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

 

[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.
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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