At the moment, I have a TNS earthing system at home via sheath cable.
The Ze measured at the supply of the house is 37 Ohms, when it should be below 0.8 Ohm
As a consequence, I am going to convert to a TT system, with fully insulated consumer units and all RCBOs.
One question, I read that the Ze for a TT system should be below 200 Ohm
Do I need the earth electrode or can I use in the TT system the same cable sheath connection that has a much lower Ze?
Thanks,
Antonio
Sounds like you need to call a sparky as you don't know what you are doing....
Philip
If that is the case call the DNO and come and get them to fix their problem. They might just make it TT but if they try that I'd also make sure that they pay for all the works required.
Better would be to get the Power Company to make good your present earthing. Sounds like a poor connection, possibly a loop-in cable from next door who have a loose connection at their terminal. RCBOs - how about double pole disconnection in event of a fault? It may be better to install a twin RCD board for the more protection, if you cannot get the DNO to repair their earth supply.
For a reliable earth,it should be less than 200 ohms if possible,but can be anywhere up to 1667 ohms if you have a 30mA RCD protection for all circuits.
For TT, yes, you need an earth electrode separate from the incoming cables sheath.
This was answered & explained in the previous thread on this. Are you fishing for whatever answer you want?
NT
Yes I was just wondering how many of us have crap earths and such which have never been touched or measured for many years. Not sure if I'd know how to do this, but in my current eyesight challenged condition, I think I'd get a man or woman in.
Brian
It's a DIY banter poster.
If we are going from the previous conversation where the DNO is refusing to provide a working earth connection, then it seems that it would be exceedingly unwise to rely on their earth provision at all - since they may disconnect it entirely at some point in the future.
Hence I would ignore the cable sheath connection entirely, install and test your own earth stake and then use that.
A description of how to install and test it can be found here:
Sounds like you need to find someone to explain the purpose of this group to you ;-)
That IS the correct answer.
The DNO won't pay.
And yes, I do have a certified electrician on the job
But I still would like to know why I need the earth electrode if I have a good TT earth with the incoming cable sheath
On Wednesday 03 July 2013 20:05 asalcedo wrote in uk.d-i-y:
Because you do not control the supplier's cable. They might dig it up and change it for a modern non metallic sheath type.
So why are you asking such questions on a newsgroup?
Thank you for the informative link.
I believe that, given the soil conditions in my property, the earth electrode may be unreliable. Probably more unreliable than the existing cable sheath earth.
It is also a matter of cost and space. If I have to install two or more earth electrodes it can get quite expensive and obtrusive in the limited space available.
If I do install the earth electrode I would certainly connect it to the cable sheath (and to the mains gas and water copper pipes).
You don't have a good TT earth, you have a substandard TN-S earth that is broken.
On Wednesday 03 July 2013 21:18 asalcedo wrote in uk.d-i-y:
That's fine - but you *must test* your rod(s) in isolation and they only pass if they alone provide the required impedance.
Everything else is a bonus but you must not count on it because it can be taken away without notice.
Agreed fully, and that is what I will do.
I think you need to assume that is not possible - i.e. you can't rely on the existing earth in any way. For starters if its reading 10s of ohms now, and it is supposed to be a direct metallic connection to the substation, you know something is very wrong. You have no guarantee that it would not simply fail altogether the moment you try to sink any current into it.
If you do connect it to your earthing system, and it fails some distance from your property, then you may find your earthing system attempting to provide earthing for half the neighbourhood!
Not necessarily - two installed one on top of the other (i.e. one long rod) takes no more space than a single. Really all we are talking about here is hammering in one, testing it, and if its not good enough connecting another top the top and resume hitting that until its gone as well. Then test that.
Avoiding the cable sheath, and connecting the main equipotential bonds as you describe will achieve what is called fortuitous earthing. I.e. it will act as an earth, but its not its design purpose, and you are not allowed to rely on it.
Your earth provision needs to function well enough without the main bonding connections, and that is how you will need to test it. (same login applies to equipotential bonding, it needs to adequately limit touch voltage differences without the fortuitous reductions achieved via the earthing system.
Many thanks again for your helpful insights that I agree with.
I have been doing quite a lot of reading on earth electrodes and you always come up with relevant comments.
Even given the fact that I might be providing earth to neigbhours, do you agree that, on balance, it is better to connect the cable sheath to the earth electrode?
I am in central London, where the soil does not freeze. Is it fair to assume that if I measure the rods this summer and they pass, they will likely have better readings at any other time of the year?
Also, would it be enough a hammer to install two rods, one on top of each other, or would it be necessary a SDS Max hammer with an SDS earth rod driver?
One more thing, I believe the RCBOs should be Type B in my case, correct?
Thanks,
Antonio
On Friday 05 July 2013 07:56 asalcedo wrote in uk.d-i-y:
Type B is the most common but there's nothing about having a TT system that precludes having Type C. If in doubt, Type B is what you want. Type C is a deliberate design decision.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.