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?

 

[NickD]

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!!

Listen to reason is exactly what I want to do, indeed what I'm bloody well pleading for. If you read what I'm asking, you'll see that saying that demand doesn't get that high (whether anecdotally or with actual data) doesn't address it. And I'm long past the point of being surprised or worried when people totally fail to get me and decide to judge me.

 

[Engineer54]

Listen to reason is exactly what I want to do, indeed what I'm bloody well pleading for. If you read what I'm asking, you'll see that saying that demand doesn't get that high (whether anecdotally or with actual data) doesn't address it. And I'm long past the point of being surprised or worried when people totally fail to get me and decide to judge me.

Well you've totally lost me now!! So what DO you actually want to know then, because you're right i've totally failed to get you??

 

[NickD]

Well you've totally lost me now!! So what DO you actually want to know then, because you're right i've totally failed to get you??

"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."

 

[NickD]

Put it another way:

I upgrade CU on existing 16mm tails cos DNO fuse is 60A. Fuse later fails (possibly due to age or defect). DNO's agent comes and bangs in a 100A cos that's what he had on the van. Client usage pattern changes (e.g. boiler fails so move to reliance on electric heating in every room). Tails get overloaded. Bad thing happens. Who is liable, the DNO agent for uprating a fuse the tails were reliant on, or me because I had no right to assume the DNO fuse would always protect the tails? (Yes I know the 60A would probably have never blown to prevent the overload anyway, but I would be far less likely to be considered liable in that event. I'm also aware the possibility of such an overload is very small - but still finite. Assume it happens and go from there.)

 

[davesparks]

What is the maximum demand after diversity for the installation? If its less than 80A then leave the 16mm in place. If its more than 80A then that must be one hell of a big house and you may need to look at getting a bigger supply brought in!

If you are really that concerned over it replace the boards main switch with a 63A DP MCB.

 

[Engineer54]

owever, 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: _____________

Because in all probability the existing DNO fuse will be rated at 60A and 16mm tails are more than capable of looking after a upgrade to a 80A DNO fuse rating. (Is that clear enough for you??)

The ACTUAL requirement for domestic DNO 100A fuse on 25mm tails is to say the least very rare and the house/property would need to be of a substantial size that has every load eating gizmo you can think of. As i say if you don't believe me (or anyone else) hire a power analyser, as suggested in my previous post.... Too many people have been influenced by the pretty pictures in OSG and GN's that are basically showing an examples, not a cover all requirement...

 

[NickD]

Because in all probability the existing DNO fuse will be rated at 60A and 16mm tails are more than capable of looking after a upgrade to a 80A DNO fuse rating. (Is that clear enough for you??)

Clear yes, but it doesn't address the issues in my first paragraph about the possibility of the DNO fitting a fuse which does NOT protect the tails, so it isn't an answer. Not that I can see anyway. Sorry.

 

[davesparks]

Clear yes, but it doesn't address the issues in my first paragraph about the possibility of the DNO fitting a fuse which does NOT protect the tails, so it isn't an answer. Not that I can see anyway. Sorry.

DNO operatives are known to have happily fitted solid links in place of fuses when one blows and its all they have with them at the time, will you allow for this in your tails?

 

[Knobhead]

Some of us believe it or not have practical experience of cables failing and the causes of failure. Unlike many that think they know what will/may happen.
Bear in mind, cables don’t spontaneously combust at 101% O/L. They have safety margins built in to the design. Added to which the chances of reaching 100% are so remote as to be zero.
You don’t have the experience to assert your arguments and so have resorted to ill founded belligerence and in doing so proved yourself a fool.

It’s your customer I feel sorry for, they’re going to get fleeced by an inexperienced and unwanted usurper to what was once a respected trade. It’s the likes of you that due to incompetence and greed have made your trade a laughing stock amongst other trades.





BTW,
1½ x ½” copper links instead of 450A fuses work wonders in fault finding on a faulty LV cable. It was OK, there was a 1200A ACB as back up if things went wrong.
It really is the “bang” test.
11KV cables are much more fun.

 

[GMES]

Right a question has been asked and several members have offered the op their answers and thoughts, he has chosen to ignore the advice so the thread is just going round in circles.
Time to close it.