Meter Tail Length - Getting the Electrical/Mathematical Equations from the DNOs.

[All4One_One4All]

Hi everyone, I'm new to this forum and this is my first post, so apologies if I get anything wrong.

I do a lot of property redevelopments, and some landlords (myself included) prefer to put the elec and gas meters outside the property in meter boxes. This can cause issues, as often the only place for the meter boxes is much more than the 3m or so from the nearest place one can put a fused switch, never mind the CU.

Over the last 30 odd years, I've worked with a number of DNO's from NW, NE, Midlands, and SE England.
In respect of meter tails, they all seem to have varying rules. For example: in most DNO's, the maximum length is 3m and many DNO's are sticklers for this; whilst others will go to 5m, and a very few will occasionally consider the property and accept much longer runs if the tails are just switched and in steel conduit (32mm) all the way to the CU. Some allow a Switch to be placed in the meter box, and others absolutely refuse to even discuss this option, offering completely unreasonable suggestions (eg putting the fused switch in a tenants wardrobe such that if it fused no electrician would be able to get to it, and no other tenants would have any electric); suggestions I think that are deliberately intended to annoy.

So I have asked DNO's the following.

1. Why are meter tails limited to 3m or 5m or occasionaly longer? What's the maths behind this rule?
2. Why can some DNOs make an exception when others won't?
3. Why do some DNO's allow the Switch Disconnector (and even the fuse enclosure) to go in the meter box when most DNO's forbid it? (After all it's MY METER BOX), and quite frankly I think they are taking the **** trying to make themselves seem more important than they really are.
4. Why do inspectors from the same DNO frequently submit very different proposals?
5. Why have I yet to find a DNO that back up their rules with sound electrical theory, mathematics, and a fully calculated solution to explain why the meter tail distance is 3m or 5m or some other distance.
6. And finally why does my current DNO allow numerous 1st floor flats over commercial ground floor properties to have meter tails exceeding 5m, and a simple SD installed in the meter box, when they refuse to even discuss this for a similar property of mine?
7. No DNO to date has BEEN prepared to mathematically explain this distance, even though I have suggested the electrical equations and mathematical techniques that might be useful (from my 1st year B.Sc. degree, so it's hardly rocket science). They quote BS7671, but they can't or won't state the paragraph! Surprisingly, the DNO with the highest potential short circuit fault current allows the longest meter tail distance (of those I've worked with).
8. Can anyone else on this forum provide the mathematics that I'm after?

Sincerely All4One_One4All

 

[Richard Burns]

The length and protection of tails can be specified as required by the relevant DNO (they should be consistent within one DNO area, but not always) this is a decision reached within the DNO and like all their requirements may not be the same for different DNOs.

I believe that the length is determined through a risk assessment rather than by mathematics.
If the tails are less than 3m they are less likely to be damaged than a longer length and 3m is a realistic length to get from cut out to a consumer unit in most situations.
The simplification for BS7671 is that they can be 3m long based on 433.2 to 433.4.
The DNO is not constrained by BS7671 but they will tend toward some of the approaches used.

 

[All4One_One4All]

Hi Richard, thanks for your reply.

If as you say, it's determined by risk (and this is the most sensible answer I've had from anyone to date), surely (to comply with all BS, EN, and ISO standards on risk assessment) it would have to be based on a property by property risk assessment?

The property I am having trouble with is intended for students. The only place for the meter is in a purpose built wall sticking out into the front garden, and for the CU to go just inside the front door on the other side of the property. The downstairs front room is a students. The DNO thought it acceptable to put the Fused Switch in that students built in wardrobe???? "Cut a hole in the side and stick it on the wall." Needless to say if this student were away, the electrician would be able to get to it. All the guidelines say that the Fused switch and CU should be in common areas accessible to all living in the property (for obvious reasons). The DNOs next suggestion was to put only three students in rather than four, an economically unacceptable solution - the DNO just seems completely and totally unreasonable. I have a switch fuse assembly that will easily fit in the meter box, but they won't allow even a switch in there! I am prepared to put the tails in 32mm galv conduit from the meter box to the CU, and this would buried in the walls and be under the floor, so I consider the risk to be minimal (in accordance with ALARP (as low as reasonably practical), but the DNO would seem to require one of two impractical solutions.
I can't even find out if there is any body/organisation I can appeal to?

Contrast with another DNO - down south (a bit more switched on down there). It's 11m horizontally and 2m vertical (total tail length 13m). They agreed my solution was the only really practical one. It's in 32 Galv conduit a minimum of 70mm from any publically accessible surface, with a switch in the meter box and the DNO used their own cartridge fuse system to protect the tails! They didn't think it was anything special! I remember the installer saying that the longer the tails, the smaller the prospective short circuit fault current and the lower the short circuit fuse rating. When I added down to inductance, he laughed, and then we both said, but it's insignifcant when one considers the length of cable that supplies the property.

So how come the DNO can put 500m of supply cable on telegraph poles outside the property, but I can't put 5m to 5.5m in 32mm galv conduit inside the property?

I think the entire bunch are just 'jobs worths'.

I don't think the local DNO want to have to do anything for the few hundred squids they are charging.

 

[Engineer54]

Further to RB'S accurate account, most if not all DNO's will insist on a switch fuse being installed on meter tails over 3/5 meters, not just a switch. If the metre installation is external to the building, then the Switch fuse will be located in the meter enclosure, there isn't much else you can do. They may however insist that they provide and install the protective switch fuse though!! Obviously at the DNO's extortionate rates...lol!!

Last but not least, ''ALWAYS'' insist on talking to the Area Manager, as it's pretty pointless exercise talking to the enquires telephone girl!! Better still, try and make an appointment to talk to the Area Manager in person, ...face to face!!

 

[Des 56]

I will play devils advocate to your frustration

and this is tongue in cheek,perhaps food for thought only


The suppliers only concern is with their equipment

We all have to play by the rules however varied and for whatever reasons they feel are justified,they are best placed to evaluate the supply set up

The cabinet may be our purchase,but its intention is for their equipment to allow the property an adequate service, they have little interest in the logistics of the customers responsibilities

The general rule is that no other equioment is permitted,this could be relaxed, but only with discretion

If its strictly enforced ,2 boxes installed instead of 1 and problem sorted

The tails length may be varied depending on the circumstances under which it is installed,as posted above,risk assessed

The 3 meter rule is fairly well known and accepted in the industry, problems about the location of any of the customers switchgear are universal to all, the solutions have been well documented,understood in advance, and should cause little alteration to a well designed install

The mathematics of the fault protection?
Working to a different set of rules than the installing spark,the supllier is best placed to decide the protection of their own equipment, even though it may not be the best practical location for the customer

My advice,take eng54s advise lol

 

[7029 dave]

meters are normally fitted adjacent the cut out,so why would you need more than 3 or 5 meters?,or are you talking about distance to cu?If your final sub circuits/cu are more than the distances quoted then yes your most certainly into sub mains/ switch fuse etc.