 277 ohms

297ohms in 2005... 277ohms in 2012

20ohms variation over 7 years?

It could be argued that it is reasonably stable...

Personally I don't think I would have left a main installation 200ohms+

{I have left a shed with a highish Ra..

cuz the economics of more rods for an 8x14 wooden shed with no metal work was pointless!}

But can you prove it is dangerous?

:C

 

[Andy™]

agreed but the recommendation is there for the reason that the connection can be unstable if over 200 AND as the conditions are excellent at the moment then I can only see it getting higher not lower.

its hardly unstable if the readings is almost the same (well, better actually) after 5 years. and if using 30mA RCD's, then id guess your Zs is well under the max Zs of 1667 ohms. so on that point, i dont think you have anything to complain about.

 

[Walter Leach]

But can you prove it is dangerous?

:C

Probably not - its a long way off 1667 ohms.

Its a place of work for 20 people rather than a shed so some difference there.

 

[SLIPSHOD & SLAPDASH]

As others said, it seems a very stable reading to me and well below the maximum so not much to worry about. Remember, the bonding connections can be made as near as practical if not possible within 600mm of the stop ***** etc

 

[Doc Hudson]

If you are concerned about the consistency of the Ra over other weather conditions. Why not put a suitable comment on the EICR with a next inspection in 6months for your own assurance. Include the cost of a return visit in your bill for this work and advise the customer accordingly.

Doc H.

 

[batty]

agreed but the recommendation is there for the reason that the connection can be unstable if over 200 AND as the conditions are excellent at the moment then I can only see it getting higher not lower.---------- Post Auto-Merged at 17:46 ---------- Previous post was made at 17:39 ----------

That also affects why the main bond should be at the incoming point and not a remote location as it would be ineffective if plastic is between the connection and the incoming location. I'm not aware of any plastic but did an earth reading from the DB to the incoming points (which are adjacent so I get virtually the same reading). 0.17ohms with the bond connected, 0.34 with the bond disconnected.

If you were the original contractor would that be your best defence?

That's not intended as hostile response - I appreciate the input..

I am finding this all very confusing the.17 ohm and

.34 ohm readings are what?

 

[m4tty]

I am finding this all very confusing the.17 ohm and .34 ohm readings are what?

From the board to the gas meter I think mate

 

[batty]

So the length of the earth conductor what's that got to do with anything?

 

[Walter Leach]

From the board to the gas meter I think mate

Correct. The Gas and Water enter the building side by side so there is very little difference in the readings back to the DB. Just proving there is continuity and no plastic along the way.

---------- Post Auto-Merged at 07:13 ---------- Previous post was made at 07:03 ----------

Thanks for all input.

Regarding the Ze - suggesting a re-visit in 6 months is a good idea to which the customer would agree - I think.

Regarding the Main Bonding - there is no reason why the connections were not made at the incoming point at the time of rewire as the building was totally renovated. Its not that hard now but it will have to be run in conduit along the external wall rather than under the floor.

Borderline C2 or C3 but overall I cant see it as Satisfactory.

 

[Adrian]

c3 for me

 

[steptoe]

that's with the ground saturated after all this rain

what difference would that make?

water is NOT a good conductor, the other point is, a bit of rain in your back garden is NOT going to affect the resistance of the ground back to star point.

It was signed off with an Ra of 297. I'm getting 277

Regarding the Ze

so what is it?

Ra or Ze ?

or are you comparing two different measurements and making a statement that they are comparable?

before you start this report you need to decide on what you are

A - measuring

B - coding

because if you start doing stuff like that then the original installer is going to laugh his head off and show you up as being incompetent,

you are trying to compare two different measurements, thats NOT a good start.

have you worked on many TT systems before?

 

[Walter Leach]

Yes I've worked on plenty of TT systems.

The post has always stated Ra apart from 1 instance above.

More interesting though is referring back to the regs table 41.5 specifically refers to Zs readings not Ze or Ra.

As all my Zs are in the order of 170ohms the requirements are in fact met and the Ra is not an issue.