tt earthing arrangement help!!

Postby martind » Sun Aug 03, 2008 6:12 pm

Hi there, this is my 1st post so please be gentle with me please.

I am a stage lighting engineer so havent much experiance with domestic wiring.

I have a tt earthing arrangement and the only earth it has is to the gas and water main bonding so i have installed an earth rod which has brought the ze down to 15 ohms. I carried out a loop reading at a socket outlet and the result was 15.66 ohms. The main incomer is also protected by a 100mA rcd. Is this all acceptable or not? Only worked with pme before.


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Postby ericmark » Sun Aug 03, 2008 10:55 pm

The problem is measuring the rod and not the gas and water pipe so the earth rod needs testing with the earth wire disconnected. There is some danger in doing this so empty house is best. In theory your allowed 460 ohms see Table 41.5 but note 2 reduces this to 200 ohms as any reading above this can be unstable. Second problem with TT is protecting the top of the rod as if there is an earth fault the gradient at the top of the rod is very steep so normally it is sunk to below ground level and a cover is put over it this effectively starts the gradient below ground and physically protects the top of the rod and cable connection while still giving access for testing.
Up to July this year 100ma S type (delayed) RCD's were the order of the day but with new regulations these have been in the main replaced with a couple of 30ma non delay.
Since the testing of the rod can produce danger we would not normally go through the testing and to anyone else I am giving this warning this procedure should be carried out only by trained people as any mistake can put 230 volts on exposed metal work.
From your post Martin I will assume you know the dangers but others reading this post may not.
In real terms martin I would think you have no problems with a 15 ohm reading and I know many electricians who should know better test Ze with all cables connected and it can cause problems with paperwork where Ze is higher than Zs and if this is the case I would put a note Ze measured to earth rod only to save any problems where people are like yourself use to TN-C-S (PME) and jump to wrong conclusions.
If Sparx answers this let his post override mine as he has a lot more experience in submitting paperwork than me.

Postby TOPSPARK » Mon Aug 04, 2008 7:07 pm

Hi martin
Topspark here a reading of 15 ohms for a tt system is a great reading but as the table for tt systems states 41.5 that a reading of 460 ohms is for non delayed rcds for final circuits not exceeding 32amps.If you dissconnect your main earthing conductor and then take the Ze this is the loop impedance of the earth rod and it is below 460 ohms then disconnection times should be met according to table41.1 as stated in regulation 411.3.2.2 of the current 17th edition of the regs book.
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Postby martind » Mon Aug 04, 2008 7:57 pm

thanks for the reply, is it ok for the zs values to be higher than the max zs for the mcbs as long as the circuits are protected by a 30mA rcd? also fitting a 17th edition consumer unit and upgrading the bonding.

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Postby sparx » Mon Aug 04, 2008 10:17 pm

Hi Martin,
Ericmark the flatterer knew this would draw me out....he is being undully modest!
I am not entirely sure which way to answer this, as Eric says there are differences under latest regs from older ones.
If you have an existing installation and just want to improve it then as he says make sure you test the rod alone, not connected to other bonding, this will give the worst case reading which will always come down when bonding reconnected, due to parallel paths. Thus rod reading must be low enough to trip RCD. It was normal to have 100mA rcd on main but not good enough for sockets so led to split load boards, rcbo's etc for some circuits. In theory a 30mA rcd could have a rod resistance of up to 1666.6ohms using 240v or a 100mA trip could have a resistance of up to 500 ohms however guidance note says over 200 ohms"may not be stable".
From your readings it would seem you have a reasonably good earth reading of 15 ohms but as Zs is only 0.66 higher I presume bonding was in place when doing Ze, check it again minus bonding to be sure.

I would not want to leave whole house on 100mA trip so without expense, time etc of changing board I would change RCD to 30mA one, not up to latest standard but much safer than now.

Went to house today where C#rry's had deliver a new cooker but would not connect it as "loop reading too high" @ 70 ohms "should be less than 0.8 ohms missus, you got a problem!" T.T. system! they didn't even look.
came from town where all TN systems.

Is TNC-S (pme) not available in your area? that would give a slightly more reliable earth path.
welcome to forum BTW,
regards SPARX
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Postby ericmark » Tue Aug 05, 2008 9:39 am

Thanks to Sparx as usual spot on, I think the 7:57 post from Martin was held up so to answer that:-
Max Zs for MCBs has 4 readings.
1) Earth loop impedance this we no longer matter as looked after by RCD.
2) Neutral loop impedance very unlikely to be a problem unless very long run around 70 meters on a ring main with 2.5mm² and 32 amp MCB.
3) The PSC but to earth and neutral from Line which is most likely covered my main incoming fuse in neutral / Line and would not be problem Earth / Line.
Up grading to 17th Edition with consumer unit means all circuits will require testing and any discrepancies noted. It takes longer to do the inspection and test than fit the whole consumer unit. Also unless you’re a registered spark you need to inform the Local Authority Building Control (LABC) before starting unless counted as an emergency then you may inform after. For insurance and house sale you will need the “Completion Certificate” which is issued by the LABC. Under 17th Edition bonding is less not more.
As to table 41.1 part of 411.3.2.2 this is really of no interest when using 30ma RCD’s with no delay as disconnection is NOT by an overcurrent device. As to table 41.5 it is Note 2 which is of most interest limiting it to 200 ohms only when 300ma and 500ma trips are used does the table come into play.
The BS7671:2008 (17th Edition) regulations are not easy reading and it is very easy to read out of context. The set of Guidance notes by the IET are split into 7 books and book 3 is the important one as to how to test and inspect you will see it referred to as GN3 and there are tests you should carry out which are not recorded and I am sure many electricians only carry out the recorded tests.
If you go to IET web site they do in PDF format the forms required for inspection and testing which unlike those included in the Part P document don’t have “On Line Version” printed across them. In the Projects part of this forum you will find links to Part P and some bits of 17th Edition.
To anyone else reading this please remember we are talking to a “Skilled Person” and what we suggest here may not be appropriate for DIY as instruments used are very expensive and require skill to use.
All best Eric

Postby martind » Tue Aug 05, 2008 5:25 pm

thanks for all the replies. I believe it is a tt system as there is just the live and neutral entering the main cutout fuse. i thought tt system came from over head power lines but it comes from underground into the house. Would the electricity board convert it to a pme if contacted?


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Postby ericmark » Tue Aug 05, 2008 5:48 pm

In my are the supply authority will normally convert to PME but not by just asking.
I have found you have to ask them what system you have. They don't like this as often they don't know. But you need to stick to your guns and say who it is up to them to tell you the system and the Ze reading. After a bit of trying to put you off they will then send someone out at which point you let it drop next door is PME (TN-C-S) at which point the electrician will take a earth loop impedance reading and if within limits will give you PME.
If you just ask for PME they will just tell you they don't have to provide an earth and thats as far as you get.
Wrong I know but that's life.
Maybe your area is more helpfull?
I have lost count of how many times I have gone to help my son knock in earth rods only to find the board have then given an earth connection.

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