TT: A bit of guidance please

[uksparks]

Hi,

Sorry about the quality of the photos, it was very dark in the garage.

Went to a job this morning, to quote on a new consumer unit, see photos below.

It's a TT earthing system, 8 way 3036 board, needs new bonding etc. Main earthing conductor is only 6mm at present, rod is just outside the garage against the wall, no idea how big it is yet.

Its got an overhead supply, two phases, but only one is used.

All the old junk on the right is coming out as its not used, this was on the second phase, it was for storage heaters.

Ill be quite honest, I have never done a TT before so would like a few pointers.

I'm going to replace the board for a split board, thats not an issue, its just the earthing thats the issue.

I did a Zs at the board just to see what the readings were like and you can see the photos below.


A couple of questions...

1. being a TT, i understand it needs an upfront RCD? Yes? 300ma?

2. I took a PSCC and PEFC reading and the PEFC was a lot lower than I had expected and want to know if its normal so to speak, obviously the fault current comes down the higher the resistance.

3. Im permitted 200 Ohms on Ze, so 8.3Ohms is good.

This was with the earth connected, it was only a brief look, so not an exact measurement.

PSCC (L - N)
0.19 Ohms
1.3kA PFC

PEFC (L - E)
8.33 Ohms
30A PFC

Is there anything I should have to think about it addition to the norm with a TNC-S / TNS supply? I can't think of anything other than the RCD.

I felt the 30A was low on the PEFC, but then again, its got an RCD.



I can only assume this was an RCD as it tripped when I did a high current Zs on the board.



Miscellaneous...

 

[uksparks]

Seriously? The absoloute minimum size for an earthing conductor is 2.5mm if provided with mechanical protection.

2.m sounds ridiculous but I'm sure you are right. Im just reading a section on it now.

I think I could be cut a bit of slack as I did openly confess at the start I hadn't done anything with TT.

 

[yellowvanman]

The 6mm conductor, I am not saying this has never worked and doesn't provide an adequate connection etc, what I was suggesting is that it is less than the minimum CSA permitted by BS7671, see table 54.7 on page 162 of the BGB. It clearly states that the if the CSA of the line conductor is between 16mm and 35mm the CSA of the earthing conductor needs to be 16mm which is why I said it's too small as the CSA of the line conductor is 25mm ok.

When sizing earthing conductors you should be reading 542.3 and Table 54.1, where it states that with adequate mechanical protection the earthing conductor can be a small as 2.5mm. If this was the case the bonding conductor must be at least 6mm (544.1.1).

 

[uksparks]

Ok so section 542.3.1 table 54.1 states 2.5mm where burried and where mechanical protection exists.

I feel you have led me on slightly saying 2.5mm is the right way forward, but having read it I will eat pie or whatever the saying is and say I was wrong, but equally, it does say 16mm if not protected which it won't be, so I will still go ahead with the 16mm anyway, not only this, its only going to go towards lowering the resistance of the earth path, ok on marginal, but it all helps.

 

[alanl]

You are going to bury a 16mm cable unprotected?
Why not just run some conduit in?
The cost would be £5 or less, you will have to dig a trench anyway.
The cable is then fully protected, and you can use a smaller cable.
Burying an earth conductor unprotected is just daft wjhatever its size.

 

[uksparks]

No im not going to bury it... The existing one is right next to the wall, so no need, if I put another in, ill do it at the front of the property, ok I may need to put some conduit it round it there.

Ill still use 16mm regardless of what it says in the regs, theres not a chance I am going to run a length of 2.5mm.

 

[davesparks]

Don't put it right next to the wall!

 

[Engineer54]

First off I am not your matey and wouldn't want to be I based my opinion of you and the others mentioned not only after your comments to my post but on others I have read on this forum over the past 6 months.

I have read my posts made in this thread and still claim I have never said 200 ohms is an acceptable value for any TT installation if you want to use my reference to BS 7671 to try and justify your behaviour towards me then why don't you just refer to 1667 ohms also mentioned in BS 7671 which in theory is the maximum.

If you think any customers are willing to pay an electrician to mess around trying to achieve an Ra to gain ZS values which may never be achievable you are mistaken not in my part of the world where TT is the dominant method. If you have a two up two down mid terraced property supplied by overhead no PME with a tarmac pavement at the front and a 6 inch concreted yard at the rear you are limited to where rods or other methods can be used you do your best to achieve the lowest Ra you can in a timely fashion.

Oh for sure your not my mate ha ha, Eh, i don't think the term was used in that context to mean anything along those lines.... I speak as i find, and you come across as someone who is only interested in the ''Easy'', ...i may be wrong but that's how you come across, especially when you throw your toys out the pram and call start calling people that have proved no-end of times that they know what they are talking about, ...Bell Ends!!

If and only if the RCD device works as required (they are not exactly the most reliable bits of kit), if it doesn't it won't matter a jot what the maximum Ra ohms is in theory or otherwise.... Hence why is is prudent to always fit an up front S type RCD devise, at least you are giving the TT installation a decent degree of protection, rather than Zip!!

I would expect customers hope that an electrician employed to do a job of work that involved their safety to do just that. I agree it isn't always achievable but i'm sure they would expect you to at least do your up most to TRY!! 6'' depth of concrete in a back yard isn't an excuse not place a decent sized rod DEEP!! After all your going to need to provide a flush concrete or tough plastic earth connection pit anyway, or is that something else you don't provide, because it's not a viable proposition for a customers two up two down terraced house??

 

[uksparks]

Don't put it right next to the wall!

No it's ok I won't, I was just saying the existing one is next to the wall. I spoke to Trev and said put it a clear meter away from a wall min, which is a good idea, I need to have a look to see where I can get one in and see where's best.

I do appreciate all the advice given.

 

[7029 dave]

UK, have not read the whole thread, if it is an main earth protective conductor, run it in 16mm, I would agree with you.

 

[Gavin A]

No it's ok I won't, I was just saying the existing one is next to the wall. I spoke to Trev and said put it a clear meter away from a wall min, which is a good idea, I need to have a look to see where I can get one in and see where's best.

I do appreciate all the advice given.

this is probably going to be obvious, but really check for services before banging an 8 foot spike into the ground, as water pipes and drains etc are only at about 4 foot, and it can really ruin your day to put a spike through one of them.

 

[Engineer54]

this is probably going to be obvious, but really check for services before banging an 8 foot spike into the ground, as water pipes and drains etc are only at about 4 foot, and it can really ruin your day to put a spike through one of them.

Where accessible it's always advisable to excavate a down metre, before driving the rod(s) in the ground if there is a possibility of pipes or drains in the vicinity. Also makes setting in the flush earth connection pit that much easier too!! lol!!

 

[Gavin A]

Where accessible it's always advisable to excavate a down metre, before driving the rod(s) in the ground if there is a possibility of pipes or drains in the vicinity. Also makes setting in the flush earth connection pit that much easier too!! lol!!

I was going to say that 1m wasn't far enough, but thought I'd check, and apparently it's actually 750mm minimum depth for water pipes in England at least. I'm sure I remember being told it was 1200mm, but that might have been Scotland, as I can imagine in Inverness or somewhere 750mm isn't going to protect from winter ground freezing.

When I was doing festivals, one of the stage crews put a spike through the mains water pipe that just happened to be running randomly through the middle of a field without the farmer even knowing anything about it being there. Not what I wanted to see on the morning before we opened, a sodding fountain slap bang in the middle of the front of the main stage.

Been a bit paranoid about it since then.

 

[ipf]

I was going to say that 1m wasn't far enough, but thought I'd check, and apparently it's actually 750mm minimum depth for water pipes in England at least. I'm sure I remember being told it was 1200mm, but that might have been Scotland, as I can imagine in Inverness or somewhere 750mm isn't going to protect from winter ground freezing.

When I was doing festivals, one of the stage crews put a spike through the mains water pipe that just happened to be running randomly through the middle of a field without the farmer even knowing anything about it being there. Not what I wanted to see on the morning before we opened, a sodding fountain slap bang in the middle of the front of the main stage.

Been a bit paranoid about it since then.

'Singing in the rain', eh? or 'Three coins in the fountain', maybe.......shouldn't have been much trouble sinking the rod, at least.

 

[Gavin A]

'Singing in the rain', eh? or 'Three coins in the fountain', maybe.......shouldn't have been much trouble sinking the rod, at least.

twas a 5 foot marquee peg to guy the stage cover down.

 

[Engineer54]

I was going to say that 1m wasn't far enough, but thought I'd check, and apparently it's actually 750mm minimum depth for water pipes in England at least. I'm sure I remember being told it was 1200mm, but that might have been Scotland, as I can imagine in Inverness or somewhere 750mm isn't going to protect from winter ground freezing.

When I was doing festivals, one of the stage crews put a spike through the mains water pipe that just happened to be running randomly through the middle of a field without the farmer even knowing anything about it being there. Not what I wanted to see on the morning before we opened, a sodding fountain slap bang in the middle of the front of the main stage.

Been a bit paranoid about it since then.

The way housing estates etc are thrown up these days in the UK, i'm pretty sure that 90%+ are nowhere near 750mm deep unless as you say the area is subject to long winter periods of 0 C degree and below!!. Anyway excavating down a metre will certainly help the overall Ra and stability of any earth electrode system. Oh, and don't expect the initial Ra value obtained to be it's final value. As the backfilled soil compacts against any exposed electrode, Ra values should fall, sometimes quite significantly...