What type of earthing system is this?

[crystal ball]

Twas the legend on the sticker

 

[flameport]

PME is surely relating to the network and not the earthing system presented at the service head.

That is correct, but for domestic installations and the majority of others, PME = TN-C-S.

 

[JohnW2]

That is correct, but for domestic installations and the majority of others, PME = TN-C-S.

Indeed** - and I have to say that I was very surprised and disillusioned when the late-lamented westie told us (presumably correctly) that, at least for domestic distribution, the 'M' of PME often meant just one extra earth - one at the very end of the distribution cable in addition to the one at the transformer!

**To be a little pedantic, I suppose it would have been better for you to say "TN-C-S = PME" (rather than "PME = TN-C-S") since, at least in the UK, domestic PME is only allowed in the presence of PME, whereas (as in my case) a PME supply does not have to be (more correctly, does not have to be used as) TN-C-S.

Kind Regards, John

 

[bernardgreen]

Overhead supplies carry the risk of a conductor snapping in strong winds ( or if a vehicle impacts a pole ) If the Neutral snaps but the Live doesn't then a then PME ( TN-C-S ) "Earth" would not be at Ground potential. Quite hazardous. Hence the reaon why in the past PME "earths" were uncommon when the local supply network was overhead cables

 

[davphi338]

What are your thoughts with the TT earthing and the metal CU unit, there is the potential for the CU to become live and the DNO fuse not to blow due to the relativly high impedance of the TT earth

I have an MK metal CU unit that meets 17th edition 3rd amendment which I was hoping the electrician can install. From what I understand it should still meet 18th edition rules with TNS earthing. Its a split load x2 63A/30mA RCDs with 100A main switch, there will be x2 32A rings, x2 6A light radials and a 40A cooker outlet. Distribution of MCB's across upstairs and down stairs. I'm presuming the main DNO fuse is 60A. Appropriate labels for 6 monthly RCD testing and correct glanding for tails etc.

Do you think a 17th edition amendment 3 CU will meet 18th edition regs with respect to TT earthing?

I'm not a qualified electrician but am qualified in electrical and electronic engineering and plan to do a lot of the first fix work for my daughter to keep costs down, its her first property and money is tight. An electrician has agreed to this and will do all the final testing and certificates but as the house purchase hasn't gone fully through yet hasn't visited the property.

Is there any point installing a separate 100mA S type RCD on the feed to the CU as I presume this would also need to be in a metal enclosure and simply moves the problem to that box?

Will there be a requirement to replace the main MCB on the CU to a 100mA 80A RCD?

Thanks for your thoughts

 

[Taylortwocities]

An electrician has agreed to this and will do all the final testing and certificates but as the house purchase hasn't gone fully through yet hasn't visited the property.

Quite honestly, you need to ask all of these questions of your electrician. He is going to have to certify that HE/SHE Designed, Installed and Tested all of this.
There are far too many variables to cover them all on an Internet forum discussion.

 

[JohnD]

that's an interesting thought.

Unless the wording has been revised, it says:

421.1.201 Within domestic (household) premises, consumer units and similar switchgear assemblies shall comply with BS EN 61439-3 and shall:


(i) have their enclosure manufactured from non-combustible material, or


(ii) be enclosed in a cabinet or enclosure constructed of non-combustible material and complying with Regulation 132.12.


NOTE 1: Ferrous metal, e.g. steel, is deemed to be an example of a non-combustible material.


NOTE 2: The implementation date for this regulation is the 1st January 2016, but does not preclude compliance with the regulation prior to that date.


An RCD is not included in that amendment.

 

[AndyPRK]

What are your thoughts with the TT earthing and the metal CU unit, there is the potential for the CU to become live and the DNO fuse not to blow due to the relativly high impedance of the TT earth

I have an MK metal CU unit that meets 17th edition 3rd amendment which I was hoping the electrician can install. From what I understand it should still meet 18th edition rules with TNS earthing. Its a split load x2 63A/30mA RCDs with 100A main switch, there will be x2 32A rings, x2 6A light radials and a 40A cooker outlet. Distribution of MCB's across upstairs and down stairs. I'm presuming the main DNO fuse is 60A. Appropriate labels for 6 monthly RCD testing and correct glanding for tails etc.

Do you think a 17th edition amendment 3 CU will meet 18th edition regs with respect to TT earthing?

I'm not a qualified electrician but am qualified in electrical and electronic engineering and plan to do a lot of the first fix work for my daughter to keep costs down, its her first property and money is tight. An electrician has agreed to this and will do all the final testing and certificates but as the house purchase hasn't gone fully through yet hasn't visited the property.

Thanks for your thoughts

That's all sounds ok to 18th to me. (also not a sparky!)
The 18th introduced SPD surge protectors. Can't recall if this is manadetry. But would be good to have in rural areas.
They cost about £60 upwards. Can either use 1 way in the CU or an external box.

 

[Taylortwocities]

This article deals with the issue of TT + metal CU requirements.
You may need a Voltimum log on to view.
https://www.voltimum.co.uk/articles/installing-metal-consumer-units

 

[EFLImpudence]

Is there any point installing a separate 100mA S type RCD on the feed to the CU as I presume this would also need to be in a metal enclosure and simply moves the problem to that box?

A single RCD in an enclosure is not, by definition, a consumer unit so may still be plastic.

Whether that's what they meant is impossible to say. No author ever clarifies what was meant when the regulations were written.
I sometimes suspect that the regulations are intentionally made ambiguous to allow get-outs.

Will there be a requirement to replace the main MCB on the CU to a 100mA 80A RCD?

Do you mean the Main Switch? If so, no and why would there be?

 

[JohnW2]

..

 

[JohnW2]

[ Apologies for mess of previous attempt at this post - the forum software made a mess of it. Trying again ... ]

Overhead supplies carry the risk of a conductor snapping in strong winds ( or if a vehicle impacts a pole ) If the Neutral snaps but the Live doesn't then a then PME ( TN-C-S ) "Earth" would not be at Ground potential. Quite hazardous. Hence the reaon why in the past PME "earths" were uncommon when the local supply network was overhead cables

We know that the (very rare) occurrence of a broken/'lost' neutral in a TN-C-S LV supply network can result in certain hazards, and that is equally true whether the supply is underground or overhead.

However, I'm not so sure that that (very rare) occurrence is necessarily more common with overhead supplies than with underground ones, since they often (always? - my experience is limited) do not use 'singles' (i.e. with L and N being separate cables). For example, the overhead supply to my house (and all others in my locality which have overhead supplies) uses 'ABC' ("aerial bundled cable") cable - in which the conductors are twisted (or otherwise joined) together. With such a cable it would be incredibly unlikely that the neutral could be 'snapped' without the same happening to the line/phase conductor(s). These photos are of the final part of the supply to my house (and other houses) - the upper four (thicker) cables being ABC, the lower thinner ones being telephone cables (and one guy wire!) ...

[see next post for the first photo which should be here]

.... and ABC cable in close up ...

Kind Regards, John

 

[AndyPRK]

Can’t read that.

SPD stuff

 

[JohnW2]

... the missing photo from my previous post ...

Kind Regards, John

 

[bernardgreen]

However, I'm not so sure that that (very rare) occurrence is necessarily more common with overhead supplies than with underground ones

The information given to me in 1980 by the then supply company ( DNO ? ) was that failed Neutrals in 4 wire overheads was frequent enough to make PME undesirable in such cases. This was odd as they offered a PME system from the 4 wire in the street.

I recall seeing 4 wire overheads with one conductor iced up while the other three were clear, I assume the Phases were carrying current and thus warmed while the Neutral had little if any current and thus no warming,

Similar information from the DNO in 2011 when arranging the supply to this cottage from the UG cable in the street,

ABC cable ( 4 wires spiraled in a bunch ) is far "safer" thn open 4 wire systems.