Omission of overcurrent at origin of installation?

[BodgeJob]

Hi all,

My question relates to the omission of overcurrent protection (specifically overload and fault protection) at the origin of an installation. Am I right in saying that DNOs allow this for meter tails, due to the maximum length of 2 or 3m, insulated and sheathed nature of the tails and the relatively low chance of a fault occurring?

For a domestic installation, the norm is to install 25mm live conductors and 16mm earthing conductor from the meter to the consumer unit, in case a 100 amp HRC fuse is installed by the DNO, and to future proof the install for future loads etc.

That's all and well, but what is the correct selection of tail CSA for extra consumer units. Maybe an extremely low load off peak contactor control DB, or a new electric shower board? In these cases, we know that overloads are impossible due to the nature of the loads; therefore we can workout the design current and choose a size of tails to use, assuming a 1 way CU is used (no spare ways).

A load of 8.5kW @ 230v = 37A, therefore we can use an mcb of 40A. 10mm^2 tails would be adequate to carry the load. However, if we split the meter tails into a henley block, those 10mm tails may now be "protected" by a 100a DNO fuse. Is this acceptable? I personally don't like the thought of this, but on the other hand I can see how stupid it would be to install 25mm tails for a 1 way 40A CU. THEORETICALLY, if the design current was 6 amps for instance, could 1.5mm^2 tails be used to supply a 1 way CU with a 10A mcb?

I understand that adiabatics may be used to see if the live tails and protective conductor could withstand the potential fault current until the DNO fuse blew etc, but is this fault protection by the DNOs fuse allowed to be omitted anyway?

With regards to consumer units where overloads are possible (socket circuits maybe), surely you could apply diversity, calculate a maximum demand, then size the tails to the expected demand of the new CU? If these live tails are again, less than 25mm^2 on a 100a DNO fuse, do we have the same problem? Obviously overload protection would have to be provided for the final circuits with the potential for overload, in the CU.

I am sorry about the length of my post. Hopefully this makes sense to people too, haha. This is my first post so thank you for having me.

Thank you.

 

[pc1966]

The regulations that apply here are:

• 433.2.2
• 434.2.1
• 434.3(iv)

You must protect against faults, and generally you also have to protect against overload. For a quick reckoning on your adiabatic limits the likes of Table B5 in the OSG can be consulted. For 100A fuse the lower limit is 6mm but only if Zs at end of cable is below 0.13 ohm, which is unlikely (this is to meet adiabatic limit, the 5s limit is met at higher Zs as you can see with 0.28 ohms for 16mm cable).

The requirement for no primary insulation (as well as lowering the risk of a fault) would force the use of double-sheathed cable, realistically you won't easily get that below 16mm unless you buy a 100m reel of smaller 6181Y. I really doubt it is worth the cost! So you might as well use 16mm tails for such a job.

 

[pc1966]

Just to expand a bit "protect against faults" can be choosing the up-stream OCPD (or using/abusing the DNO's fuse) for protection, or it can be taking special care that a fault is very, VERY, unlikely.

The 2nd aspect to the DNO's fuse that is not always obvious is they do not have to meet the 5s ADS limit of the wiring regs! Their fuse is to protect their network from overload damage, not to limit the exposure to dangerous voltages in the event of a fault, so you can have a fuse & Ze combination that fails BS7671 but is adequate from their perspective.

 

[Lucien Nunes]

I think the combination of the OP's insight and the replies by @pc1966 more or less exhaust the subject. I would draw attention to the way that for meter tails, the considerations are partly electrical and partly administrative. On the one hand, the electrical situation is fairly clear:

• Overload protection can be upstream or downstream (e.g. one small final circuit circuit can use small tails)
• Fault protection must be upstream but can be sized adiabatically, so 100A DNO fuse doesn't need tails with 100A continuous rating.

But since the components either side of the tails belong to (and may only be fitted by) different parties, they have different aims and requirements:

• DNO works to ESQCR and seeks to protect their equipment and other customers against faults on this installation and to avoid expense of spurious callouts.
• Installation built to BS7671 seeks to protects its users against shock and fire while avoiding nuisance trips etc.
• A wise installer seeks to protect their customer against organisational headaches such has having to arrange paid callout to upsize tails when an extra circuit is to be added.

It's often impractical for the DNO to fuse the premises service cable against overload at the point it taps off the main, so the protection is dowsntream at the service terminal. Although that fuse is there mainly for the DNO's benefit, it's reasonable and sensible for them to allow it to be used to protect their customer's short tails that are at minimal risk of fault. Otherwise the illogical situation would arise of the customer always having to fit a smaller fuse immediately downstream (to protect the DNO fuse against being blown) so the installation could never be allowed to draw as much current as the supply can provide. Ultimately the tails end up being sized to match the fuse which is sized to protect the supply, moderated by the electrician's common sense as to whether there is any reason not to use the largest practical size to minimise the risk of a costly callout to upgrade in the future.

 

[BodgeJob]

Thank you very much for your extremely informative and timely replies. So there is the choice to either comply with ADS using the DNO's fuse OR take "special care that a fault is very, VERY, unlikely"?

Just to expand a bit "protect against faults" can be choosing the up-stream OCPD (or using/abusing the DNO's fuse) for protection, or it can be taking special care that a fault is very, VERY, unlikely.

So if "protect against faults" could be satisfied by using double-sheathed cable and making sure the length does not exceed 2 or 3m from the origin, does this mean the DNO's fuse will not need to be considered for fault protection for that part of the installation and therefore the tails not sized with regards to carrying such fault currents possible at that part of the installation? As long as the meter tails are protected/sized for overload protection downstream (in the CU)?

 

[pc1966]

Thank you very much for your extremely informative and timely replies. So there is the choice to either comply with ADS using the DNO's fuse OR take "special care that a fault is very, VERY, unlikely"?

Ideally you do both!

So if "protect against faults" could be satisfied by using double-sheathed cable and making sure the length does not exceed 2 or 3m from the origin, does this mean the DNO's fuse will not need to be considered for fault protection for that part of the installation and therefore the tails not sized with regards to carrying such fault currents possible at that part of the installation? As long as the meter tails are protected/sized for overload protection downstream (in the CU)?

Mostly.

The issue of "very, VERY, unlikely" is not just they have extra insulation from the double-sheath, it also comes down to the whole situation they find themselves in.

If you take the classic situation where protection is downstream of the cable you are looking at the connections from a bus-bar chamber to the fused-switches, etc, and here you have an enclosed box that is only accessed by qualified people. However, meter tails are normally found around the meter and CU in a domestic property. The same sort of place where many folks store all sorts of crap and so the issue of physical strength, vulnerability to outside influences (such as somene pulling out said crap), and related soundness of support/termination at the end points comes in to play.

For example, if you were running smaller tails in trunking from an enclosed junction block to a CU or similar where the OCPD provided protection for final circuit faults then probably most engineers would agree that it is fine. If you have an exposed Henley block feeding small tails a couple of meters along a cupboard wall to a similar CU it would be much harder to convince anyone that damage is extremely unlikely.

 

[Dave OCD]

In years gone by many of us would feed an additional small CU [say for a shower or outbuilding for example] with 6mm tails, often directly connected in with the existing 16mm tails in the existing CU main switch [a fiddly thing to achieve sometimes ]. This was considered perfectly ok but we just wouldn't do it now.