Protecting 16mm tails

[NickD]

Got a domestic install with client requesting CU upgrade. Tails are 16mm, EC 10mm, and buried in structure between meter and fusebox. Rating of DNO cutout fuse not known. Ordinarily I would upgrade the tails & EC to 25/16 as a matter of course (domestic load includes an outdoor hot tub as well as the usual sockets/cooker/immersion type stuff, don't recall if electric shower present) but routing them through the existing route of the 16mm tails looks like a non starter and I can't see a practical alternative route. The meter box is small and absolutely chocker with no space in there to add a separate fused cutout. Adding a fused cutout next to the replacement CU (in downstairs cloakroom) would probably be doable but cluttered/messy.

Installing a 63A MCB in the new CU, suitably labelled, immediately downstream of the main switch and feeding all RCDs/final circuits...good/stupid idea?

 

[cprfenom]

Im not getting why you ask a question, then argue to ---- about the replies.

In this situation it really does seem that Tony has hit the nail on the head. Its as if you want someone to justify your point of view. If there is no evidence that the current tails are under any pressure, why don't you leave them be, or when you ask for an isolator to be put in in order to change the consumer unit, ask the engineer to add a 60/80 and not the standard 100A they always seem to put in.

Don't ask questions if your point of view is set in stone...

 

[Knobhead]

Forget any arguments you think I'm making then and please just answer my question; do you consider it is entirely justified and good engineering practice to rely on the present rating of the DNO's fuse to protect the consumer's tails?

One of the first questions you were asked was “what size is the DNO fuse?”
You don’t know.

Will the DNO suddenly decide to up the fuse?
You don’t know.

Will the load dramatically increase?
You don’t know.

About the only thing you do know is that there has been no problem to date.

 

[NickD]

Im not getting why you ask a question, then argue to ---- about the replies.

In this situation it really does seem that Tony has hit the nail on the head. Its as if you want someone to justify your point of view...

No, absolutely not, not at all. I don't actually have a point of view on the answer to this. I would be very happy indeed to walk away from this confident that leaving the 16mm tails in situ, relying long-term on the rating of the DNO fuse for their protection, was a totally reasonable and defensible engineering proposition.

Let's try again...

The stated purpose of the DNO fuse is to protect the DNO's cable upstream of it, and any protection it gives to the consumer's system downstream of it is mere happy accident. It is unclear (to me, anyway) what the duty of care of the DNO is if it goes to uprate the fuse when the consumer's system in fact relies on the present rating of the fuse for its safety. The DNO is a large complex organisation with which I am not particularly familiar. I can have no real control over what it does with the fuse in the future, and I would be kidding myself if I were to make blanket assumptions about the DNO's operational processes and practices (including about them complying with what may appear to me to be common sense).

However, what makes it legitimate and reasonable for me to rely on the present rating of the DNO fuse to protect the consumer's tails for the long term, is the following: _____________

Please fill in the blank. Thanks.

PS If you believe "The tails have been OK up to now" is a good answer then with respect I think you've missed my point.

 

[dcelectrical]

am I being thick here, I have never known the DNO to come in and change fuses, they generally ask us to change them when a builder pops them and nicks the one from the flat next door before walking away whistling the tune of rawhide.

 

[Knobhead]

The stated purpose of the DNO fuse is to protect the DNO's cable upstream of it, and any protection it gives to the consumer's system downstream of it is mere happy accident.

Pardon?

Would you care to explain how a downstream device protects the supply cable.

 

[NickD]

Would you care to explain how a downstream device protects the supply cable.

On balance, no. (I think it's something to do with it going bang though.)

 

[Des 56]

The problem we have here is this quote from the opening post
quote
Ordinarily I would upgrade the tails & EC to 25/16 as a matter of course


The post then goes on to describe the difficulty in routing the proposed tails

I may be wrong but it seems to me that the question of the supplier fuse (given the above quote)is never taken into account and it is only now mentioned because of routing problems for those tails

It is for this reason that I think the post is looking for justification of the change ( when he knows full well its not required) and he is unfortunately clutching at straws to allay the criticism that has been given

 

[NickD]

I think the post is looking for justification of the change ( when he knows full well its not required)

If you want to think that, that is of course your prerogative; and it would seem the logical next step is to blow me out of the water by filling in the blank with a great answer, which totally convinces with reason and evidence why the tails can be left as they are as long as the present DNO rating protects them.

Alternatively, if I have no such agenda and am genuinely asking the question, the logical next step is to fill in the blank with a great answer, which totally convinces with reason and evidence why the tails can be left as they are as long as the present DNO rating protects them.

 

[Knobhead]

On balance, no. (I think it's something to do with it going bang though.)

The service fuse will be a BS88, about a subtle as a brick under O/L. It won’t limit the fault current.

If you consider that in urban areas the feeder cables may be protected by 400A or even 630A fuses a 35mm[SUP]2[/SUP] concentric doesn’t stand a chance.

That weakness is part of the protection. A fault on a small service cable the cable will act as the fuse and blow the fault clear.

It’s as rough as the proverbial badgers arse, but it works.

 

[NickD]

The service fuse will be a BS88, about a subtle as a brick under O/L. It won’t limit the fault current.

If you consider that in urban areas the feeder cables may be protected by 400A or even 630A fuses a 35mm[SUP]2[/SUP] concentric doesn’t stand a chance.

That weakness is part of the protection. A fault on a small service cable the cable will act as the fuse and blow the fault clear.

It’s as rough as the proverbial badgers arse, but it works.

I bow to your superior knowledge. Could you be so kind as to deploy it in the direction of answering my question?

 

[Engineer54]

I think you need to convince yourself, rather than any of us trying to convince you. Hire a power analyser and do your worst for a week on your own property. I can guarantee that when you see the results of the max demand values recorded, you'll be then wise enough, NEVER to install 25mm tails as a matter of course again!!!

 

[NickD]

I think you need to convince yourself, rather than any of us trying to convince you. Hire a power analyser and do your worst for a week on your own property. I can guarantee that when you see the results of the max demand values recorded, you'll be then wise enough, NEVER to install 25mm tails as a matter of course again!!!

The plural of anecdote is not data...

 

[Knobhead]

About time this was closed before we get to the blood and snot.

 

[NickD]

About time this was closed before we get to the blood and snot.

"You have offended my family, and you have offended a Shaolin temple....."

....maybe by someone with the wit to do it filling in that blank....?

 

[Engineer54]

The plural of anecdote is not data...

Trying to be clever won't get you anywhere!! You're only making yourself look bloody foolish, and digging a deeper hole for yourself. If you don't want to listen to reason, get on with it!! lol!!