Earthing Problems

[Barking Mat]

The only time I have ever found it necessary to get creative with earth electrodes was when the client had an indoor swimming pool and to avoid issues with the supply neutral, we had to employ a TT earthing system to protect the whole house installation.
To ensure the earth touch voltage could not exceed 50 Volt we had to provide a earth electrode that didn’t exceed 20 Ohm.

To the OP, if you are committed to creating a TT installation with sufficiently low Ze to enable your MCBs to operate within 0.4 seconds in the event of your RCD failing to operate; are you aware of the maximum permissible impedance needed to ensure that your MCBs will operate?

The impedance of your electrodes will vary and may also depend upon the reliability of the earth conductor termination to the electrode or any other metal to metal joints.

If your 30 mA RCD is in good working order your main earth conductor only needs to flow 0.03 Amp to operate.

Good luck with your project.

Welcome to the forum.

 

[Onoff]

Welcome to the forum.

Guns N' Roses?

 

[Barking Mat]

Guns N' Roses?

That's 100% lost on me.

 

[Onoff]

That's 100% lost on me.

It's the pub talking, neurons firing at random!

 

[dobbslc]

Guns N' Roses?

That's 100% lost on me.

Welcome to the jungle baby (shouting in a screechy voice)

 

[Hotstuff]

Just back from the pub so aware of very little!

I'm not sure if I have the tables showing the current curve characteristics of these Type 1 & 2 C50 breakers!

BS 3871?

Vaguely recall the yellow / white writing on them dictated something too? Interrupt rating?

Pit should be dug this weekend and maybe I'll get the plate buried. Not sure if I'll have the brass nuts and bolts to bolt the "strap" to it.

Hope this morning doesn’t find you suffering after last night’s journey on the “night train” lol.

Your C50 MCBs would indeed be BS3871 and probably a type 2 curve with a duty PSC rating of just 4.5 KA.

The time current curve charts are to be found within BS7681. I only referred to them as it would give you an idea as to how low your installation’s Ze has to be in order for a short circuit/overload protection device to operate in time in the event of an earth fault.

The other thing to consider is the maximum permissible touch voltage of 50 Volt.
If you are designing your system to operate even if your main RCD fails, do consider the danger of faults not clearing and the electrical potential of your earthing system being raised.

I’ve seen all the problems associated with TNC-S earthing systems and completely understand your concerns of poor supply neutral impedances but in all honesty and with respect, I would strongly recommend that you invest your money and efforts in a nice new consumer unit complete with RCD/RCBO protection and arrange for your DMO to confirm that TNC-S is available in your area and get it connected.

Your DNO will probably ask for a satisfactory electrical installation condition report prior to connection.

I retired early a couple of years ago but had been a QS for an electrical contractor for about 20 years. Sadly I no longer have access to a technical library so I can’t help with better guidance as to regulation refs or exact locations of charts.

I hope my ramblings make sense and trust that they don’t come across as condescending.

 

[Onoff]

Your DNO will probably ask for a satisfactory electrical installation condition report prior to connection.

That's one reason I won't ask them yet!

 

[Onoff]

Today I made.....the digger driver a cup of tea.

9" wide, 4' hole dug. Lots of flints and clay came out. Funnily enough the digger started to struggle 4' down on the flint layer which stopped my rods going much further. The centre of the 2' square plate will be 3' down.

Going to clean up the plate, make a copper strap up and see what we've got after back filling.

I'm hopeful. This will give a massively increased surface area over the rod (s).

 

[Hotstuff]

That's one reason I won't ask them yet!

Totally understand mate, it’s a proper ball ache once you have to involve the DNO in any project.

It could be that after testing the Ze, a previous householder connected the main earth conductor into that nice shiny new service head.

 

[Onoff]

Some flattened 28mm copper pipe, bolted with 3 M8 brass bolts. I fluxed and tinned the pipe, bolted it up and went over it with the torch again. Couldn't really get enough heat so popped to the BiLs who has oxyacetylene.

I went over the copper plate with a cup brush in the grinder. Didn't really bring it up as I'd expected. Then gave it a wash with a strong citric acid solution and rinsed. Again, still not shiny...

Will the verdigris affect it that much?

 

[awemawson]

If it was a mirror finish it would be covered in verdegis within a year of burying I would expect

 

[Onoff]

Bolted and soldered. Cleaned and masked up then sprayed the joint with some black bitumen. A trade off as in losing a bit of surface area but hey ho...

 

[Onoff]

Pre the burial ceremony this morning I had an "I wonder" moment. I'm not overly convinced / happy that the copper plate is clean enough. Went and put the low impedance ohmmeter on it:

(Can't for the life of me think why SWMBO always complains I have tools everywhere...)

On the exposed copper (didn't take the lead measurement / not nulled, so even lower):

On the green:

On the white:

This suggests to me I need to do some more surface preparation?

Saying that, switching to insulation resistance and doing the 500V IR test and it made no difference to what I was touching, just blasted straight through.

Am I overthinking it, should I just get it buried?

 

[Seadog]

Try it wet.

 

[Onoff]

Try it wet.

Didn't try that. Poured some vinegar and lemon juice over each side and hit it with a 4" cup brush, rinsed and it's come up more "coppery". It's going in shortly, not that I'm looking forward to sifting the flints and backfilling:

 

[Hotstuff]

Try it wet.

I don’t know why but this reminds me of a joke; apologies in advance:

A man walks into a shoe store, and tries on a pair of shoes.

"How do they feel?" asks the sales clerk.

"Well, they feel a bit tight." replies the man.

The assistant promptly bends down and has a look at the shoes and the mans feet.

"Try pulling out on the tongue." offers the clerk.

"Nath theyth sthill feelth a bith tighth." He says.

 

[Onoff]

Plate in & started backfilling, tried to get the flints out and put the clay back first as that holds water. Slow going, there's an ever increasing pile of flints each end!

 

[Onoff]

@Barking Mat , earth plates are pretty common in rural France I believe?

 

[Barking Mat]

@Barking Mat , earth plates are pretty common in rural France I believe?

I've never heard of them personally, but I'm no expert.

 

[Farside]

@Barking Mat , earth plates are pretty common in rural France I believe?

Referring back to my UN post, you could backfill with charcoal and salt.