What Sizecable And Which Consumer Unit For A Tt System

[steptoe]

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.

thats what I thought,

but it seems that Mr Skelton seems that the star point and the impedance has some bearing on this,

I just got confused between him intermixing the resistance of the ground with the impedance of the supply,

in his instance how would you measure the Ra for [say] a large industrial/commercial building before the supply is available.?

is it even possible?

is Damian saying that once I measure the Ra it will change once the DNO install their substation.?

 

[Sidewinder]

I don't know what he is suggesting Steps as he's not around.

 

[Marvo]

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.

I assume this is tongue in cheek..... I didn't know you could buy a 2 foot earth rod. If it is serious then I'd question how you could get physical stability in a rod that's only 2 feet into the ground. Electrical stability is not possible without physical stability. Also seasonal fluctuations would likely be unacceptably high if the ground freezes in winter. The shortest earth spike we use is 2.3 meters and I can't remember the last time we only needed a single rod to achieve an acceptable Ra reading.

 

[steptoe]

I assume this is tongue in cheek..... I didn't know you could buy a 2 foot earth rod.

If it is serious then I'd question how you could get physical stability in a rod that's only 2 feet into the ground. Electrical stability is not possible without physical stability. Also seasonal fluctuations would likely be unacceptably high if the ground freezes in winter. The shortest earth spike we use is 2.3 meters and I can't remember the last time we only needed a single rod to achieve an acceptable Ra reading.

its not tongue in cheek,

its actually a rod that was cut, assessed the requirement before the rod was hammered in, and an RA suggested as shallow as possible due to the risk of other services,

Ra was tested before rod was cut, has remained stable and acceptable since.

 

[Marvo]

If there's a risk of hitting services we'd either dig by hand for the first 1 - 1.5 meters or we'd use an earth mat, either way the spades would be out. I couldn't consider a 2 foot rod as a stable option no matter how low the impedance on the day so I guess we'll agree to disagree.

 

[SLIPSHOD & SLAPDASH]

I dont do much TT work but recently been involved with a lot of temporary generator installations. They have a sensing circuit which will not allow the genny to start & will shut it down if the ra is too high. I dont know the figures but will try to find out, however quite often when we turn up to site the contractor has fired up the genny to get the kettle boiled. The earth rod is normally not more than 100mm deep . Most of these jobs are on the highways and can last 6 months. We have to drill a hole inbetween the curbstones or sometimes the tarmac and hammer in a rod (after cat scanning the area).

My point here is these jobs have the gennys running 24/7 & the earth is monitored but is never unstable enough to shut down the motor even with what I would gave thought a poor electrode.

 

[steptoe]

If there's a risk of hitting services we'd either dig by hand for the first 1 - 1.5 meters or we'd use an earth mat, either way the spades would be out. I couldn't consider a 2 foot rod as a stable option no matter how low the impedance on the day so I guess we'll agree to disagree.

I can understand that,

but why?

on what grounds?

if the rod is giving a consistent Ra, and the only physical interference is nature, why would it not be satisfactory?

perhaps not ideal, but much of what we do isnt.

EDIT,

its NOT about the impedance of the rod, its about its resistance.

 

[SPECIAL LOCATION]

 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

For the sake of the OP I will correct some misinformation that has been stated above... 

This guidance is incorrect as there is no regulation stating that is should be done.....

This is one of the problems with the industry today.....

People stuck in their ways..

not willing to look at other suitable solutions to a problem they need to solve..



:Salute

 

[D Skelton]

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,

because I read your question to be in a sarcastic tone, a reasonable assumption to make given a couple of lines directed at me in your first response, namely; "hahaha, that is a lot of misinformed twaddle." and "complete and utter nonsense".

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,?

If it was truly an honest question then I apologise, but as I said above, maybe you can see why I took it the way I did.

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

The first part of my post #36 says it. The distance to the TX won't have an impact on the Ra, but it will have an impact on your earth fault loop path. You might get a lovely Ra result, but what happens at the height of summer and the top soil has all dried out when you're a half a mile away from the nearest transformer? Maybe I wasn't clear enough when trying to make this point?

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,?

In red

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.

As I said in post #36, it won't.

For the sake of the OP I will correct some misinformation that has been stated above... 

This guidance is incorrect as there is no regulation stating that is should be done.....

This is one of the problems with the industry today.....

People stuck in their ways..

not willing to look at other suitable solutions to a problem they need to solve..



:Salute

Touché Sir.

As I'm sure you're well aware though, the big fat smiley face at the end of that line kinda gives away the fact that it is advise. Also make note that nowhere do I say that this is the way it must be done, so no, not misinformation

 

[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.

To me this did not state that the Tx will not have an influence on the Ra.

With the question marks, it appeared that a question was being asked, rather than a statement made.

 

[Sharpend]

Damien

I have read and re read the thread, and I fail to see how Steps reply about twaddle was in a sarcastic tone? He merely made a statement in reply to your statement.

I think that you are taking things too personally, or perhaps are a tad over defensive? I'd say that in most if not all cases, comments posted are directed in relation to what another has commented upon, not at the person themselves.

That is not too say that some comments aren't at the person but these are usually where it is blatantly clear that the poster is either totally ignorant of their inability to carry out electrical work safely without risk to themselves or others, or they come to the forum with attitude in their posts.

I will say that if a poster brings an idea or topic to the forum for discussion or debate then it is only polite that they fully finish they're understanding of the topic on the thread, otherwise how are others to learn from their knowledge or them to develop their knowledge further? Sometimes we all need to accept gracefully that we may be wrong with our understandings?

Perhaps if you answered steps question then knowledge would be exchanged?

 

[Steve3948]

I sgree with Marvo I dont think a 2ft Rod is acceptable and a mat or tape would be much better, after all if a 2ft Rod was acceptable you would be able to buy them off the shelf

Well you can but I think they are for use with the Ra tester only

 

[D Skelton]

Damien

I have read and re read the thread, and I fail to see how Steps reply about twaddle was in a sarcastic tone?

It's not that response that I'm talking about, that one was just a bit rude. The second response of his at post #34 is that one I'm referring to. He says that I'm speaking a load of misinformed twaddle and that I'm speaking nonsense, that in itself is enough to say that he must be informed. If you are accusing someone of being misinformed then surely you have to be informed enough to make that accusation. Therefore I was in disbelief that someone so 'informed' would ask the question he did, that's why I read it sarcasticly. Anyway, steptoe said that it was an honest question and I apologised and attempted to answer it.

Back to the point in hand however. The notion that we can happily rely on 2 foot rods to achieve stable ohmic values is just nonsense. The most stable values I have ever achieved have been when we set up TT systems with TN values (I've only done this twice). In order to get the low values we use multiple rods, sometimes four or five in crows foot configuration and each is driven deep (normally 12-16ft) to guarantee that stability. In these cases the Ra just doesn't fluctuate at all and as I've already said, I've seen four footers fluctuating hundreds of ohms at a time.

Edit: in my experience, TN values on TT systems are only achievable on the inland side of the South Downs from me (clay bed). I could whack in a thousand rods where I am (solid chalk) and not get anywhere near a TN value. So yeah, as others have said as well, the ground conditions will always be a factor when attempting to design a good, stable TT system.

 

[steptoe]

#30 by  D Skelton, you stated that proximity to the Tx could be affecting how good or stable the Ra was on a rod,

How do you explain that.? 

 

[D Skelton]

#30 by  D Skelton, you stated that proximity to the Tx could be affecting how good or stable the Ra was on a rod,

How do you explain that.? 

Well now you point me back to that post, it certanly does seem like a very poorly worded line. I can definitely see why you would question that. I think what I was trying to get across was that a million things could affect the effectiveness of that rod, such as the distance to the Tx, not specifically that the distance to the Tx would have a direct impact on the individual Ra of the rod which of course it won't. I'll happily hold my hands up, I worded that very badly.

 

[Cirrus]

TT systems account for circa 99% of my domestic work and as a standard fit we use 30mA dual RCD C/U's, 10mm earth and 4' earth rods and regularly see Ra readings of 20 ohms - 190 ohms. Only once have I had to install multiple spikes (4 to be exact) due to unknown subtrate giving higher than acceptable readings.

 

[Steve3948]

TT systems account for circa 99% of my domestic work and as a standard fit we use 30mA dual RCD C/U's, 10mm earth and 4' earth rods and regularly see Ra readings of 20 ohms - 190 ohms. Only once have I had to install multiple spikes (4 to be exact) due to unknown subtrate giving higher than acceptable readings.

The gravel areas like Kempsford, Westwell, Holwell, Ashton Keynes are all areas where a TT can be a right pain in the whatsit.

 

[Manator]

Steptoe, what Sidewinder is saying is that the only influence the Tx can make on any installed rod is if that rod was installed as part of, or in close proximity to the Tx earthing field.

If you think about the earthing methods, then as Sidewinder suggests, Ze can be influenced, but Ra would never be.

 

[steptoe]

Steptoe, what Sidewinder is saying is that the only influence the Tx can make on any installed rod is if that rod was installed as part of, or in close proximity to the Tx earthing field.

If you think about the earthing methods, then as Sidewinder suggests, Ze can be influenced, but Ra would never be.

Agreed, I can understand that, it was the post of D Skelton that said the Tx proximity could affect the Ra that I was questioning, and had I been missing something all these years.

 

[D Skelton]

Agreed, I can understand that, it was the post of D Skelton that said the Tx proximity could affect the Ra that I was questioning, and had I been missing something all these years.

I didn't say it would, I just worded the line badly. It wasn't the point I was making.