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Topic Title: TT and pole electrodes
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Created On: 03 September 2013 10:56 PM
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 03 September 2013 10:56 PM
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peteTLM

Posts: 3082
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Looked at a house today thats supplied TT. The last DNO ukpn pole is 600mm away from the house wall, and the cutout on the other side, and has its own earth conductor running down the pole and into the ground at the base. Couldnt really see what was going on at the top of the pole.

The house electrode is no more than 200mm away from the pole electrode.

Is this a problem? Im ok with TT, but not really comes across such close proximity

P

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry


Edited: 03 September 2013 at 11:16 PM by peteTLM
 03 September 2013 11:20 PM
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John Peckham

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Pete

Is there a transformer on the top of the pole?

If not the supply may be PME and UKPN may give the consumer an earth. If not the consumers electrode will need to be sited away from the supply electrode by a couple of rod lengths to prevent overlapping zones. The problem will be knowing how long the suppliers electrode is?

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John Peckham

http://www.astutetechnicalservices.co.uk/
 03 September 2013 11:26 PM
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AJJewsbury

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It sounds like the consumer's electrode is going to be significantly influenced by the DNO's CNE potential - i.e. the installation is going to behave rather like a PME system (if with a high Ze) - and so be subject to some of the dangers normally associated with a PME supply.

If it's a simple internal domestic installation with bonding up to PME standards (e.g. bonding & earthing conductor at least 10mm2) I see little grounds for worry. If however it's in a situation where PME might not be advisable (e.g. lots of outdoor stuff, swimming pool, caravan supply, etc) - a move of the consumer's electrode might be in order.

- Andy.
 03 September 2013 11:32 PM
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peteTLM

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John,

no transformer on that pole or the one before. Couldnt eyeball the other poles but can do later in the week.
The feed is ABC and poles cant be more than a couple of years old. Cutout is at least 25 years old as is the service cable, so i guess they changed and upgraded the service, but somehow forgot about the very last bit.
16mm G/Y down the pole.

PME wouldnt be great, too many swimming pools and stables, but TNS would be nice - no chance there

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 03 September 2013 11:40 PM
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alancapon

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Not necessarily. It is usual with an overhead system which may be significantly unbalanced or a long single phase run to apply earths to the neutral at regular intervals to reduce the neutral to earth voltage at a customer's premises. This does not necessarily make the network suitable for supplying a PME connection. It is known as "TT with MEN" (Multiple Earthed Neutral). With underground networks it is possible to have "TNS with MEN" as well, which is again not the same as PME.

Regards,

Alan.
 04 September 2013 11:32 AM
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Delbot321

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I used to work for one of the supply authorities and was involved in upgrading large amounts of the LV overhead network. All the ABC (Aerial Bundle Conductor - twisted together conductors overhead) we put in was suitable for PME, however when we replaced the supply cables to properties we never converted any consumers earthing system, we simply reconnect the supply. Some consumers had already been converted to PME on the old 4 wire (or 2 or 3 wire) overhead and some were still on TT. If they wanted to convert to PME in order to remove their main RCD at a later date that was up to the consumer and subject to the usual limitations on the use of PME.

It sounds to me as if the customer has a TT supply from a historical connection, the DNO have upgraded their network and reconnected the customer as was. The earth down the pole is one of their many earth connections for distributing PME (Protective MULTIPLE Earthing)

I've never come across the scenario Alan describes above, however I've only worked on the network for one DNO and as we all know there are many with a variety of different perceptions and arrangements so the arrangement Alan described could also be the case.

Could always ask the DNO - I know it's a nuisance but they have a legal duty to tell you what earthing arrangement is available (or what is not) and this will at least give you a definitive answer.

Question is why do you need to know?
As has already been said it's not good having the electrodes with overlapping zones of influence. If it's to make changes then you may need to modify it or if it's for an EICR then simply comment and advise on it as appropriate.
 05 September 2013 10:26 PM
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davezawadi

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Consider this, it is certainly no worse than PME with an RCD, and probably better. This is really why such supply upgrades by the DNO are completely safe, you have adequate protection whatever is going on. I would just not worry about it, and an EICR with satisfactory RCD results seals the deal. It is not your problem to consider anything the other side of the meter (whatever you may think, you will probably get nowhere if you make a complaint, SEP).

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David
CEng etc, don't ask, its a result not a question!
myurl
 07 September 2013 01:11 PM
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weirdbeard

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Originally posted by: alancapon

Not necessarily. It is usual with an overhead system which may be significantly unbalanced or a long single phase run to apply earths to the neutral at regular intervals to reduce the neutral to earth voltage at a customer's premises.



Hi Alan, what's the reason for reducing the neutral to earth voltage, wouldn't this be advantageous only when there is a neutral to earth fault within the customers TT installation?
 07 September 2013 01:28 PM
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weirdbeard

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Originally posted by: peteTLM



Is this a problem? Im ok with TT, but not really comes across such close proximity


Hi pete, I think it's probably ok regs wise - I think this is why double/omni- pole switching is required for isolation purposes within a TT installation.
 07 September 2013 01:44 PM
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alancapon

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Overhead networks whilst not necessarily longer than underground cable feeds have traditionally used smaller conductors, as they are air cooled. You need to think of volt-drop in a different way to appreciate the problem. When we talk about a volt drop of 10 volts, we have dropped half of this in the phase conductor, but at the "load end" of the circuit, we will also see a voltage rise in the neutral conductor of a similar amount. With a three phase network, the various neutral currents sum together (under most circumstances), reducing the "voltage rise" in the neutral.

It is usual (certainly in the British Isles) to try and reduce the neutral voltage to as few volts as possible. This is often done by creating a parallel path using the earth and regular electrodes, on the supply. If the supply is three phase and out of balance, the other option is to use a "static balancer" to equalise the three phase to neutral voltages. These used to be quite common, but are becoming rarer as a solution.

Regards,

Alan.
 07 September 2013 02:22 PM
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weirdbeard

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Alan, I'm still confused why the network need to take steps to reduce the neutral to earthing voltage within a consumers TT installation - there should be no risk of any N-E voltage or L-E, - unless theres a double fault within the installation, ie breakdown of basic insulation faulting to cpc and the CPD failing to operate?
 07 September 2013 02:35 PM
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alancapon

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Normal single phase load will cause a rise in the neutral - earth voltage, due to the resistance of the neutral conductor.

Regards,

Alan.
 07 September 2013 02:42 PM
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weirdbeard

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But Alan....when would this N-E be an actual problem within the consumers TT installation, the network, as far as i know doesn't take steps to reduce the potential L-E voltage, such as providing the installations earth connection themselves where its a TT installation, why is the neutral different?
 09 September 2013 10:47 AM
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AJJewsbury

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when would this N-E be an actual problem within the consumers TT installation

I guess it's based on the old British tradition as treating N as "safe", even though it's now officially classed as live conductor. SP switching, appliances (e.g. old exposed bar electric fires, lampholders, toasters) effectively having N exposed to touch even when switched off, N bars in CUs not being touch proof. It doesn't take too many volts a.c. to give a tingle under the wrong conditions - maybe not immediately life-threatening, but not good customer relations for either the electrician or the DNO.
- Andy.
 09 September 2013 07:44 PM
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weirdbeard

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Originally posted by: AJJewsbury

when would this N-E be an actual problem within the consumers TT installation


I guess it's based on the old British tradition as treating N as "safe", even though it's now officially classed as live conductor. SP switching, appliances (e.g. old exposed bar electric fires, lampholders, toasters) effectively having N exposed to touch even when switched off, N bars in CUs not being touch proof. It doesn't take too many volts a.c. to give a tingle under the wrong conditions - maybe not immediately life-threatening, but not good customer relations for either the electrician or the DNO.



Forgive me if I'm wrong, but I don't understand why a DNO, for a TT system where they don't supply the earth connection, would bother taking steps on their side to guard users of the installation from direct contact between a live conductor or equipment connected to one, and earth?
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