IET logo
 
IET
Decrease font size
Increase font size
Topic Title: The DNO and systems of earthing
Topic Summary:
Created On: 01 May 2013 08:38 PM
Status: Post and Reply
Linear : Threading : Single : Branch
1 2 Next Last unread
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 01 May 2013 08:38 PM
User is offline View Users Profile Print this message


Avatar for UKPN.
UKPN

Posts: 452
Joined: 17 January 2012

Contrary to popular belief, the DNO has many types of earthing in its
network.
But for purposes of this post I will talk about two=SNE and CNE.
SNE, normally cable sheath, is offered via a clip and earth block on older properties and is there for the consumer to use without obligation
on the DNO. The DNO has no obligation to maintain it.
Many older cables are not an earth path at all, and any exposed sheath
will be shrouded for safety.

CNE, (PME) is provided on new properties and the DNO is under obligation to maintain it subject to being offered in the first place if it comes under the safe installation rule.
The DNO are under no obligation to update a SNE system to PME but
will, if the network is CNE.

The DNO do not guarantee any external loop impedance, consumers/
designers must decide if the values suit their purpose.

Regards.
 01 May 2013 08:55 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 18929
Joined: 23 March 2004

Great - so you are saying that any SNE system - usually a TN-S earthing arrangement to the service is liable to fail at any time, and that if the consumer uses it, it's totally at his risk, despite you providing it in the first place.

Don't you think you should be alerting relevant people, and loudly - ie like most of your consumers so they can take appropriate action then.

That's what you may want to happen - I suspect there might just be a bit of an outcry though.

I wonder where the legal people will take the case that results in a fatality due to a failure of your SNE arrangements that couldn't be identified by a consumer or had no warning mechanism in place to identify a potential for that failure.

It'll be an interesting one.

Regards

OMS

-------------------------
Failure is always an option
 01 May 2013 09:16 PM
User is offline View Users Profile Print this message


Avatar for leckie.
leckie

Posts: 1605
Joined: 21 November 2008

UKPN, sorry for being thick, but do you mean that the dno will not guarantee .35 ohms Ze on a pme system?

Edited: 01 May 2013 at 10:20 PM by leckie
 01 May 2013 09:18 PM
User is offline View Users Profile Print this message



statter

Posts: 118
Joined: 06 February 2013

It looks to me like the loss of an earth connection on the sheath of a cable such as an old PILCSTA which ran underground for any distance and typically providing a TN-S supply would be a breach of regulation 13.2 b of the ESQCR. There is a carve out in reg 2 for some older stuff but regulation 13 applies irrespective.

I think it would be reasonable for a DNO to check earthing of these 'metallic screens' from time to time so that any loss of earth connection would be detetected in a timely fashion.

I suspect a court would take a dim view if no inspection regime was in place.

.... so whilst the DNO has no obligation to maintain the connection to a cable sheath it does have an obligation to make sure that sheath is earthed if the cable runs underground.
 01 May 2013 09:25 PM
User is offline View Users Profile Print this message



statter

Posts: 118
Joined: 06 February 2013

.......... oh and sorry forgot about 24.4 and 24.5 which require an earth terminal on all new installations and all installations where the service connection is replaced (overhead or underground) irrespective of age, type, location etc.

So if an old service is replaced DNO must make available an earth terminal (prob by TN-C-S) but in some areas this is very difficult (e.g. in rocky conditions with not much topsoil) in which case there is no alternative but to provide a new TN-S earth (or possibly PNB which is TN-C-S anyway) if its the only supply on the transformer.
 01 May 2013 09:37 PM
User is offline View Users Profile Print this message



mikejumper

Posts: 1672
Joined: 14 December 2006

Originally posted by: UKPN
SNE, normally cable sheath, is offered via a clip and earth block on older properties and is there for the consumer to use without obligation
on the DNO. The DNO has no obligation to maintain it.

Imagine if pubic transport operated like that.

'The train on platform 1 is there for your use without obligation but if a wheel flys off at 100mph then bad luck because there is no requirement
for us to maintain our trains'
 01 May 2013 10:01 PM
User is offline View Users Profile Print this message



John Peckham

Posts: 7258
Joined: 23 April 2005

UKPN


Are you saying that the DNOs are exempt from the Electricity at Work Regulations 1989?

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 01 May 2013 10:21 PM
User is offline View Users Profile Print this message


Avatar for leckie.
leckie

Posts: 1605
Joined: 21 November 2008

Blimey, looks like UKPN are on strike! No replies!
 01 May 2013 10:29 PM
User is offline View Users Profile Print this message



statter

Posts: 118
Joined: 06 February 2013

I'm not sure that the elec at work regs help that much. IMO they are primarily concerned with ensuring electrical safety at work. It's. Not at all clear that this extends to service cut outs especially if cable sheath is covered and not exposed. The elec at work regs do put an onus on an employer and self employed to make sure installations are properly earthed though.
 01 May 2013 10:32 PM
User is offline View Users Profile Print this message



Zuiko

Posts: 518
Joined: 14 September 2010

Don't you think you should be alerting relevant people, and loudly - ie like most of your consumers so they can take appropriate action then.

What appropriate action? To provide safe disconnection during an earth fault? The customer is obligated to do that in the first place!

I don't see the fuss - the consumer should ensure that their installation is correctly connected to earth. If the DNO provide PME then by all means use it, but that does not mean that it is the DNOs responsibility to ensure that the installation is safe and fit for purpose.
 01 May 2013 10:32 PM
User is offline View Users Profile Print this message



DOUGIE1000

Posts: 4113
Joined: 13 August 2005

Can I ask John have you ever had a pme with a greater than 0.35ohms, I can't ever recall having been on this situation,

Tn-s yes but pme?

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 01 May 2013 10:34 PM
User is offline View Users Profile Print this message



Zuiko

Posts: 518
Joined: 14 September 2010

UKPN, sorry for being thick, but do you mean that the dno will not guarantee .35 ohms Ze on a pme system?



The .35 ohms is regularly swerved. Let's face it, if the jointer measured 0.4 ohms at the cutout , that's going to be fine. If it starts heading north of 0.5 ohms then the DNO is not going to give you PME.
 01 May 2013 10:36 PM
User is offline View Users Profile Print this message


Avatar for slittle.
slittle

Posts: 3377
Joined: 22 November 2007

I beg to differ with the all knowing UKPN but if a dno provides an SNE earth then they are obilged to maintain it.

It's too late tonight for me to find the words that guide me but I'm sure I can lay my hands on it

And I've had many jobs over the last few years where they have repaired a lost SNE earth


Stu
 01 May 2013 10:36 PM
User is offline View Users Profile Print this message



Zuiko

Posts: 518
Joined: 14 September 2010

have you ever had a pme with a greater than 0.35ohms, I can't ever recall having been on this situation,




I've come across it hundreds of times. In fact, there was an operational procedure at my old DNO that suggested ignoring up to 0.7.
 01 May 2013 10:40 PM
User is offline View Users Profile Print this message



DOUGIE1000

Posts: 4113
Joined: 13 August 2005

Originally posted by: Zuiko

have you ever had a pme with a greater than 0.35ohms, I can't ever recall having been on this situation,

I've come across it hundreds of times. In fact, there was an operational procedure at my old DNO that suggested ignoring up to 0.7.


You as lines man might have come a cross it out on the field as n-e could could be at cableheads and you would be measuring it further away from the cablehead, but at the cable head I can't ever recall getting a high zero on pme.

The cable head is where its joined I guess after all.

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 01 May 2013 11:19 PM
User is offline View Users Profile Print this message



Zuiko

Posts: 518
Joined: 14 September 2010

you see it more in rural areas; the 35mm^2 / 35m rule of thumb is often broken; quite common to see LV tees over 100m with >220V no load at the head and >0.5 ohms ELI
 02 May 2013 12:18 AM
User is offline View Users Profile Print this message



alancapon

Posts: 5700
Joined: 27 December 2005

Originally posted by: UKPN
. . . SNE, normally cable sheath, is offered via a clip and earth block on older properties and is there for the consumer to use without obligation on the DNO. The DNO has no obligation to maintain it. . .

I am not convinced about that. The DNO do not have to provide an earth connection to begin with, but if it is provided, then they have to maintain it.

. . . The DNO do not guarantee any external loop impedance, consumers/
designers must decide if the values suit their purpose. . .

That could be interesting too. If the DNO state that their fuse will protect the first 3m of meter tails, then offer a service cable / cutout fuse combination that will not do the job, then I wonder how that would go in court.


Regards,

Alan.
 02 May 2013 06:28 AM
User is offline View Users Profile Print this message


Avatar for dbullard.
dbullard

Posts: 1166
Joined: 02 March 2006

I have a few "in the sticks" PME conversions where the DNO have just taken the CU earth into a set of Henleys on the incoming neutral supply and sealed with their lead crimps, and the overhead supply is quite a long one from the pole mounted transformer about 700 yards away.


I have a PAT testing job coming up soon in one of the property's that has this cobbled together arrangement, and the Ze is almost from memory around .035 or certainly heading that way unless it has been changed since last year

I get the impression that UKPN is a bit of a "Windup" makes a post and runs .................


Regards

Daren

edit : fat fingers eating toast and drinking coffee !!!!

-------------------------
..... Dont pee in my pocket and tell me its raining ......


www.quest-electrical-sw.co.uk
 02 May 2013 10:08 AM
User is online View Users Profile Print this message



AJJewsbury

Posts: 10971
Joined: 13 August 2003

UKPN, sorry for being thick, but do you mean that the dno will not guarantee .35 ohms Ze on a pme system?

I understand that 0.35/0.8/21 aren't guarantees, just "usual" upper limits (i.e. good things to base generalised designs on).

What the DNO are obliged to do is ensure that the supply voltage doesn't vary by more than -6% and +10%, which in simple cases, puts a limit on the resistance of their supply lines - they can't start off with more than 253V (as the consumer will get this when the load is zero) and they can't let it drop to less than 216.2V (when the consumer is drawing full load) - a difference of 36.8V. So for a 100A supply the supply L-N impedance can't exceed 36.8V/100A = 0.368 Ohms - and of course in TN-C-S L-N is the same as L-PE, so that's the Ze limit too. For lower loads, the loop impedance could be allowed to be higher.

- Andy.
 02 May 2013 10:19 AM
User is offline View Users Profile Print this message


Avatar for BigRed.
BigRed

Posts: 552
Joined: 10 November 2006

Has UKPN being hijacked by David Cockburn? DNO's have to maintain existing earths, pure and simple, why they don't tell the front office guys this is only for their profiteering. Mentionong OFGEM and ESQCR usually gets someone with more than one braincell on the phone....
UKPN, i'm surprised and shopcked at your lack of knowledge of esqcr. Go read a copy before digging holes and jumpng in them.
IET » Wiring and the regulations » The DNO and systems of earthing

1 2 Next Last unread
Topic Tools Topic Tools
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.