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

 

[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.
[automerge]1590393361[/automerge]
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.