TT Earthing Integrity

[esra_ptrap]

On a TT earthed system is there an easy way to ensure the integrity of the earth if you cannot locate the earthing rod?

The system has a split load CU with a 100mA time delayed RCD protecting the whole installation and a 30mA RCD protecting all 4 ring mains so I'm relatively happy that the installation is safe.

There is what appears to be a 6mm2 earth cable from the earthing block which runs off under the upstairs floor but, as I cannot locate the earthing rod, it would be good to find out if the installation is adequately earthed.

Also, there doesn't appear to be any bonding to either the incoming water and gas pipes at the point at which they enter the property (i.e. where it should be) though I haven't checked for continuity between the CU earth and the pipework so it could be bonded anywhere I guess and without being able to disconnect this (if it is connected) I can't see how I could check the main earth.

How would the experts do it?

 

[plugwash]

you could use a loop impedance tester or a 4 terminal resistance meter with a pair of temporary electrodes (when you are using a rod in a genset only system you have to use the temporary electrode soloution but in a grid supplied install the loop tester is a much easier soloution)

however this will only tell you if the earth is electrically good right now it won't tell you whether its in good condition or a botch job thats about to fall apart

if in doubt best option is probablly to add a new rod or enquire to your suplier about PME conversion.

 

[megawatt]

How would esra rule out earthing through the gas/water main during testing?

 

[plugwash]

it wouldn't thats the problem it would just say that the cable you tested was connected to earth with a certain resistance not how its connected to earth.

 

[plugwash]

well you could find out if a wire connected to your pipework with a resistance meter but you couldn't eliminate it being connected via disused pipework or pipeword that is islolated from your plumbing installation by plastic pipe.

 

[ban-all-sheds]

How would esra rule out earthing through the gas/water main during testing?

Disconnect the main bonding....?

 

[megawatt]

BAS: Esra wrote:

Also, there doesn't appear to be any bonding to either the incoming water and gas pipes at the point at which they enter the property (i.e. where it should be) though I haven't checked for continuity between the CU earth and the pipework so it could be bonded anywhere I guess and without being able to disconnect this (if it is connected) I can't see how I could check the main earth.

I guess you missed this, you need to get to bed earlier

I think esras issue is that there is only a single earthing cable from the CU assumed to be connected to an earthing rod but may be also connected to pipework somewhere other than where it should be or, only connected to gas/water and not connected to an earth rod at all

Other than pulling floor up and tracking the earth cable I can't see an easy way to do this as, if it is connected to incoming water/gas services a loop test may well find a good earth through this route without there being an earthing rod connected which was okay for many many years but, as we know, ain't considered okay now.

The path of least resistance IMO is, if a satisfactory earth is found during test, stop looking for a rod and move on though I would run bonding from the CU to the points on the gas + water where they enter the property anyway just in case some fool later removes the current bonding if they come across it under floorboards and don't know what it is

Esra, you need to be aware though that if you do as I've suggested and there isn't a satisfactory connection to an earthing rod you are in breach of current regs. Your call though ... Good luck

 

[plugwash]

iirc the big problem with using service pipework as an earth is the risk it will be replaced with plastic.

if in doubt adding another rod in a location you know about is the best option but new rods absoloutely must be tested as contact with the ground isn't something you can exactly rely on

 

[esra_ptrap]

Thanks all for your replies.

Plugwash: My initial thought was to assume that there was no current earthing rod and to install a new one but I have read many posts suggesting that two earthing rods in different locations can be dangerous?

Presumably, if I fit a new rod, I should disconnect the existing 6mm earth from the CU and run a new one to the new earth and then bond the water/gas from the CU also etc?

I must admit that I'm leaning towards Megawatt's thinking given the fact that it's a fairly new installation and the standard of the work that can be seen is really good it would be unlikely that they would rewire a property without a proper earth.

 

[ban-all-sheds]

given the fact that it's a fairly new installation and the standard of the work that can be seen is really good it would be unlikely that they would rewire a property without a proper earth.

Sure enough to gamble your life, or the life of others, on it?

BTW - like your username!

 

[esra_ptrap]

I don't know WHAT you mean

I'm more comfortable gambling with other people's lives than my own if I'm being honest BAS

I was just about to post anyway because we've just discovered via a conversation with one of our neighbours (a builder who knew the people who built our extension) that they had laid a large earthing plate under the main garage floor slab when the foundations were put in which explains why I couldn't find any external wires. With this knowledge I think the earthing cable from the CU probably travels to this plate inside the boxing in we have around a soil pipe.

He also said that, if we were to dig around the incoming gas/water mains we would discover that they ARE plastic so we must have an earthing rod ... DOH!

All that needs to be done now is the main bonding from the CU to the gas/water at the point they enter the property and all is hunkey-dorey