A real moan tonight...

[bernardgreen]

However, with overhead ABC, or underground, cables I would imagine that selective loss of the neutral conductor is extremely rare.

Remove "extremely" and I would agree.

 

[SUNRAY]

Investigations revealed the blast was caused after a piece of cable was cut from an overhead line. This affected the earthing of the electrical network in the area and resulted in some unusual electrical activity, which affected some of the gas pipes within the properties. This resulted in the fires and explosion.

Yorkshire Post Saturday, 11th February 2012,

Initial fires were caused by overloaded Main Bonds to metal water mains,

As I interpret this post it is a good reason to not import the DNO's earth and not bond the DNO's neutral to the customers earth.

 

[winston1]

Do you know anything at all Winston? It certainly appears to me that you don't.

I'd have though by now you would have learnt that I have a lot of experience and can identify and test most circuits with a reasonable degree of accuracy and by now.

As I mentioned earlier there is an excellent log book for the building which the customer encourages the use of as a note book and there are several sets of dead tests showing ~0.35Ω of L & N end to end resistance and my first thought at 11pm on a callout in the cold and damp was Winstons repeated and incorrect bleating so I just had to do this:View attachment 203839
So once and for all will you PLEASE keep your factually incorrect and rude/arrogant statements to yourself.

I find it so sad that while dealing with potentially life threatening situations one of the first thing that goes through my head is the comment from a silly little know nothing.

OK so this was wired unconventionally. I will rephrase my statement:

Lights are not normally wired on rings. I'm surprised you think these are.

Are you going to remove the return wire to save confusion later?

 

[SUNRAY]

Yes, I'm also a bit uncertain as to what SUNRAY meant. As you say, the 'M' of the 'PME' (which is required with TN-C-S) means that there are 'multiple' connections between the supply neutral and earth electrodes (even though, if what westie used to tell us is correct, that 'multiple' often/usually meant only one 'earth' in addition to the one at the transformer).

Furthermore, it was only at the 11th hour that the requirement for all TN-C-S installations to have a local TT rod (connected to the TN-C-S 'earth') was removed from the draft of the current version of BS7671 - so I presume that this was not considered to be a 'hazard' (which sounds reasonable, since hardly any credible domestic TT electrodes would have a low enough impedance to result in enough current to flow to damage, let alone 'set alight' any credible cable).

In practice, main bonding cables from TN-C-S 'earths' to low-impedance extraneous-c-ps, are far more likely to carry very high currents than is a connection to a domestic TT electrode.

Kind Regards, John

I have always been under the impression a TT earth is never connected to an imported earth, we had major discussions

Edit: on a big campsite over whether each pillar should have its own spike and whether they should all be joined

I assume the supply is TN-C-S but the cupboard containing the Isolator, Henley block, earth bar and 4 sets of tails doesn't include the cut-out and meter which is a fair treck round the garden and not one I elected to do in the horrible weather.

I did manage to speak with the crew on site earlier this afternoon who have said there were no earth connexions in their network due to metal thefts. If that is correct it is just about possible [at a very big push] that the only earth feeding what I count as 5 or 6 farms and 4 houses was a length of 1mm² T&E. I will be replacing it with 2.5mm² singles and give it a decent inspection then.

 

[DetlefSchmitz]

OK so this was wired unconventionally. I will rephrase my statement:

Lights are not normally wired on rings. I'm surprised you think these are.

Are you going to remove the return wire to save confusion later?

Trolling again! Go back to the Screwfix forum...

 

[SUNRAY]

OK so this was wired unconventionally.

Ring final circuits have been conventional for well over half a century.

I will rephrase my statement:

Lights are not normally wired on rings.

Thank you for admitting you are wrong yet again.

Lights are not normally wired on rings. I'm surprised you think these are.

Working on lighting rings is not actually that uncommon.

Are you going to remove the return wire to save confusion later?

Why on earth should I?
1) The customer has not asked me to alter it.
2) There is no reason to create additional work to make changes to a circuit which doesn't appear to have any faults.
3) There is no confusion unless 'know nothings' start playing with it .

 

[winston1]

Ring final circuits have been conventional for well over half a century.

Indeed, but not on lighting circuits.

 

[bernardgreen]

Switch off Winston.

 

[JohnW2]

I have always been under the impression a TT earth is never connected to an imported earth, we had big discussions

In many countries it is compulsory and, as I said, it was only at a very late pre-publication stage that the requirement of a TT earth for all electrical installations in the UK was removed from the final draft of the current version of BS7671.

One of the practical problems that was extensively discussed when we thought it was going to become a requirement was the potentially dangerous situation that can arise, temporarily, when some, but not all, properties using the same part of a distribution network have their TN earths connected to local TT ones.

I did manage to speak with the crew on site earlier this afternoon who have said there were no earth connexions in their network due to metal thefts.

You mean the DNOs (local) distribution network? If so, they surely can't allow that situation to persist (if it's true), can they?

Kind Regards, John

 

[JohnW2]

Remove "extremely" and I would agree.

Well, even if it's left there, the word is obviously open to a wide range of individual interpretations; we're real back to the 'risk perception' business again!

Anyway, whatever adjectives one uses, I would have thought that only extremely improbable circumstances could result in the N conductor, but not the L one, of an ABC cable or an underground multi-core cable to become 'broken'.

Kind Regards, John

 

[JohnW2]

Quite right, it may have been noticed all the breakers in the CU are off and aqequate testing for dead was performed. Not importing the DNO's earth into the building would have removed so much danger, as would not exposing within the building. This would effectively negate any percieved requirement to use a device to disconnect anything. Excuse the patchwork but a basic version of a nicer way to put this SWA in:

All agreed (and, despite your modesty, the 'patchwork' is fairly impressive!).

Of course, the common situation is that of a house with a TN-C-S earth connected via SWA to a TT'd outhouse. The SWA armour has to be connected to one (but not both!) of those 'earths' - and, if one wants fault protection by ADS to be primarily provided by an OPD, then it has to be the TN earth - leading to the situation such as you've described, with close proximity of the two earths at the 'outhouse end'.

If, with that situation, one takes the SWA straight into the CU, then if it's a metal CU attempting to use a brass SWA gland would, of course, be particularly iffy - and a plastic stuffing gland far more sensible/safe.

Kind Regards, John

 

[JohnW2]

In many countries it is compulsory and, as I said, it was only at a very late pre-publication stage that the requirement of a TT earth for all electrical installations in the UK was removed from the final draft of the current version of BS7671.
One of the practical problems that was extensively discussed when we thought it was going to become a requirement was the potentially dangerous situation that can arise, temporarily, when some, but not all, properties using the same part of a distribution network have their TN earths connected to local TT ones.

In case anyone doesn't know, or has forgotten, the text in red in the following was present in earlier drafts of BS2761:2018, but did not appear the the final draft, which is what was published as the current regulations (BS7671:2018)...

542.1.201 The main earthing terminal shall be connected with Earth by one of the methods described in Regulations 542.1.2.1 to 3, as appropriate to the type of system of which the installation is to form a part and in compliance with Regulations 542.1.3.1 and 542.1.3.2. Additionally, there shall be an earth electrode, supplementing any earthing facility provided by the distributor, in accordance with one of the requirements of Regulation 542.2.3, to prevent the appearance of a dangerous touch voltage in the event of the loss of the main connection to Earth.

Kind Regards, John

 

[bernardgreen]

Anyway, whatever adjectives one uses, I would have thought that only extremely improbable circumstances could result in the N conductor, but not the L one, of an ABC cable or an underground multi-core cable to become 'broken'.

Metal thieves at substations tend to go for the Neutral as the risk of fatal shock is much lower than if they went for a phase conductor,,

 

[AndyPRK]

Could someone give me a one line summary of the fault.

Loss of supplier N ?
Did the polarity reversal have any Relevance?

Also please consider if someone put one hand on the cu and the other hand on the termination box, I guess they would get a belt across the chest

 

[SUNRAY]

You mean the DNOs (local) distribution network? If so, they surely can't allow that situation to persist (if it's true), can they?

Kind Regards, John

Indeed not, and they quite rightly they isolated the sub within minutes of being on site.