Asking DNO re supply characteristics

[SimonH2]

If you contacted the DNO and asked :

• What's the earthing arrangement at the site. It appears to be PME (TN-C-S) but there is no labelling so no guarantee that it isn't a "DIY job".
• What's the type and rating of the supply fuse ?

What sort of answer would you expect ?

 

[AndyPRK]

You could get. Dunno. We will charge you to come and look.

 

[SimonH2]

That would be too logical and obvious. As you might guess, I didn't get the answers I expected

 

[JohnW2]

That would be too logical and obvious. As you might guess, I didn't get the answers I expected

I think we need to ask what sort of answers you expected

From what I've heard, I think they usually have reasonable records relating to what has been PMEd - but virtually no useful records beyond that - particularly not in relation to the ratings of service fuses in individual installations (which, in some cases, is seemingly a function of what fuses 'the man from the DNO' had in his toolbox when he last replaced it!).

Kind Regards, John

 

[SimonH2]

Well, I've put together a few bits and pieces from various threads and ...
Determined that if the service is PME then the DNO is responsible for providing and maintaining an earth. But "DIY conversions" aren't unknown so just the presence of an earth connection to the service head doesn't mean it's officially PME. And as the installation has had "a few changes" over the years, and I've seen evidence that it once had an earth rod (separate from the lightning conductor), I thought it might be interesting to see if the DNO could confirm it is PME.
And since the service fuse isn't accessible to see the rating without breaking the seal, the responsible approach would be to ask the DNO. There's been a few comments in threads along the lines of "and what was agreed with the DNO regarding connected loads etc".

So I asked the DNO (ENWL here) and got a reply that was basically :
"Most of the information can be provided by your electrician."
On the earthing, they then say basically "if it's PME then these values, if not PME then other values".
On the service fuse, it says "We do not keep these details and your electrician will be able to determine what type and amp it is."

I didn't think this was very helpful So rather tongue in cheek, I replied along the lines of :
Please confirm that the presence of an earth connection from the service head meads that the service is officially PME
I assume that your statement constitutes approval to break the seal and remove the fuse to visually inspect the fuse. Please confirm this ?

I got replies from two people (the original person and someone else). Anyone like to speculate on what they said ?
Well probably not

One helpfully said that they would forward the request to the connections team to see what was originally installed - but they have no record of any later changes. That latter bit doesn't sound very useful - if true then it means they really have a big gap in their records of what the network is.
They also said that no, our electrician (I haven't told them it's me) cannot pull the fuse - I need to contact our supplier and make an appointment.

The person who sent me the original "not useful" reply said :
No we don't know anything about the fuse installed, and we've no records of ever having attended the site. And no you absolutely can't pull the fuse and have a look.
And also said that basically they have no idea what kind of earthing is installed. And goes on to explain that earth rods are installed when there's no existing earthing at the site, and the site owner is responsible for the cost of installing one, etc.

TBH I wasn't expecting a useful reply. But it does seem strange that they can't answer basically safety related questions about the service they provide

As it is, I think I might know someone with authorisation to pull the fuse and reseal it as a foreigner without having to go through the palaver of getting our supplier to contact the DNO etc.

 

[ericmark]

I got "We will send some one out to check FOC" and he arrived and measured then gave me a TN-C-S supply.

 

[SimonH2]

Well that sounds rather more positive than I got.

I'll add that a friend used to live a few miles from there, connected to a similar type of rural network. Some years ago the DNO came round converting them to PME and disconnecting earth rods (but not the bonding to the metal water pipe). So my expectation would be that this site was also converted, but of course we all know what assumption can do

What I would take from the replies I got is that if anyone sticks an earth connection into the service head, the DNO will then maintain a PME earth - regardless of whether they wanted to provide it or not. I do wonder if they really wanted to give that impression

 

[JohnW2]

What I would take from the replies I got is that if anyone sticks an earth connection into the service head, the DNO will then maintain a PME earth - regardless of whether they wanted to provide it or not. I do wonder if they really wanted to give that impression

That probably makes sense since (IF the supply to the property is PMEd - see below), in the absence of the sort of records that we suspect/know they don't have, if the design of the service head is such that there is provision for an earthing conductor to be connected (without breaking any seals) to an incoming (PMEd) neutral, and an earthing conductor is connected there, then it would be next-to-impossible for a DNO to 'prove' that they didn't make that connection.

You speak as if 'having to maintain a PME earth' is some sort of burden - but it surely is a total 'non-burden', since it merely means that they have to maintain the neutral connection to the property (which they obviously have to, anyway)?

However, as above, the important issue is whether or not the actual supply has been "PMEd" (which, as I said, seems to be the one thing that DNOs do {perhaps understandably} have records of). If there were a claim that an earthing conductor emerging from a service head were a 'PME earth', but the DNO knew that the supply to the property had not been PMEd, then they would know that (a) it was 'DIY work' (well, at least, non-DNO work) and (b) that it was 'illegal' (TN-C-S is only allowed with PME) and hence theoretically potentially 'dangerous'.

Kind Regards, John

 

[333rocky333]

One of the questions in the 18th exam relates to 132.2 and includes getting that info by Enquiry, which obviously seems not that easy.

 

[JohnW2]

One of the questions in the 18th exam relates to 132.2 and includes getting that info by Enquiry, which obviously seems not that easy.

Indeed. I would imagine that, in general, they could not answer most of the questions with total confidence from their 'records'. Even if they knew that the supply to the property was PMEd, they probably could not be sure that it was being used as TN-C-S. I imagine that the closest to certainty they would have would be in the case of relatively new builds, when they knew that a PMEd supply had been utilised and hence that, by implication, the new property would presumably have been wired as TN-C-S.

Kind Regards, John

 

[bernardgreen]

There seems to be a lot of confusion about how to provide a safe and reliable "earth" to an electrical installation.

Originally ( back before the 1950's ) the safety earth was a real Earth derived from a very reliable ground rod. This "rod" was in most installations the metal water pipe, frequently the Neutral at the substation was bonded to the water pipe network producing a very low impedance third wire to the property. This was a third "wire" that could carry enough fault current to ensure fuses melted.

Faced with the introduction of plastic water pipes and the loss of the third "wire" the electrical compnaies had to find an alternative means to ensure safety. Some companies added a third wire to the distribution system. Adding a third wire was fairly easy in areas with over head distribution but expensive in towns with buried cables. Many compnaies expected cusotomers to arrange for an electrician to install a reliable ground rod.

The "Earths" in many properties were now dependent on a local ground rod of dubious quality and impedance. A rod with a few ohms of impedance would not sink enough fault current to ensure a fuse would blow.

Something other than a fuse was needed to cut the power if there was a fault.

This was the Voltage Operated Earth Leakage Breaker ( VOELB ) which compared the voltage on the Earth wire inside the property with the voltage on the ground rod. If this was 50 volts or more then the VOELB would operate and disconnect Live and Neutral. Many properties still had a metallic water supply pipe used as an Earth and thus connected to the Earth wire inside the property. This connection of Ground via water pipe to the internal Earth wire effectively bypassed the VOELB. There could be very high fault currents flowing into the water supply pipe without the VOELB seeing enough voltage to operate.

Something better than the VOELB was needed.

This was the RCD which operates if the current on the Neutral supply wire is different to the current on the Live supply wire. It will react to almost any leakage of current to ground and does in 99.9% of faults cut the power before the potential on the internal Earth wiring reaches a hazardous level.

The RCD allows for the connection to Ground have an impedance as high as 2,000 ohms ( absolute worst case ) before the operation of the RCD cannot be assured.

Now it gets complicated.

The complication is when the "official" Earth for the property is achieved by TN-C-S ( PME ) and there is also another low impedance route from the internal Earth wire to Ground. Most commonly this other route is a metalic water or gas supply pipe that has been bonded to the internal Earth at the Main Earthing Terminal. This is safe until there is a fault in the supply network which causes the supply Neutral to rise just a few volts above the potential on the bonded supply.

 

[SimonH2]

In our case, plastic water pipe but metal pipes to a big metal oil tank sat on lumps of saturated wet timber. Some interesting safety implications there under fault conditions.

 

[bernardgreen]

My previous house had PME from an over head supply and a Ground rod in the bank of the stream at water level. I never meaured the impedance of the ground rod but during periods of a bouncing supply Neutral a clamp meter on the cable to the rod was fluttering rapidly on the 10 amp range.

I always thought it strange that the supply operator strongly advised a PME supply from an ancient 4 wire overhead supply ( one post was dated as 1939 ).

 

[SimonH2]

Yes, it does seem an odd arrangement. A house SWMBO looks after lost it's supply not long ago, and I suspect it was the neutral broken - I could see the broken connection at the top of a pole where the supply split to several houses.
Perhaps we all need our earth rods and a VOELB (or rather, a currect operated one) to protect against PME failure

 

[bernardgreen]

A lost Neutral ( metal theft at substation ) caused this damage,

The source of the fire is the Main Earth Terminal

( source Daily Mail article by By James Tozer UPDATED:06:49, 4 May 2010 ) available HERE with many terchnically in-accurate comments