UK to TT or not to TT

[Mfctitch]

Morning everyone
It seem I have another outside job.
this time the run is 44 metres with about 2 metres going into a chase in a concrete path so calculated for underground.
the job is to supply a double socket at the end of the garden but eventually this will be where a large shed is going to go into which they will want sockets lights and a heater
Board has one spare way but dual RCD
So thinking of putting in a 63 amp fused main switch
then of course an RCD weather proof double socket.

But I've allowed for 38 amps i.e garage unit
so as a section is underground its coming up at 10mm rather then 6mm if all over ground.
by the way i'm using 3 core SWA
That's not the problem.
I just know you like lots of info.

The question is would you TT or not?
and why would you TT or not TT?
I like watching John Ward and he's not a fan of TT so if you would TT would you test it as a ZE an the sub board or test it using the 3 pins

Thanks again for your help

 

[buzzlightyear]

what is the main system now in and any extraneous pipes going to the new shed .

 

[ChrisElectrical88]

Is the DNO supply TNS or PME? Wooden or Metal shed?

 

[Mfctitch]

what is the main system now in and any extraneous pipes going to the new shed .

Ha ha
I knew I'd leave something out TNC-S
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Is the DNO supply TNS or PME? Wooden or Metal shed?

wooden shed no metal work

 

[pc1966]

The main argument for going TT is when you have a TN-C-S supply and have some issue with bonding extraneous conductive parts. If you are using 3-core 10mm SWA then your CPC size is adequate for bonding on all system.

So really the only remaining issue would be if there was a reason to worry about the bonded metalwork presenting a hazard to true Earth if you were unlucky enough to have a PME fault?
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Final thought: make sure there is enough of the SWA feed to the socket left that it can be used to the final garage CU location. You could do a resin joint later if needed but life is much easier and more reliable if you don't have to!

 

[Mfctitch]

The main argument for going TT is when you have a TN-C-S supply and have some issue with bonding extraneous conductive parts. If you are using 3-core 10mm SWA then your CPC size is adequate for bonding on all system.

So really the only remaining issue would be if there was a reason to worry about the bonded metalwork presenting a hazard to true Earth if you were unlucky enough to have a PME fault?

very true but in that case the whole house would be affected not just the shed, so no to TT in your view?

 

[buzzlightyear]

The main argument for going TT is when you have a TN-C-S supply and have some issue with bonding extraneous conductive parts. If you are using 3-core 10mm SWA then your CPC size is adequate for bonding on all system.

lose a nuatral then your buggered ,TT IT .44 mtrs away .

 

[Mfctitch]

The main argument for going TT is when you have a TN-C-S supply and have some issue with bonding extraneous conductive parts. If you are using 3-core 10mm SWA then your CPC size is adequate for bonding on all system.

So really the only remaining issue would be if there was a reason to worry about the bonded metalwork presenting a hazard to true Earth if you were unlucky enough to have a PME fault?
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Final thought: make sure there is enough of the SWA feed to the socket left that it can be used to the final garage CU location. You could do a resin joint later if needed but life is much easier and more reliable if you don't have to!

Thanks but have allowed for and extra 6 metres which will be coiled up as they don't know exactly where the shed is going

 

[bigspark17]

Ha ha
I knew I'd leave something out TNC-S
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wooden shed no metal work

why would you TT this then?

 

[pc1966]

very true but in that case the whole house would be affected not just the shed, so no to TT in your view?

Generally if you have a reliable TN-derived earth then you should use it.

The issue is when you have something conductive outdoors that is on the CPC and if it were to go to ~100V under the rare PME fault condition it would be a hazard. Just now the focus has been on EV chargers for this case (with someoene washing the car when a fault occurred, etc), but in the past it was prohibited for building sites, etc.

Typical outdoor power tools like lawnmower, hedge-trimmer, etc, are not an issue as they are mostly double-insulated.

 

[Mike Johnson]

Hypothetical question: what would be the problem if you did TT the supply at the shed?

 

[Mfctitch]

So at the moment it seems that we mostly are in the john ward camp.
and that my calculations and equipment are ok
but just for argument's sake if I went TT how would you test it as I've been shown 2 ways
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Hypothetical question: what would be the problem if you did TT the supply at the shed?

Is that aimed at me or everyone?
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Just putting it out there I have to sign off as the dog is getting impatient for his walk sorry.
I'll return later hopefully to find more questions and some answers

 

[pc1966]

Hypothetical question: what would be the problem if you did TT the supply at the shed?

Assuming this is a general question, then my answer is you are trading off:

• Reduced risk of a PME-fault making bonded (Earthed to most folk) equipment live.
• Increased dependency on the RCD action to clear any fault (which is less reliable then the MCB trip action).

Now I don't really have any numerical values of the probability of either fault leading to death/injury so I can't honestly advise either way. But if going TT then I personally would spend the extra to have a 100mA S-type incomer and separate 30mA RCBOs on the final circuits to avoid any single point of failure in earth fault protection.
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but just for argument's sake if I went TT how would you test it as I've been shown 2 ways

With TT you have two things to test as both are needed for fault clearing:

• Earth rod impedance
• RCD trip action

Testing the RCD is standard for any MFT, the issue really is the earth rod and making sure it is reliably below 200 ohms (for most systems at 100mA/30mA RCD action).

Testing the rod without AC power is more fundamental, but needs suitable test equipment and many MFT lack that capability, and usually involves a bit of poking rods in the ground at a fair distance to see if you get the plateau for the "fall of potential" result. But there are various other test approaches (mostly more expensive in equipment to carry out).

If you have AC power then simply using your MFT to do a Zs measurement with the rod as the earth is easiest, but needs more care and may lead to the unfortunate case of finding out it is not good enough and having to isolate it again before getting another rod (assuming the first was not driven down far enough).

That is another risk with going TT - you need a earth rod (or equivalent reliable connection to Earth) and some folk just hammer it in without checking if there are buried pipes or cables in the area!

 

[Strima]

38 amps for a shed?

 

[pc1966]

Sauna? Interesting plant cultivation? Brewery?

 

[Mike Johnson]

I never thought for a moment that someone would instal an Earth Rod and not check its impedance or location, if attached to a RCBO of suitable value in the DB would that not cover any problems that may be encountered?

Interesting plant cultivation, I assume you mean Orchids.

 

[radiohead]

lose a nuatral then your buggered ,TT IT .44 mtrs away .

These sort of statements always confound logic.
What if you TT and someone cuts the earth lead to the rod? What if the RCD you are totally reliant on for earth fault protection fails?
No electrical system is immune from faults and a TT has every bit as much potential for danger as a TNCS. And what difference does the distance from the supply make? 4m is safe but 44 isnt?

 

[Taylortwocities]

Like the majority, I see little point in abandoning a perfectly wonderful earth loop impedance of less than half an ohm (from the TNC-S) for a weedy TT rod that will maybe provide something like 200 ohms except in a dry summer or when the gardener has chopped the cable when clearing the bind weed. (I've done it).

No extraneous conductive parts? stick with TNC-S. (IMO)

 

[Mike Johnson]

Those "weedy" TT rods are relied upon in the majority of Europe where the Electrical Authorities do not supply an Earth.

 

[pc1966]

I never thought for a moment that someone would install an Earth Rod and not check its impedance or location, if attached to a RCBO of suitable value in the DB would that not cover any problems that may be encountered?

Of course the rod, location, protection of cable, etc, would all be thought through and tested when done professionally

The issue is not one of "Is a proper TT installation good enough?" as clearly it is. The debate comes from looking at the possible failure modes and the likely lack of any further testing of RCD/RCBO for possibly a decade or more once installed, and comparing it to the TN-C-S case also available.

Yes, the RCD have test buttons. Yes folk are told to check them regularly. But...

Interesting plant cultivation, I assume you mean Orchids.

Those are interesting plants for sure, and potentially profitable as well.

 

[davesparks]

lose a nuatral then your buggered ,TT IT .44 mtrs away .

Distance is irrelevant to this
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Those "weedy" TT rods are relied upon in the majority of Europe where the Electrical Authorities do not supply an Earth.

Great, but how is that relevant?

'someone else did it so it's OK for me to do it' sounds more like a child in a playground than an intelligent adult.

 

[Mike Johnson]

It's relevant because most of Europe use it as an acceptable means of supplying an Earth, so it should not be discounted, now put your toys back in your pram.

 

[buzzlightyear]

These sort of statements always confound logic.
What if you TT and someone cuts the earth lead to the rod? What if the RCD you are totally reliant on for earth fault protection fails?
No electrical system is immune from faults and a TT has every bit as much potential for danger as a TNCS. And what difference does the distance from the supply make? 4m is safe but 44 isnt

none above whether a fault accrues you are relying on a earth from the DNO on their system when a neutral goes down and then what..

 

[radiohead]

none above whether a fault accrues you are relying on a earth from the DNO on their system when a neutral goes down and then what..

Run that by me again Buzz!

 

[buzzlightyear]

View: https://youtu.be/JRHyqouJPzE
their you go.

 

[radiohead]

The thing is Buzz it's no more likely to happen than any other potentially dangerous defect on any system. I've seen failed RCD's on TT systems which would cause a shock hazard in the event of an earth fault. The risk of a broken neutral on TNCS systems it miniscule, it simply isn't worth worrying about for an external shed installation which isn't even considered sufficiently hazardous to be classed as a special location in Bs7671.

 

[pc1966]

Exactly, all about balancing the risks.

 

[Mfctitch]

Evening
So finally back.
so to recap it looks like no TT
but I'd still like to know which method people would use for testing a TT system
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View: https://youtu.be/JRHyqouJPzE
their you go.

love watching his stuff
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Assuming this is a general question, then my answer is you are trading off:

• Reduced risk of a PME-fault making bonded (Earthed to most folk) equipment live.
• Increased dependency on the RCD action to clear any fault (which is less reliable then the MCB trip action).

Now I don't really have any numerical values of the probability of either fault leading to death/injury so I can't honestly advise either way. But if going TT then I personally would spend the extra to have a 100mA S-type incomer and separate 30mA RCBOs on the final circuits to avoid any single point of failure in earth fault protection.
[automerge]1590320598[/automerge]

With TT you have two things to test as both are needed for fault clearing:

• Earth rod impedance
• RCD trip action

Testing the RCD is standard for any MFT, the issue really is the earth rod and making sure it is reliably below 200 ohms (for most systems at 100mA/30mA RCD action).

Testing the rod without AC power is more fundamental, but needs suitable test equipment and many MFT lack that capability, and usually involves a bit of poking rods in the ground at a fair distance to see if you get the plateau for the "fall of potential" result. But there are various other test approaches (mostly more expensive in equipment to carry out).

If you have AC power then simply using your MFT to do a Zs measurement with the rod as the earth is easiest, but needs more care and may lead to the unfortunate case of finding out it is not good enough and having to isolate it again before getting another rod (assuming the first was not driven down far enough).

That is another risk with going TT - you need a earth rod (or equivalent reliable connection to Earth) and some folk just hammer it in without checking if there are buried pipes or cables in the area!

sorry missed this before I posted
My MFT has the ability to use the 3 leads and I've done it once and it did seem very long winded that's why I asked just to check that I'm not being lazy and testing like you would do a Ze is just as good?
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38 amps for a shed?

your question mark leads to to think that my method of just adding the 32 and 6 amp is a little over the top and I might be erring too much on the side of caution.

 

[Mike Johnson]

Testing an TT earth rod, page 55 : https://dam-assets.fluke.com/s3fs-public/166x____umeng0100.pdf

 

[Mfctitch]

Testing an TT earth rod, page 55 : https://dam-assets.fluke.com/s3fs-public/166x____umeng0100.pdf

Someones up early
thanks for that but I have the same info for my megger my question was would you do it the long way round or is doing it as a Ze just being lazy

 

[pc1966]

Megger do a much more in-depth guide to earth impedance testing, here is one of them (just follow the download button):

Earth electrode and looping, Theory and applications

Earth electrode and earth loop impedance testing Theory and applications - As two of the most common testing subjects debated amongst electricians.

uk.megger.com

However, I remember seeing another Megger guide that covered earth red specifically and how to check situations like sub-station where you may not get far enough away for fall-of-potential to be accurate, etc.
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Here it is. Well worth keeping a copy (and reading it) if you ever do need to put in earth rods and it really matters (e.g. if you need well below 200 ohms for a special installation with high current RCD on TT, or it has to be below 10 ohms for lightning guide lines, etc):

http://www.biddlemegger.com/biddle-ug/GettingDownToEarth-MC.pdf

 

[Mike Johnson]

Someones up early
thanks for that but I have the same info for my megger my question was would you do it the long way round or is doing it as a Ze just being lazy

IMO there is only one way to do the test correctly.

 

[Strima]

your question mark leads to to think that my method of just adding the 32 and 6 amp is a little over the top and I might be erring too much on the side of caution.

What is the intended use of the shed? A couple of sockets for power tools and a light fitting will only have low demand.

The average 3 bed house won't even draw that much current, yes I know there will be exceptions such as electric showers, water heaters etc.

thanks for that but I have the same info for my megger my question was would you do it the long way round or is doing it as a Ze just being laz

The Ze method of testing an electrode is a recognised method in GN3. Infact this is often the only way I can test a lot of street cabinets as they're often surrounded by tarmac/concrete so I can't get the test electrodes in and the trailing leads would pose a trip hazard to members of the public.

 

[davesparks]

It's relevant because most of Europe use it as an acceptable means of supplying an Earth, so it should not be discounted, now put your toys back in your pram.

Other countries do it therefore we should lower our standards to match theirs?

What is a typical Ra for these rods used in Europe? What is their fault current handling ability?
What size is the typical incoming supply?
What is the typical PFC of their supplies?

 

[pc1966]

IMO there is only one way to do the test correctly.

What is that?

The Megger guide shows half a dozen with various pros and cons! But as @Strima has already said, what is practical to do might be the overall determining factor, and to some degree GN3 is about practical ways to ensure a system is OK.

 

[Mfctitch]

What is the intended use of the shed? A couple of sockets for power tools and a light fitting will only have low demand.

The average 3 bed house won't even draw that much current, yes I know there will be exceptions such as electric showers, water heaters etc.


The Ze method of testing an electrode is a recognised method in GN3. Infact this is often the only way I can test a lot of street cabinets as they're often surrounded by tarmac/concrete so I can't get the test electrodes in and the trailing leads would pose a trip hazard to members of the public.

I have no idea what they are going to run from there. All I do know is that they will want to run the small pool pump with is on a 13amp plus they have mentioned heating so that a 3Kw panel heater so I'm assuming a computer and the bits that go with it, maybe a small fridge and a kettle and some lights.

and thanks for the reply on the Ze

 

[radiohead]

I only have the means to carry out an earth loop impedance test on an electrode and consider that adequate for compliance.

 

[Mfctitch]

Well it seems that we have covered this thread.
and it looks like I wont be turning this job into a TT but in future if I have to turn a job into a TT I'll be testing as per Ze.
I have another question that has come up after a comment from Strima but I'll start a new thread for that one so just saying thanks again for your input

 

[davesparks]

Using a loop impedance test is accepted if the installation is protected by RCD.

I think this is because the RCD will still work OK if the measured value isn't entirely correct.

For anything where the resistance needs to be measured reliably/accurately an earth electrode test is needed.