Another TT question !!!

[ian932]

1st off Happy New year all not been on for a bit.

My question is I've done a CU upgrade and also fitted a new earth rod as readings were about 400ohms got it down to 20ohms by driving it into the existing rod hole once removed, think it may be touching the old lead water pipe now.

Sorry I'll get to the point the gas is bonded in 6mm and cutomer will not pay for 10mm upgrade as its a half day of work lifting boards etc. so here is the question if table 54.1 in the 17th states that the main earth conductor can be as litlle as 2.5mm if protected against mechanical damage & corrosion then is it ok to leave the 6mm? and if you think the regs require it to be 10mm can it be listed as a Code 2 on the install cert?

 

[jl-heating]

it should be in the price as when you surveyed the work,

i dont see how you can upgrade a CU to latest regs without a complete upgrade.

do they not realize that the installation is only as good as the protective earth. ??

so might be down to you if you missed it on survey.

ps: i have a simple check list i fill in on a quote and find things like this are a costly lesson . but it only happens once.

quote from another forum might shed some light.

The designs in the guide are always compliant but not necessarily the most economical possible".

The 4 mm

 

[betty swollocks]

if they wont pay leave it as is and note it on cert

by improving the ze and replacing the cu you have no doubt improved the installation and thus are leaving it in a safer condition than when you arrived so nobody can beat you with it

 

[Andy™]

i normally leave earth at 6mm if TT, sometimes TN-S if it cant be upgraded.

even if the earth was to be undersized, at least you would have made the system safer

 

[batty]

You could always get away with 6mm earths to gas and water on tt in old regs. As it is all Rcd protected I would think this should be adequate but I shall have to have a look.

Batty

 

[poheZ]

simply note it down as a deviation and see if you can cross bond the gas to the water?

sometimes i put customer notified and will upgrade alongside refit..blah blah

or you could proove the size of the conductor is adequate by calculation

 

[Evans Electric]

I'll say this yet again, I don't know who decided that we are The Electric Police, if they don't want to pay you then forget it, just put it on your cert . Sounds like you have done a good job, improved safety, you can do no more, move on ,don't worry, be happy oooh think I'm breaking into song here !!

Deke

 

[ian932]

Thanks guys for the positive feed back. Applaud SmileyGuiness DrinkGuiness Drink

JL-Heating - FYI, I have a number of smaller builders who have a set of standard unit price's for my work which they quote in their price and then when they get me on site we check if additions need adding such as bonding etc. and this possibility is covered in his quote terms. I usually advice them to add a PC sum to the quote of around 20% of the electrical price for such events, this helps the punter get a feel for what maybe a worst case price.

So when I started this job my initial inspection highlighted the need to upgrade the gas bonding and price was given via builder (contractual chain thing). But punter has run out of funds to the point where he is doing some of the work the builder was going to do. By the time all this was sorted out I had done the CU upgrade.

Main earth conductor to rod is 16mm, I was only making ref to the table 54.1 to highlight the point that if you can use a 2.5mm then surely a bonding conductor up stream did not have to be larger.

The lesson on quoting was not required ta but made good reading Applaud Smiley

 

[batty]

Ian if you look in osg it says min 6mm for earth bonds to water and gas for tt so whats the problem.

Batty

 

[ian932]

Ian if you look in osg it says min 6mm for earth bonds to water and gas for tt so whats the problem.Batty

Cheers Batty, take it thats in the 17th OSG - I'll check it out Guiness Drink

 

[batty]

I am getting a bit confused with this it always used to be 6mm for tt. OSG says min 6mm but if you look in regs page 134 it says 10mm and that is what is says in green building reg book. I think I shall have to do a bit more reading.

Batty

 

[ian932]

Ian if you look in osg it says min 6mm for earth bonds to water and gas for tt so whats the problem.Batty

Just checked the OSG and yes it does show it as 6mm in figure 2.3 but if you look in the IEE Guide to Building Regs Figure 3.2.3 and table 3.32b they say if the line conductor is 25mm the MPBC needs to be 10mm and refers you to reg 544.1.1 and table 54.8 in bs7671 which also states 10mm.

It looks like it might be a miss print in the OSG is that maybe what it was in the 16th ? mines in the van and its to cold to go out and look.

Now I'm totaly confused :_|

 

[ian932]

I am getting a bit confused with this it always used to be 6mm for tt. OSG says min 6mm but if you look in regs page 134 it says 10mm and that is what is says in green building reg book. I think I shall have to do a bit more reading.Batty

You wrote the same time as me and I concur, the key is note 3 on page 29 of the OSG where it say's not less than half the area of the earthing conductor. So if its 16mm earth conductor it has to be 10mm.

 

[betty swollocks]

thats the way i've always approached it, if the main line conductor is 25mm then the earth must be at least half, so 16mm, equipotential bonding at least half again, so 10mm

same principal if the line is 16mm, 10mm earth, 6mm bonding( although in practice to be honest i always fit minimum of 10mm bonding)

 

[binky]

thats the way i've always approached it, if the main line conductor is 25mm then the earth must be at least half, so 16mm, equipotential bonding at least half again, so 10mmsame principal if the line is 16mm, 10mm earth, 6mm bonding( although in practice to be honest i always fit minimum of 10mm bonding)

That's what most people tend to do, but 6mm is fine for TT under old and new regs, despite lots of misleading diagrams in various publications.

 

[ian932]

That's what most people tend to do, but 6mm is fine for TT under old and new regs, despite lots of misleading diagrams in various publications.

I agree diagrams are making the situation worse but surley reg 544.1.1 is clear that where PME conditions do not apply it must be no less than half the CSA of the earthing conductor. Therefore 16mm earthing conductor = 10mm MPBC. The only time I can see us getting away with 6mm is if the system has only got 10mm mains tails.

Where does it say in the new regs we can use 6mm on a TT ?

 

[DYCHE4230]

I agree diagrams are making the situation worse but surley reg 544.1.1 is clear that where PME conditions do not apply it must be no less than half the CSA of the earthing conductor. Therefore 16mm earthing conductor = 10mm MPBC. The only time I can see us getting away with 6mm is if the system has only got 10mm mains tails.Where does it say in the new regs we can use 6mm on a TT ?

PAGE 16 of the OSG says min 6mm for main bonding but this would only be suitable if you main earth conductor was 10mm or less...I agree if 16mm main earth conductor then main bonding has to be 10mm...but if the customer aint paying then I aint changing it!