UK to TT or not to TT

[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.