Earth rod advice

[nicam49]

Hi All, my daughter's new (but old-1920s) house only has a very thin-1-2mm earth wire down to what I presume is the original earth rod. It's a TT circuit, so just L and N are supplied. I'd like to install a better earth via a new earth rod....any tips/advice please?
Cable size 10mm ? Thanks

 

[Taylortwocities]

Hmm,, lets look a little further at the installation first.

What sort of earth leakage protection is in place? A new type RCD with a trip current of 30mA perhaps? Or one of the old (and redundant) Voltage Operated ELCB devices?

If you think about it, the earth conductor is required to carry the earth fault current to trip the (whatever) device is there to provide safety to your daughter (and other family members?). If the fault trip current is limited to 30mA then a great big chunk of 10mm copper is going to be rather more than is needed.

Secondly, how would you test and check the integrity of you new earth electrode? Do you have test equipment that could measure the earth impedance of the electrode?
I'll guess that you probably don't, or you would not be asking on a DIY Internet forum.

Providing earthing incorrectly, particularly in a TT installation, can be a fatal mistake. Please get a qualified elecrtician to carry this work out.

 

[JohnD]

Hi All, my daughter's new (but old-1920s) house only has a very thin-1-2mm earth wire

it could have been part of the first telephone installation.

 

[nicam49]

Hi, thanks for your replies!-really appreciated. The Wylec CU is a simple NH804, 8-way with NO rcd or provision for one. There IS a separately mounted external cut-out that is black with a yellow button on it, that I'm now thinking must be that Voltage Operated ELCB devices you mentioned.- couldn't identify it myself. So, with a modern CU (which has rcds) the existing earth rod wiring may not be so inadequate after all....?
Thanks again for your thoughts and advice.
Just seen on Google Images a Crabtree VOELCD, and it looks very much like the one we've got fitted...

"Providing earthing incorrectly, particularly in a TT installation, can be a fatal mistake. Please get a qualified elecrtician to carry this work out."

point taken

 

[Taylortwocities]

You really need to get a qualified electrician to look at that. The old VOELCBs have been found not to work effectively, do not protect people and basically do not work, particularly in houses that have things like earthed metal pipes around. They were removed from the regulations more than 30 years ago.
If you want to know more then this video tells it all

If you want to skip the tekky bit, just look at the first and last 2-3 minutes.

But basically it needs replacing, probably with a modern RCD and almost certainly with a new earth electrode.

In summary, your daughter's installation provides no protection in the event of an electrical fault. You (she) should have a competent electrican sort it out for her.

 

[sparkwright]

Again, have to echo this is not DIY work, partly because a Ze test will be required to determine if the earth is any good.

Get someone in to do it.

 

[nicam49]

Wow! interesting youtube vid there...scary when explained like that. Really knowledgeable guy. I'm 16th edition certified, but not up on 17th, and was intending to fit a twin rcd CU myself-around £80 from Screwfix- but am vague regarding earth bonding regs and whether even gas pipes should now be bonded or not..seemingly more reliance now on RCDs working reliably and less emphasis on good low earth impedance now that most stuff is double-insulated.

 

[JohnW2]

I'm 16th edition certified ....

I have to say that I'm rather surprised, given some of the questions you've been asking. Things electrical have not changed that much is the past couple of decades or so.

... but not up on 17th, and was intending to fit a twin rcd CU myself-around £80 from Screwfix ....

That's going to cost you a lot more than the price of the CU in terms of notification fees.

.... - but am vague regarding earth bonding regs and whether even gas pipes should now be bonded or not..

The concepts of Main Equipotential Bonding have never changed. Most supplementary bonding is no longer required.

Do you have the necessary test kit to do a CU change?

Kind Regards, John

 

[EFLImpudence]

Just to point out:

Any and all "extraneous-conductive-parts" (parts liable to introduce a potential) - entering the premises must be bonded to the Main Earthing Terminal with a Main Bonding Conductor .
Local extraneous-c-ps entering a special location (bathrooms etc.) might require bonding together and to exposed-conductive-parts with Supplementary Bonding Conductors.

It does not matter whether these extraneous-c-ps are gas pipes, water pipes or whatever, it is whether they are extraneous-c-ps which determines if they require bonding; not that they are gas pipes, water pipes or whatever.

 

[JohnW2]

Just to point out: .... Any and all "extraneous-conductive-parts" (parts liable to introduce a potential) - entering the premises must be bonded to the Main Earthing Terminal with a Main Bonding Conductor .

Indeed - and, as I said, that principle of Main Equipotential Bonding has never changed, and I presume that has been recognised by many editions of the Wiring Regs.

Local extraneous-c-ps entering a special location (bathrooms etc.) might require bonding together and to exposed-conductive-parts with Supplementary Bonding Conductors.

Another Indeed - as I said, the requirement for most Supplementary Bonding (much more prevalent in early eds of the regs) has now gone, with the one exception you mention (and, even then, most bathrooms etc. will not actually require it)

It does not matter whether these extraneous-c-ps are gas pipes, water pipes or whatever, it is whether they are extraneous-c-ps which determines if they require bonding; not that they are gas pipes, water pipes or whatever.

... and the third Indeed - and, again, these principles have never changed.

Kind Regards, John

 

[nicam49]

Thanks all for your very helpful and informative replies. Never encountered T-T system and earth rods before...(this is an overhead-supplied power cable in Wales.) Did the 16th edition just before it changed to 17th out of interest...was a computer tech in another life. Apart from that, did a bit of house rewiring a couple of decades ago, (John!! ) and now am obviously really rusty on the subject.
Thanks again!

 

[ericmark]

Normally JW does good videos, however in this one although he is right that the ELCB-v is not reliable he talks about current, well they don't work on current they work on voltage. The resistance of the coil is quite high so very little current will flow, what they do is measure the voltage between the house earth bonding and true earth, if this exceeds 50 volt then it disconnects the supply, so if we consider 50 volts as being safe, it does not matter what other paths there are to earth, either the alternative path will hold the voltage below 50 volt so it is safe, or the device will trip.

Although they may protect people in the house, they will not protect anyone outside the bonded zone, and they will not stop current flowing to earth, and the latter is a fire risk. It will also mean any ELCB-c after the ELCB-v will fail to work as in essence the house is IT.

I had not seen one of these for years, then a couple of years ago I went to a woodland house and tested it for the owner. The RCD would trip by pushing the test button, but would not trip using a RCD tester. It transpired the hut was fed from an old stone cottage, in the cottage was an ELCB-v, the ELCB-v would protect anyone in the stone cottage but not in the hut fed from it. I fitted earth rods and RCD's. The ELCB's was in a very hard to reach point, seems a slate meat safe had be built over the top of it, and to remove the slate would have been very hard, so the ELCB-v was left in place, but now it is just a simple isolator, it does not work as a ELCB-v.

I think it was common when upgrading to RCD to leave the ELCB-v in place, it was a handy isolator allowing one to fit a modern consumer unit without needing to draw the DNO fuse.

With the ELCB-v because very little current would flow, it was a voltage device the earth rod would not raise in voltage with a fault, so there was no real need for an earth pit to keep animals away from the rod, however the new RCD can allow a full 30 mA to flow, not just 1 mA as in the case of the ELCB-v so the rod needs to be deeper and in a pit to stop anything touching ground close to the rod.

The ELCB-v in the stone cottage would trip when using an earth loop impedance tester yet with a RCD the tester did not trip it. In other words the 10 mA used by the earth loop impedance tester was enough to trip it inside the cottage.

It does need changing to a current device, mainly to protect from fire, as if there are alternative earth paths you can get high currents passing to earth without tripping the ELCB-v which could melt gas pipes and the like, and clearly if it melts a gas pipe then there is a high chance of fire.

 

[bernardgreen]

he talks about current, well they don't work on current they work on voltage.

The mechanism is electro-magnetic and not electro-static. Hence they ae operated by current. They are rated to operate at 50 volts ( or in some cases lower voltages ) which is the voltage required across the coil to drive the tripping current through the operating coil.

 

[ericmark]

The mechanism is electro-magnetic and not electro-static. Hence they ae operated by current. They are rated to operate at 50 volts ( or in some cases lower voltages ) which is the voltage required across the coil to drive the tripping current through the operating coil.

Agreed but the current is very low not enough to kill his cow near the earth rod.

 

[bernardgreen]

In the dim and distant past I had to correct a electrican who believed that the impedance of the earth to the voltage operated device was not important as it "measured voltage and with no current flowing there could be no votlage drop in any impedance in the earth lead"