16mm tails with 100A fuse ???

[Wilko]

Job today to change out old fusebox and I'd noted the 16mm tails but on pulling the fuse this morning it was 100A which I wasn't expecting. Fairly recent new smart meter installed by XYZ who are also the DNO ... Shouldn't the fuse be limited to 60A in this case? Thoughts and advice appreciated

 

[Toneyz]

Is this a joke? Or have you come to an agreement with the DNO about preventing the fuse ever being replaced with 100A by their operatives if the cutout is replaced or if someone notices that the wrong size fuse has been fitted?

The DNO operatives should be trained that they shouldn't be inserting 100A fuses without assessment .

 

[westward10]

But cable sizes are calculated based on the most onerous conditions they are installed in, so if the last few inches have a lower ccc due to being enclosed then the whole length of the cable must have that lower ccc.

If the termination at a CU is to be considered as being enclosed for the purpose of cable rating then do you also extend that to a switch, socket or joint box, and if not then why not?

I think you need to consider what, if any, the effects of being in that enclosure are. the difference between being enclosed in a conduit or trunking and being in a CU should hopefully be obvious.

These are my chains of thought and I don't think it is so off the wall. What are the obvious differences between conduit/trunking containment and a CU enclosure.

 

[Rpa07]

10 - 15 metres

 

[davesparks]

These are my chains of thought and I don't think it is so off the wall. What are the obvious differences between conduit/trunking containment and a CU enclosure.

I don't think it's particularly off the wall, but it does need some serious consideration and thought.

In a conduit or trunking the cables are bunched together with no, or minimal, airflow around them, the volumetric ratio of cables to air is also quite different.
Cables in a CU have, or should have, better spacing and therefore a little airflow around them.

 

[davesparks]

The DNO operatives should be trained that they shouldn't be inserting 100A fuses without assessment .

they are trained as such, and they do assess, but that assessment is based on their equipment. The fuse is not the customer's, it is not there to protect the customers equipment and is not sized according to the customers equipment.

A customer can request that the fuse be replaced with one of a lower size, and the DNO will fix the correct warning labels and update their records accordingly when they carry out the replacement.
They don't permit anyone else to make an alteration to their equipment.

 

[Deleted member 26818]

At last some sense.
The DNO fuse is there to protect their equipment, nothing else.
It's the consumer's responsibility to provide protection for their installation, not the DNO's.

 

[Wilko]

A customer can request that the fuse be replaced with one of a lower size, and the DNO will fix the correct warning labels and update their records accordingly when they carry out the replacement.
They don't permit anyone else to make an alteration to their equipment.

Thanks Dave - that makes sense. I will advise Customer to either upgrade the tails to 25mm or request DNO to de-rate fuse to 60A, as it better reflects the capacity of her installation.

 

[Deleted member 26818]

Thanks Dave - that makes sense. I will advise Customer to either upgrade the tails to 25mm or request DNO to de-rate fuse to 60A, as it better reflects the capacity of her installation.

Why?
Is there no protection at the CU?

 

[Wilko]

Also a good point Spin.

BTW, Apologies to all for my attempt at humour in #33.

Putting my thoughts in order then - Overload in / after the CU is eliminated with circuit design and loading. Tick. By calculation the 16mm tails have sufficient size to disconnect a 100A fuse in short circuit conditions at that site. Tick. That leaves overload of the 16mm tail ccc that maybe of concern. Some debate at to what the ccc of 16mm really is... I'm thinking there is a small traverse of the ceiling space above the front door, which would make it installation method B which gives 76A (from 4D1A).

As the house maximum demand is below this no further action is required (?).

 

[Deleted member 9648]

Also a good point Spin.

BTW, Apologies to all for my attempt at humour in #33.

Putting my thoughts in order then - Overload in / after the CU is eliminated with circuit design and loading. Tick. By calculation the 16mm tails have sufficient size to disconnect a 100A fuse in short circuit conditions at that site. Tick. That leaves overload of the 16mm tail ccc that maybe of concern. Some debate at to what the ccc of 16mm really is... I'm thinking there is a small traverse of the ceiling space above the front door, which would make it installation method B which gives 76A (from 4D1A).

As the house maximum demand is below this no further action is required (?).

Leaving the technicalities aside, if there appears to be no thermal damage to the tails and they have been there for all time it would be reasonable to assume they are adequate for the present load.

 

[westward10]

Delete.

I don't think it's particularly off the wall, but it does need some serious consideration and thought.

In a conduit or trunking the cables are bunched together with no, or minimal, airflow around them, the volumetric ratio of cables to air is also quite different.
Cables in a CU have, or should have, better spacing and therefore a little airflow around them.

I agree with conductors in conduits/trunking but I see little difference within a dist board, suggesting the presence of airflow, if any at all is subject too many factors and variables. To me containment within a board is no different to conductors in a well proportioned trunking.

 

[Deleted member 9648]

I can see the point that is being made, but I take a less technical and more pragmatic view. I've been in the game 40 odd years and have never seen an instance of thermal damage to a short section of conductor in an enclosure when there wasn't similar damage outside the enclosure.....poor termination heat aside. I cannot recall a single instance of technical advise literature highlighting an issue here.
We've got enough to worry about day to day without trivialities like this getting in the way.

 

[Wilko]

Leaving the technicalities aside, if there appears to be no thermal damage to the tails and they have been there for all time it would be reasonable to assume they are adequate for the present load.

Agree.
Should have done this earlier - sorry, still learning - on initial survey I noted number of circuits, points and appliances etc. Using OSG App A the future demand (a new induction hob is planned) after diversity calculates at 52A, well below the tails rating. From inspection yesterday the tails are in pristine condition, supporting this.

 

[telectrix]

why not just bung in a 60A KMF between meter and CU. this will make up for the short tails and also give a point of isolation independent of the DNO fuse.

 

[Ian1981]

It's worth taking note of page 332 of bs 7671 which states
Where the over current device is intended to afford fault circuit protection only, In can be greater than Iz and I2 can be greater than 1.45 Iz.
The protective device must be selected for compliance with regulation 434.5.2 in which we've already established is fine.