What Sizecable And Which Consumer Unit For A Tt System

[SPECIAL LOCATION]

If this is an old install somewhere in the dim distant past an earth of some sort should have exsisted....

I doubt it has never had an earth during its entire lifespan so far...?

Wherever or whatever it is..

IMHO it would NOT have been a GAS pipe..

May have been a water pipe or a now hidden buried earth rod..

But NOT a Gas pipe....

So the Gas wire is most likely a bonding wire....

Whoever installed that should have verified an earth somewhere on the installation?

Have you tried testing the earth with your Gas bond disconnected.?

I think as Batty says you need to trace from the voltage operated earth leakage breaker...

P.S.

welcome to the forumbulator..

Guinness   Guinness   Guinness   Guinness

 

[D Skelton]

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 

As you say, there is no need for an s-type up front, but it would be prudent to fit one if relying solely on the RCDs for fault protection.

can anybody advise?

You've already had suggestions and comments from others so I have very little to add. One thing I will say though is that when you do end up driving a rod in, make it a 5/8" 8 footer at least, not a 3/8" 4 foot twig

 

[roys]

Oh and make sure you are well clear of the gas and water main when you hammer the rod in.

 

[steptoe]

As you say, there is no need for an s-type up front, but it would be prudent to fit one if relying solely on the RCDs for fault protection.

You've already had suggestions and comments from others so I have very little to add. One thing I will say though is that when you do end up driving a rod in, make it a 5/8" 8 footer at least, not a 3/8" 4 foot twig

Why 8' ? 

It may not require it, or it may require multiple rods, not deeper.

I'd say drive one 4' rod in, do an Ra, If it's unsatisfactory then it needs tested to verify if it needs to go deeper, or multiple. 

 

[D Skelton]

Why 8' ? 

It may not require it, or it may require multiple rods, not deeper.

I'd say drive one 4' rod in, do an Ra, If it's unsatisfactory then it needs tested to verify if it needs to go deeper, or multiple. 

Depth is key to stability. You're far less likely to be able to guarantee a stable Ra value with only four feet of rod. Round my way down here on the South coast we have about a foot of top soil before you hit solid chalk. If you desperately want a sub 200ohm Ra you need multiple rods, if however you want a sub 200ohm and stable Ra, you need multiple deep rods. 9 times out of 10 though, for me, one deep rod is enough. As long as I can guarantee stability, and as long as the Ra is under 500ohms, the value is almost meaningless. I have however seen 4 foot twig's Ra values fluctuating wildly depending on the time of year and level of rainfall and if you want to keep a sub 500ohm value, the chances are no soil condition will guarantee this with such a small length of rod.

Of course we can only speculate as we don't know the ground conditions where the OP's job is, but what I can say is that I would never be caught driving anything less than 8 feet into the ground. Far better to be safe than sorry I'd say, especially when entrusting the safety of the entire installation to just two small pieces of equipment, well documented for being prone to faliure

 

[steptoe]

Depth is key to stability. You're far less likely to be able to guarantee a stable Ra value with only four feet of rod. Round my way down here on the South coast we have about a foot of top soil before you hit solid chalk. If you desperately want a sub 200ohm Ra you need multiple rods, if however you want a sub 200ohm and stable Ra, you need multiple deep rods. 9 times out of 10 though, for me, one deep rod is enough. As long as I can guarantee stability, and as long as the Ra is under 500ohms, the value is almost meaningless. I have however seen 4 foot twig's Ra values fluctuating wildly depending on the time of year and level of rainfall and if you want to keep a sub 500ohm value, the chances are no soil condition will guarantee this with such a small length of rod.

Of course we can only speculate as we don't know the ground conditions where the OP's job is, but what I can say is that I would never be caught driving anything less than 8 feet into the ground. Far better to be safe than sorry I'd say, especially when entrusting the safety of the entire installation to just two small pieces of equipment, well documented for being prone to faliure

:slap

That is a lot of misinformed twaddle.

You have obviously never worked on slate ground then If you are able to drive 8' rods in everywhere,

I have a 2' rod on a job with an Ra of <70ohms,

Has never been over 100 in the ten years it's been in, tested yearly.

As for needing to be 8' to be stable, complete and utter nonsense, very very seldom have I needed to join rods to get an acceptable stable Ra, 

Just how do you preliminary survey these jobs to know you will need such deep rods?

My tester only has 16" probes, I'd hate having to drive 8' ones in just for a survey,

I'd bet my customers would love that too.! 

 

[ProDave]

You need to know your local conditions.  Here we have a high water table on peaty ground. It's hard enough getting a 4' rod all the way in with the amount of stones and boulders in the ground. I'll bet you would never get an 8' one all the way in.

 

[Andy™]

usually get <80 with a 4ft up here

 

[D Skelton]

:slap

That is a lot of misinformed twaddle.

You have obviously never worked on slate ground then If you are able to drive 8' rods in everywhere,

I have a 2' rod on a job with an Ra of <70ohms,

Has never been over 100 in the ten years it's been in, tested yearly.

As for needing to be 8' to be stable, complete and utter nonsense, very very seldom have I needed to join rods to get an acceptable stable Ra, 

Just how do you preliminary survey these jobs to know you will need such deep rods?

My tester only has 16" probes, I'd hate having to drive 8' ones in just for a survey,

I'd bet my customers would love that too.! 

Well, thanks for the constructive response. Stick the knife in a little deeper why don't you?! You do love a bit of sniping that's for sure! A harmless comment that I thought might even be slightly constructive according to your standards, but NO! Spit the venom, Damian's WRONG again! Haha

I have never worked on slate ground no, so I cannot comment, but do you think driving rods in to chalk is a piece of cake?! No, that's why I use a very handy tool called an earth rod driver. 8 feet of rod can be driven into chalk in 30 seconds easy.

With regards to your 2 foot rod, a million factors could be affecting that, such as how close it is to the TX. As for it having such a stable value, have you tested it quarterly?

My original point was that if the OP is happy to leave the safety of his entire installation in the hands of two 30mA RCD's, devices that are well known to be prone to faliure, then a four foot twig will probably be more than enough, he will afterall have 1667 ohms to play with. If however he wants to add some back up protection in the form of an s-type up front, then he's only got 500 ohms to play with, therefore he'd need to be able to guarantee a level of stability that a four foot twig, in all likelyhood, just won't provide.

You clearly have very little understanding of the difference between ohmic values and stability when it comes to TT earthing. The only way to guarantee stability is to go in deep. I have measured both initially low but unstable Ra values and high but stable values, the key difference between them being depth.

As for misinformed twaddle, you need to look in the mirror pet.

 

[Steve3948]

When I did a few years on an MOD base also with chalky ground we also used an earth rod driver, and I witnessed one of the funniest things I have seen.

I was working with an older sparky, I would have been around 22 and he was 45 ish so was in charge, we kept adding and adding rods, then testing then adding more rods but the reading was no better until a bloke walked past and said is that aerial thing anything to do with you guys, we both looked over and sticking out of the ground almost vertical was at least 4 of the rods. Pete started the rod driver and yes a bit more started to emerge.

Red faced he said we better dig around where it comes out of the ground and cut it off, we were then instructed to lay 30M of Copper tape and see how that fared. I have mentioned it before the reading had to be below 10 ohms, due to it being something to do with radio.

 

[NozSpark]

Holy cow... do you think that I should replace all those 4' rods that ive installed then?

Even the one that came in at <15 ohms out in the sticks?

 

[D Skelton]

Holy cow... do you think that I should replace all those 4' rods that ive installed then?

Even the one that came in at <15 ohms out in the sticks?

No, just pray that their values remain stable.

I was testing a twig just the other day, about 20 miles inland from me which is pretty much clay bed, lovely stuff for TT systems, first time testing it so no clue as to it's stability and I got 5 ohms. It happens, easily. Have you been back to check the value of your 15 ohm rod?

 

[steptoe]

Well, thanks for the constructive response. Stick the knife in a little deeper why don't you?! You do love a bit of sniping that's for sure! A harmless comment that I thought might even be slightly constructive according to your standards, but NO! Spit the venom, Damian's WRONG again! Haha

I have never worked on slate ground no, so I cannot comment, but do you think driving rods in to chalk is a piece of cake?! No, that's why I use a very handy tool called an earth rod driver. 8 feet of rod can be driven into chalk in 30 seconds easy.

With regards to your 2 foot rod, a million factors could be affecting that, such as how close it is to the TX. As for it having such a stable value, have you tested it quarterly?

My original point was that if the OP is happy to leave the safety of his entire installation in the hands of two 30mA RCD's, devices that are well known to be prone to faliure, then a four foot twig will probably be more than enough, he will afterall have 1667 ohms to play with. If however he wants to add some back up protection in the form of an s-type up front, then he's only got 500 ohms to play with, therefore he'd need to be able to guarantee a level of stability that a four foot twig, in all likelyhood, just won't provide.

You clearly have very little understanding of the difference between ohmic values and stability when it comes to TT earthing. The only way to guarantee stability is to go in deep. I have measured both initially low but unstable Ra values and high but stable values, the key difference between them being depth.

As for misinformed twaddle, you need to look in the mirror pet.

Yep, I obviously don't have much of a clue,

That's why I'm going to honestly ask the question,

Why does the proximity of the Tx have any bearing on the Ra.?

 

[kerching]

Well the last 11kV sub i did had a 25mm 6491x wire stripped, run around the outside of the base, rodded at each corner with a 1m rod and back into the sub

Just saying

 

[D Skelton]

Yep, I obviously don't have much of a clue,

That's why I'm going to honestly ask the question,

Why does the proximity of the Tx have any bearing on the Ra.?

It won't??

Edit: You know what, I'm done arguing the toss with you. Unless you're going to engage in decent debate rather than just sniping at me for no good reason, I won't be responding. I've no time for people who's heads are up their own backsides.

Stick to your 4 foot rods mate, really, go about your business, I haven't got a problem.

 

[steptoe]

It won't??

Edit: You know what, I'm done arguing the toss with you. Unless you're going to engage in decent debate rather than just sniping at me for no good reason, I won't be responding. I've no time for people who's heads are up their own backsides.

Stick to your 4 foot rods mate, really, go about your business, I haven't got a problem.

I don't know, honestly,I don't know why the proximity of the Tx has any bearing on the Ra,

Now, I haven't snided or argued with you on any threads that I know of, or can think of, so why are you now suddenly having a go at me,

I asked an honest question on a subject I obviously didn't understand, and you decide I'm having a go at you so don't answer, that's a tad unfair don't you think,?

I think its a little unfair of you to make an obviously informed statement then refuse to explain how or why,

I thought the purpose of these sort of forums was to discuss ideas, facts, and exchange opinions as to the why's and wherefores of various things,

I only asked as I didnt know why the Tx had a bearing on the Ra of a TT rod, I understand why it would make a difference to the Ze of a TN system, but I don't understand the bearing on a TT system,

A knowledge's shared and all that, isn't that why we are all here,?

 

[M107]

When driving the rods for sigs kit we used something like this..

http://www.harger.com/products/idx.cfm?subc4id=120&p=1&oiid=15&thid=492&footid=0&footinfoid=0

As  for the depth of rod's........ I'm down here in the south,Newbury/Thatcham & going north is 1mtr of good soil under foot then under  that it's ancient riverbed of gravel (my extension foundations were near on 3mtrs deep & we were still in gravel) & yes doubling the rods is sometimes required, but not had any problems if they are length ways  or spaced apart.

If I get to Andover/Kingsclere/Basingstoke I'm on chalk & the only time I've had to go deep (ooh err missus) with extended rods is when on the downs/high ground because yes the ground dry's out very quick even after a down pour.

Other than that, one rod (5/8 or other wise) to date has been fine & stable.

What does get on my ***'s is the Hampshire diamonds (that's flints to you foreigners) that you'll always hit when knocking in a rod.

 

[Sidewinder]

It won't??

Edit: You know what, I'm done arguing the toss with you. Unless you're going to engage in decent debate rather than just sniping at me for no good reason, I won't be responding. I've no time for people who's heads are up their own backsides.

Stick to your 4 foot rods mate, really, go about your business, I haven't got a problem.

Damien,

How can the local DNO Tx have any bearing on the reading taken by en earth electrode resistance tester to establish the Ra of the electrode when the supply coming from that said Tx has nothing to do with the measurement being taken?

Please think about this then come back?

Now a Ze measurement the Tx, CAN have an influence on, an Ra, impossible, unless you can change the laws of physics, or you are putting the rod within the spheres of influence of the earthing network for the local DNO Tx which, you must not do.

 

[steptoe]

Damien,

How can the local DNO Tx have any bearing on the reading taken by en earth electrode resistance tester to establish the Ra of the electrode when the supply coming from that said Tx has nothing to do with the measurement being taken?

Please think about this then come back?

Now a Ze measurement the Tx, CAN have an influence on, an Ra, impossible, unless you can change the laws of physics, or you are putting the rod within the spheres of influence of the earthing network for the local DNO Tx which, you must not do.

OK

can you explain that is simpleton words please?

obviously Damian has fell out with me, so wont tell me,

how does the impedance of the star point to the neutral of the installation affect the resistance of the ground?

is, I suppose, the question.

 

[Sidewinder]

Steps,

IF you are taking an Ra measurement, then the incoming supply has nothing to do with the measurement if you are using an electrode resistance tester.

It's supplied from it's own supply and the Ra measurement to the rod can be taken even before the DNO supply is provided.

Hence the DNO Tx has nothing to do with the Ra measurement.

The Spheres of influence bit, is to do with the voltage gradient etc. in the event of a fault on the HV or LV DNO network.