I am changing from TN-S to TT earthing system at home.
I will use an unprotected, non buried, earthing cable from the earth rod to the earth block.
No phase conductor in the house is larger than 10mm2
To be in compliance with the regulations, which minimum size of earthing cable do I have to use?
Thanks,
Antonio
Hi,
On Thursday 25 July 2013 21:59 asalcedo wrote in uk.d-i-y:
As a TT earthing conductor will carry very little current, it comes down to mechanical protection of the cable. From the IEE/IET OnSite Guide:
Buried: Unprotected: 25mm2 Protected against corrsion: 16mm2 Protected against corrosion and mechanical damage: 2.5mm2
Not Buried: Unprotected: 4mm2 Protected against corrsion: 4mm2 Protected against corrosion and mechanical damage: 2.5mm2
How will the conductor be protected betwene the top of the rod (ouside I presume) and the house wall where it will not be subject to damage?
Cheers,
Tim
Hi Tim,
Thank you again for your help.
I agree with your conclusions and that is what I told my electrician.
He said "regulations 542.3.1 cable not protected mechanical damage must be 16mm2" But I think he did not realize that 542.3.1 applies to buried conductor. So I ordered 10 mm2 to have a bit more mechanical protection without going too big and unsightly.
The earth electrode/s will be placed inside the house, in the garage, near the incoming supply and consumer units. Most likely by a wall. The cable from the earth rod will run along that wall until the consumer unit.
I was going to use cable clips. Is that enough or do I need to use conduit or trunking?
Depending on the success with the earth rod impedances I may need just
2m of earthing cable if one electrode at the ground floor in the garage is enough. Or I may need up to 14m if two electrodes are required (the second electrode would be located at the other extreme of the garage that is excavated at the same level as the basement of the house)Is 10 mm2 enough in all cases?
Regards,
Antonio
What Tim said... copy of the OSG table information here:
On Friday 26 July 2013 10:17 asalcedo wrote in uk.d-i-y:
I would run a piece of 90 degree bend conduit down the wall and 10cm below floor level. Make a little pit for the earth rod and stick a lid on it - such lids exist for exactly this purpose and are marked "earth rod".
That way the cable and the rod clamp is protected in an environment where mechanical damage is highly likely and furture sparkies will be able to find it.
Great.
It is certainly less conspicuous than the plastic inspection pit 'Plastic Inspection Earth Pit | Screwfix.com'
The garage floor has either concrete or stone, I'll see if this smaller terminal cover that you mention can be fit without too much trouble
My electrician still says that 16mm2 is required because my meter tails are 25mm2 and:
"Reg. depends of the live conductor
live 6mm2 earth 6mm2 live 10mm2 earth 10mm2 live 16mm2 earth 16mm2 live 25mm2 earth 16mm2 live 32mm2 earth 16mm2"
I am going to tell him
The size of the meter tails is not very relevant for the earthing conductor. The size referred in that table above is not that of the meter tails but of the conductors in the system after the consumer unit.
Am I correct? What else can I tell him?
Thanks.
You need to be aware that not having it done by a qualified person may invalidate your insurance. Also if you come to sell the property you will probably be asked to certify that all electrical work has been done by a qualified person. In any case can you really afford to take a risk with such a safety critical task?
Those are figures from Table 4.1, and are correct for TN-S and TN-C-S supplies. However for TT you need table 4.2.
Well it is - but only in cases were the external earth impedance is less than an ohm.
The sizes quoted apply to that of the line conductor of the system - so that will be the tails connecting the main cutout to the meter, thence the CU.
That you think he might be consulting table 4.1 in the OSG, rather than
4.2 as is appropriate for TT supplies.
I have never heard of that argument being used in real life... nor have I seen any disclaimers in insurance policies. Note also that many include damage to a house as a result of "accidental" damage, which would suggest that are not excluding householder c*ck-ups from allowable claims.
Correcting faults in earthing and bonding is a non notifiable activity.
There is never a requirement that work be carried out by a "qualified" person. There may be a requirement that notifiable work be carried out by a "registered competent person" or that LABC are involved or a third party inspection is done. (note the last option is new to Part P 2013)
I would say most definitely, far safer to do the work yourself, so that you know its done to an adequate standard, than rely one someone else who does not need to live there.
ISTM that the professional electrician in this circumstance has already demonstrated a lack of familiarity with TT systems.
On Saturday 27 July 2013 13:58 Peter Crosland wrote in uk.d-i-y:
I have never seen an insurance policy that says that - have you?
Just out of curiosity (maybe I missed something), are you willingly changing from TN-S to TT, and why would you want TT? Or is the electricity supplier going to cut off the earth connection?
From previous threads, he is currently TN-S, but the earth impedance is
37 ohms, and the supplier is refusing to fix it. So its either a case of nag em anyway until they do, or go TT since that level of Ze with TN-S is about as much use an ash tray on a motobike.
Oh, now that you mention it, I do remember seeing that a little while ago, but I hadn't made the connection. (No pun intended.) Thanks.
Am I right in thinking that no-one would want a TT given a choice of that or TN-S or TN-C-S?
Insurance policies invariably include a clause that requires the policyholder to disclose anything that might, and I stress might, affect the risk insured. It is a frequent, if not the most common, reason for claims not being paid.
Changing form one type of earthing to another is not IMHO correcting a fault therefore I would suggest it is notifiable.
To suggest what happened with a past installation does not mean a properly qualified person is going to make a worse job in future seems a specious argument to me but I may have misunderstood what you mean. Likewise to suggest that just because a professional is not going to live in premises that they will do a worse job is simply absurd.
All insurance contracts are classified as ones of "the utmost good faith". This means that there is an overriding requirement to notify the insurer of any fact that might affect their perception of the risk. This imposes a very serious requirement on the insured and it does not have to be spelt out specifically in the policy wording.
I know someone with a TN-C-S supply that has a TT install. He did have a TT supply and the REC changed the supply. He kept the installation as TT. He did however have a good electrical installation and he knows what he is doing.
TBH as long as the electrical installation is sound it matters not what sytem you use IMHO.
As a generalisation perhaps, although one might argue that a well maintained TT system has some advantages over TN-C-S - especially if you want to connect up outbuildings, and / or run your own generator for standby purposes.
It *is* correcting a fault, because he currently has no functioning earthing system at all - that is a serious fault, and needs rectifying as a matter of urgency.
Its not notifiable in the part P sense - but I am not sure if you are suggesting one should notify the insurance company? Personally I can't see the need since so many home owners would not have any clue about technical aspects of their supply such as the style of earthing used, it would seem unreasonable for an insurer to expect to be told should it change. Its like suggesting that you should notify them if the water company increases your water pressure.
I think you have missed the thrust of my argument. I was not talking about the past installation, but the comments made by his electrician who was giving advice on the requirements for the TT system. The advice he gave, was not applicable to TT installs but was based on guidance for TN systems. This would suggest to me that he may not have much experience in this particular area.
Depends on the "professional" obviously. However my point was the counter one - if its your own place, then you are going to make damn sure the work is done to a high standard.
I think that has changed this year.
The Consumer Insurance (Disclosure and Representation) Act 2012 came into force on 6th April 2013. It removes the duty on consumers to volunteer ?material facts? if not asked.
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