What Sizecable And Which Consumer Unit For A Tt System

[6thsense]

I am hoping to use my own house for my assessment to join either ELECSA and intend to replace the BS3036 fuse board with a dual RCD 10 way CU. I have run some tests today and found that the only earthing I currently have is on the incoming gas pipe. ie no earthing...nice challenge for my 1st job!!!

I understand that I need to install an earth rod but have very little garden that is not covered in concrete, the only place I can install one would require to run around 25mtrs of cable to reach the CU. I have read the on site guide which recommends 16mm cable when using 25mm tails, but have also read that the cable can be a minimum of 4mm for a TT system

Most people have given me the stock answer of 16mm as the Tails are 25mm however I do not believe that this has to apply to a TT system although I am happy to be proven wrong. Considering the length of the cable needed would I best to stick with a 16mm or use something smaller?

My other question relates to the 10 way CU.   I have read that TT systems require a 100mA time delayed RCD as the main switch with a 30mA RCD for circuits that require additional protection (bathroom, socket outlets etc). However, I intend to protect all the circuits with a 30mA RCD as none of my cables are installed in walls at more that 50mm or in earthed metallic covering. On this basis, could I use a standard 10way dual 30mA RCD consumer unit or is it a requirement for TT systems to have the 100mA time delayed RCD?

Ps: I have used the adibiatic equation for the earth cable but the calcs came out as 0.028 (based on a 9amp PFC), I am no expert on these equations but it does not seem right!!  

Apologies for the essay but can anybody advise?

Many thanks

 

[Andy™]

:facepalm:      headbang  

do you own a copy of BS7671? everything you asked is in there

 

[NozSpark]

Before answering your question could I ask what training you have?

as you seem to have the theory, but very little understanding

Before joining a Part P scam it might be better if you shadowed someone for a while

 

[Manator]

Hello 6thsense, do you see dead people all the time, just like in the film?

 

[Andy™]

and how do you know you have a PFC of 9a? with that value, i would be very concerned. now a PEFC of 9a is possible with TT...

 

[ProDave]

You won't know your PEFC  until you get round to installing that earth rod.

Have you asked your supplier if PME is available?

 

[Doc Hudson]

I have run some tests today and found that the only earthing I currently have is on the incoming gas pipe. ie no earthing...nice challenge for my 1st job!!!

What tests have you actually run, using what test instruments and what were the results. Do you currently not have a 100ma td RCD or is it an old VOELCB. How old is the property, photographs would be interesting.

Doc H.

 

[Marvo]

Can you post some pictures of the supply head? You may need to ask Admin for permissions as you only have 2 posts..

The earth is an integral part of the incoming supply. I have great doubt that the DNO would have installed a supply to your house without an earth. If the earth was supplied but is no longer functional then get the DNO to fix it, if there genuinely is no earth then likewise get them to supply one, either way it's their responsibility.

 

[6thsense]

Thanks for the advice so far. I trained and worked as a sparks some 20yrs ago but have just updated my quals to 17th edition.  I have never come across a TT system hence seeking some advice.

The 9amp PFC is the reading from the system in its current state, ie with earthing to the incoming gas supply only. I have read BS7671 for advice but I am sure that you will agree that regs aren't written in plain English and tend to contradict themselves. The on site guide is clearer but mentions the 100mA time delay RCD but then shows a diagram with a dual 30mA RCD.

 

[Andy™]

as someone 'competent' (well you should be if your wanting to be assessed to sign off your own work and be left to work alone whilst getting paid by customers for it) then you should know the requirements of the regulations. TT is slightly different from TN with RCD requirements but its all in the regs and fairly straight forward and that part doesnt really contradict its-self

out of curiosity, what training do you have? im guessing short course

 

[m4tty]

Marvo . The dno will deny supplying an earth and say they have no records and to get the electrician to put a rod in round here anyway.

6sense. Dual rcd board fine. 25mm tails. 16mm to rod and cant you sds some concrete bang a rod in and get a proper concrete pit. Wilts sell them. Do the full set of tests before changing the board and report any probs and were try to help

 

[Adrian]

dual 30ma rcd board is fine no need for a 100 ma  main switch

4mm to rod is all that is required 

can you drill concrete path and drive the stake through  that 

marco they do not have to supply an earth 

 

[Andy™]

The earth is an integral part of the incoming supply. I have great doubt that the DNO would have installed a supply to your house without an earth. If the earth was supplied but is no longer functional then get the DNO to fix it, if there genuinely is no earth then likewise get them to supply one, either way it's their responsibility.

DNO only have to give you L&N. if they do give an E then it must be maintained however

 

[6thsense]

I cant add a photo but the current system has 2 Chiltern 60amp VOEL circuit breakers. 

It sounds as though the DNO should be supplying an earth so I will get onto them before breaking concrete and putting in my own earth rod. 

Thanks for your advice though

 

[Marvo]

DNO only have to give you L&N. if they do give an E then it must be maintained however

Thanks Andy I stand corrected then but cannot imagine why they'd provide the supply without one.

 

[Manator]

Thanks Andy I stand corrected then but cannot imagine why they'd provide the supply without one.

That is the nature of TT in this country Marvo. It is more common in rural area's with overhead supplies. However more and more are adapting to PME supplies.

 

[Andy™]

Thanks Andy I stand corrected then but cannot imagine why they'd provide the supply without one.

unusual to get a new supply without TNCS, but many older installations only every got a L&N. and thats not just rual areas - its fairly built up where i am, but there are still many estates without an earth abd are all TT. TT can sometimes be made to TNCS depending on network wiring

 

[Marvo]

That is the nature of TT in this country Marvo. It is more common in rural area's with overhead supplies. However more and more are adapting to PME supplies.

Thanks, I found the opening post confusing and I didn't realise it was a rural network. We also have some strange rural distribution arrangements. This sounds like an old installation though so I still don't understand how it could have not had some earthing arrangement.

 

[Manator]

It was a get out clause for the distribution networks, they could remove an earth if they wanted!

As Andy said in his post some heavily populated area's also had TT supplies.

 

[batty]

The existing earth will be in the bottom of the voltage operated circuit breaker. i would be very surprised if that goes to the gas. It will probably need upgrading though could be as small as 2.5mm.