3m max meter tails - DNO rule or BS7671?

[John D v2.0]

Hi all, I was aware DNOs usually have a 2 or 3m rule on tail length, which I assumed for their own reasons (simplicity/flexibility and consistency I guessed). However I was having a read of UKPNs doc and they pin the blame on BS7671 for that rule. That was unexpected.

BS 7671 Regulation 433.2.2 allows a maximum distance of three metres between overload protection devices and their source, therefore the customer shall provide and install an intermediate switch fuse or isolating switch if the distance between the meter and consumer unit is greater than three metres.

From:
http://library.ukpowernetworks.co.u...Customer+Supplies+up+to+100A+Single+Phase.pdf

The part of the reg they cite apparently applies only to circuits without adequate *fault* protection.

To determine the adequacy of fault protection purely for BS7671 you would just need the total Zs, the adiabatic parameters for the cables involved and the fuse characteristics?

So it stands to reason that they are saying their fuse doesn't provide adequate fault protection for any length of tails. They are also saying it's not the "overload protection device" for the tails for BS 7671 purposes either.

In that case what is it for? Unspecified DNO protection purposes? Or is it just there to allow meter operators to safely isolate the supply by removing it?

Thoughts?

 

[EFLImpudence]

The part of the reg they cite apparently applies only to circuits without adequate *fault* protection.

You are correct.

To determine the adequacy of fault protection purely for BS7671 you would just need the total Zs, the adiabatic parameters for the cables involved and the fuse characteristics?

Correct.

So it stands to reason that they are saying their fuse doesn't provide adequate fault protection for any length of tails. They are also saying it's not the "overload protection device" for the tails for BS 7671 purposes either.

But, if overload occurred on the tails, it is whether they like it or not. They can't alter the physics.
Or they don't want the responsibility.

Or, as you have discovered, a mistake was made in reading BS7671 and it has been lost in the mists of time and now everyone thinks it is necessary.

In that case what is it for? Unspecified DNO protection purposes?

Why would your same rated fuse be better than theirs?

Or is it just there to allow meter operators to safely isolate the supply by removing it?

No, not that. It is on the consumer's side of the meter.
That would not be for the consumer to provide anyway.

 

[EFLImpudence]

There is another point, of course, which is that, in the case of meter tails, a reduction in c.s.a. has not occurred (it is far more likely in these days of fashionably wasting copper that an increase of c.s.a. is the case from DNO cable to tails) therefore 432.2 does not come into the matter at all.

BS7671 has no consideration of the position of unnecessary OPDs.

 

[ericmark]

I had always thought that regulation was to allow you to connect to the buzz bars with a cable appropriate to the supply fuses in a distribution unit? So buzz bars may be 1000 amp but cable from buzz bars to fused isolator are only 16 mm² as the fuse in the fused isolator is only 100 amp.

The rule I will admit does not say that is why it exists, and I have said and been criticised for saying it would also seem to cover unfused spurs from a ring final, so unfused spur would have a maximum length of 3 meters.

As you say it hardly matters if 3 meters or 30 meters a fault can overload the cable. But we do seem to see this with BS7671. You will likely remember the provision with 16th edition for a shower in a bedroom, with the 17th edition this went, and we got the 3 meter rule for any room containing bath or shower, since it is unlikely to get a bathroom large enough to be able to get 3 meters away to fit a socket, it is really for when a shower is fitted in a bedroom, but it no longer says that.

It is easy now with metal CU's to feed them with SWA cable, the problem is how to connect that cable to the DNO's supply, at one time there was a move to isolators, so the HSE rules about isolating else where could be complied with, where an isolator is not provided there is no option you must draw the fuse to work on the consumer unit if you follow HSE rules. What ever you do, you will be breaking a rule. Two wrongs don't make a right, so drawing the DNO fuse to work on the consumer unit is still wrong, however I have not seen a court case where there is not some other things as well as missing seal, be it very low power being used or black marks there has always been some thing as well as broken seal.

When I started the DNO would inspect the house before connecting, it seems today they no longer do that? The procedure in one place was the isolator would be fitted by user, and switched off and sealed, until the supply had been connected, so it was the user removing the seal which allowed the supply to be connected, so the supplier (not the DNO it was in fact Baileys) was only responsible to the isolator. Laing was then responsible to test from the isolator onwards. OK this was not a 100 amp supply, it could be up to 1000 amp, but it has happened where a unused cable was connected in error and then the sparks really fly, so person connecting has to ensure it is safe to connect.

This could be why we have 3 meter rule, close enough to check? In my day the DNO insisted there was an electrician on site before they connected, the CU isolator could be off, and tails provided where the meter was to be fitted, but if the electrician was not there the supply would not be connected, it did some times seem silly, wait there all day, the DNO would arrive, ask are you the electrician you would say yes, then they connect, no paper work to say safe or anything, you just had to be there.

 

[JohnW2]

I had always thought that regulation was to allow you to connect to the buzz bars with a cable appropriate to the supply fuses in a distribution unit? So buzz bars may be 1000 amp but cable from buzz bars to fused isolator are only 16 mm² as the fuse in the fused isolator is only 100 amp.

I'd be grateful if you could help me understand what you are saying here. In particular, what do you mean by "bus bars may be 1000A"?

Kind Regards, John

 

[JohnW2]

... Or they don't want the responsibility. .... Why would your same rated fuse be better than theirs?

That's the bit that has never made any sense to me. Certainly if the cutout fuse is 60 or 80A, that will give adequate protection to 16mm² tails of any length - and I really don't see what is gained by adding another 60A or 80A fuse - other than, as you say, some attempt on the part of the DNO to pass the buck of responsibility to someone else's fuse!

Kind Regards, John

 

[ban-all-sheds]

Hi all, I was aware DNOs usually have a 2 or 3m rule on tail length, which I assumed for their own reasons (simplicity/flexibility and consistency I guessed).

You forgot hard-of-thinkingness. Or was that what you meant by "simplicity"? I believe that is now a deprecated non-PC term, and "learning difficulties" is preferred.

However I was having a read of UKPNs doc and they pin the blame on BS7671 for that rule. That was unexpected.

Ah - "unexpected". You did forget hard-of-thinkingness.

BS 7671 Regulation 433.2.2 allows a maximum distance of three metres between overload protection devices and their source,

Untrue. Or at best egregiously disingenuous.

therefore the customer shall provide and install an intermediate switch fuse or isolating switch if the distance between the meter and consumer unit is greater than three metres.

Says it all.

What protection does a switch provide? Is it

a) None
b) Nada
c) SFA
d) All of the above​

?

The part of the reg they cite apparently applies only to circuits without adequate *fault* protection.

To determine the adequacy of fault protection purely for BS7671 you would just need the total Zs, the adiabatic parameters for the cables involved and the fuse characteristics?

So it stands to reason that they are saying their fuse doesn't provide adequate fault protection for any length of tails.

Indeed.

Which is patently bo****ks.

They are also saying it's not the "overload protection device" for the tails for BS 7671 purposes either.

In that case what is it for? Unspecified DNO protection purposes?

Downstream protection for their supply cable.

 

[ericmark]

In the switch room the supply from the transformer next door went through a large isolator then to a box with copper covered aluminium bars around 6 foot long and 1/4" by 2" with a series of holes pre drilled. then above and below this we has a series of fused isolators and distribution units, some of these would only be 100A rating, although the step down transformer could supply 750 amp per phase the 3 meter rule means the cable from the buzz bars to distribution unit could be just 16 mm² because there was a moulded breaker which was only 100 amp as the main isolator into the distribution unit.

With a crane supply where we wanted to charge the banksman's radio the supply to main board likely 500A but the radio needed just 5A in fact we used a 60A fuse feeding a 16A RCBO the fuse reduced the let through current to under 4.7kA the rating of the RCBO. The cable was just 2.5 mm² from the crane supply, it complied as under 3 meters and unlikely to be damaged.

 

[John D v2.0]

Thanks for all the replies. And I didn't even spot the fact that they think an isolator will fulfil the aforementioned reg!

Regarding ericmarks post, perhaps "3m" is like the bible's "40" Did the olde english counting go 1, 2, lots, so everything became translated to 3?

Why would your same rated fuse be better than theirs?

No, not that. It is on the consumer's side of the meter.
That would not be for the consumer to provide anyway.

sorry by "it" I meant their fuse, the DNO one.

The DNO cable is supposed to be 35mm alu according to UKPN area, so it is a reduction in physical CSA strictly speaking. Maybe by definition it is, when it enters the installation. In our case the incomer is 3 phase PILC.

That's the bit that has never made any sense to me. Certainly if the cutout fuse is 60 or 80A, that will give adequate protection to 16mm² tails of any length - and I really don't see what is gained by adding another 60A or 80A fuse - other than, as you say, some attempt on the part of the DNO to pass the buck of responsibility to someone else's fuse!

Well not quite in other situations, haven't checked that one. If you want a disconnection time of 5 seconds and your Ze is 0.8 on a 100A fuse, you won't get fault protection however short your tails are.

 

[ericmark]

Regarding ericmarks post, perhaps "3m" is like the bible's "40" Did the olde english counting go 1, 2, lots, so everything became translated to 3?

Going to sleep now Yan Tan Tethera

 

[JohnW2]

If you want a disconnection time of 5 seconds and your Ze is 0.8 on a 100A fuse, you won't get fault protection however short your tails are.

I was, of course, talking about overload protection of the cable. As for fault protection, if the Ze is 0.8, then I think that even a 60 fuse would be pretty marginal in terms of 5 secs disconnection!

Anyway, my point w as that, whatever size teh service fuse, and whatever degree of protection it provides, putting a second fuse of the same rating in series with it is not going to make a blind difference.

Kind Regards, John

 

[EFLImpudence]

Thanks for all the replies. And I didn't even spot the fact that they think an isolator will fulfil the aforementioned reg!

Neither did I.

The DNO cable is supposed to be 35mm alu according to UKPN area, so it is a reduction in physical CSA strictly speaking. Maybe by definition it is, when it enters the installation. In our case the incomer is 3 phase PILC.

Either way, 25mm² copper tails with 100A fuse is perfectly compliant so would not be classed as reduced c.s.a.
DNOs use 16mm² for 100A supplies.

 

[333rocky333]

I had always thought that regulation was to allow you to connect to the buzz bars with a cable appropriate to the supply fuses in a distribution unit? So buzz bars may be 1000 amp but cable from buzz bars to fused isolator are only 16 mm² as the fuse in the fused isolator is only 100 amp.

Still often see it, a massive great bus bar with a little 4mm cable coming off to a small isolater usually for something like the the fire alarm nowadays, as I recall it has to have its own supply, though I was always taught that as well as the cable length restricted it had to have mechanical protection like metal conduit or Metal trunking. 434.2.1 (ii)
Hence the switchgear often was Metal couplered direct onto the busbar chamber.

When I first started you could get DI meter tail cables in 6mm size and less i think and a similar thing was done using henley blocks, where a large 16mm DI cable went to the henley block and smaller 6mm DI cables came out into small 15 amp switch fuses, Henley blocks had a mix of small holes and larger holes to assist this, I was never sure this was right due to the lack of mechanical protection and sure there was once something in the regs 433.2.1 about, where every change of CSA occurs there must be a Fuse or similar.
Have even seen in the past where they insisted on using the same 16 mm cable, which would not fit in the small switch fuse terminals, so they would cut off strands to get it in.

 

[John D v2.0]

Anyway, my point w as that, whatever size teh service fuse, and whatever degree of protection it provides, putting a second fuse of the same rating in series with it is not going to make a blind difference.

True! Although they don't say put the same rating, they actually say in that doc
"""
The customer should seek professional electrical advice to determine the protection
equipment that is appropriate for the customer given the impedance at the cut-out and the
design of their installation as UK Power Networks is unable to advise on these aspects.
"""
Which basically means you might not get 100a through one switch fuse if your electrician can't get the disconnection time low enough.

 

[EFLImpudence]

That situation would be the same if the tails were not over three metres in length, in which case the DNO fuse is not adequate.