Tt Zs Query Re Main Bonding Main Earthing

[Wizbit]

Hi, grateful for any thoughts..my brain is slowing..

scenario = House with TT..doing EICR before possible major updates. (To give you an idea of the condition the house does not even have RCD fault protection, and what was acting as a Voltage ELCB has been at some point disconnected from any earthing route and is now simply acting as an isolation switch!) I digress..

Got an Ra of around 80ohms..condition of rod (if it is even a rod...unknown for now as it looks as if its now lies under an extension.) Main Bonding to Gas, but none to Water that I can yet find. Example of a Zs on the Ring Main was 2.2ohms (BS 3871 type 1) obviously too big but fine for a TT IF they had RCD. Being a newbie to TT systems I thought I'd try a Z's with the gas bonding disconnected..result >80ohms. This was the same on all circuits. 

Main bonding naturally provides some parallel earth in any system, but this difference would suggest to me that in the event of a fault current wanting to find the least resistance to earth that any earth fault in the house would always go down the Gas bonding?..surely effectively making the gas supply the main earthing?.Ie a no no! 

I guess it must be the same on all TT systems that have an Ra above a few ohms?. but any thoughts would be most.. :Welcome:

 

[Technician]

Only a suggestion but if the electrode is under an extension,

you may not have any idea of its age and therefore its

condition.  I would respectfully suggest that you try to find

out what condition it is in because, (at the moment at least)

you do not know if it is an effective earthing arrangement.

HTH before you plan anything else.

Drain swamp to locate positions and numbers of crocodiles

BEFORE wading into swamp.

 

[Wizbit]

Oh don't worry technician..those joys are all to come!..yep the first thing I'm going to be doing is tracing the Rod and ensuring that's all happy and stable before starting any work..mind you .it wouldn't surprise me seeing everything else  if its actually going down to the incoming water.. It's a 1950s estate all on TT. My Q is just a wondering type..it seems to me that if the gas bonding is doing that much of a job of taking the Zs down that however respectable an Ra I manage to get in the end, a fault is always going to end up choosing the gas pipe?..or am I over thinking on a Weds morning! :Salute

 

[SPECIAL LOCATION]

Your observation about the actual real route any electricity will take in the event of a fault is correct......

i.e. if the least resistive path is a water pipe, gas pipe, structural steel, human body.. etc.. etc..

then by ohms law the majority of the current will flow down that path...

BUT..

It is not the direction that the current flows that defines what is what you call the "main earth"....

The Main earth terminal will always still be the point at which protective conductors, bonding conductors, functional earths and the means of earthing (e.g. electrode / sheath / combined neutral+earth).are connected.

I think you are mistaking the concept that any incoming service pipes must not be relied on solely as the means of earthing..

As if at a later date the supply pipes are changed to plastic, then the installation would have no effective means of earthing.

Under all fault conditions you have no control over how the electricity flows once it is traveling though the earth to make its way back to the supply.

 

[Technician]

Wizbit...I have no doubt that you have considered all this.

I came at it from the standpoint of Chapter 54. 

542.2.3 does not include gas bonding within the category

of "suitable earth electrodes".

IIRC the use of water mains as an earthing arrangement

was suitable until the 1960's (Acknowledgements Doc Hudson)

but SL's post says it all.

 

[Steve3948]

The earth rod/mat/tape/connection should be accessible for inspection iirc

 

[Wizbit]

Thanks both. Oh yes, no worries I'll be making sure the rod is sourced and sorted and accessible etc before doing any of the work required  O)  (new CU etc ..there are loads of issues with this house..will keep me off the streets for a while!) 

the reason for my original post was just wanting to check if it was an 'acceptable' situation to have..ie you cannot use a Gas pipe as a means of Earthing, but within a TT it is surely inevitable that however good the Ra is, fault currents will choose the main bonding route as that is such a lower resistance..and I know there's nothing you can do about it and nothing against it Regs-wise..just wondering if I should be bothered by it?

cheers all

 

[Technician]

Just remember that protective bonding is a measure used in

fault protection and that, when conducting an inspection and

test, protective bonding connections are slackened off in order

to remove parallel paths which are provided by protective

bonding, which includes the gas bonding connection.

 

[Wizbit]

Thanks techie.. Yep I guess along with 411.3.1.2 etc that within the whole context of ADS the fact that bonding is just a part of ADS then makes sense to what I was musing on...cool..thanks all!