Question about tails.

[sjm]

Hi Guys.

Quick question about length of meter tails and need for additional fuse protection.
I have a job coming up where the existing tails are about 5-6 meters long 25mm csa and run in ducting undergrond then up into the cu cupboard.
The system is 3 phase TT so I'm going to install a 100ma type"s" RCD at the origin.
When I mentioned additional fusing to the NIC tech helpline I was told that DNO fuse was sufficient for overcurrent protection of the tails.

So my 2 questions are;
1 Is this correct?
2 Where does the idea of 3 meters max come from?
Or is it like the idea that we can't fit plate switches in Bathrooms, or that 3036 fuses MUST be changed or any of the other myths that get repeated and become folklore.

Thanks in advance
Steve.

 

[spark 68]

So my 2 questions are;
1 Is this correct?
2 Where does the idea of 3 meters max come from?
Or is it like the idea that we can't fit plate switches in Bathrooms, or that 3036 fuses MUST be changed or any of the other myths that get repeated and become folklore.

Thanks in advance
Steve.

It is a DNO rule, and even this can vary depending on the individual DNO.

 

[JUD]

^^^^^^Ditto^^^^^^

If NIC say it's OK then it must be as they seem to over-rule everybody

Call the DNO and ask what their limit is. More than likely 3 meters.

 

[sjm]

It is a DNO rule, and even this can vary depending on the individual DNO.

My point exactly. Which DNO is this a rule for if it varries from DNO to DNO.

As an example I asked Western power several years ago and was told "you're the electrician work it out". So what rule??

Just another thought.
If it's a DNO rule why aren't we allowed to put said fusing in their meter box?

 

[DurhamSparky]

ive seen meter tails installed at over 10-12 meters!! on new builds as well... no switch fuse just a REC isolator

 

[sjm]

ive seen meter tails installed at over 10-12 meters!! on new builds as well... no switch fuse just a REC isolator

Exactly so where does the idea come. Or why are these designers not taking any notice of THE RULE of the DNO's

 

[DurhamSparky]

way i see it............. DNO responsibility ends at Service Head....

how many jobs you seen with 2/4/6mm cables stuffed in meters powering 60A loads...!! do they care then???

why should they care if you install 10 meters of 25mm cable..... they wont..

i follow BS7671 as thats what i have been trained in... in that BOOK it mentions nothing apart from mechanical protection and cable sizing for load requirements...!

 

[JUD]

Exactly so where does the idea come. Or why are these designers not taking any notice of THE RULE of the DNO's

Probably because the DNOs attitude is.....

My point exactly. Which DNO is this a rule for if it varries from DNO to DNO.

As an example I asked Western power several years ago and was told "you're the electrician work it out". So what rule??

 

[Guitarist]

I think that maybe the "rule" came about from the confusion around the 3 metre reg itself. We can fit smaller cable than fuse protecting it provided that the "correct" fusing is carried out within 3 metres.
This allows DB feed cables to be attached to much larger busbar systems, and 3 metres is deemed as a reasonable distance to prevent breaking into said cable and connecting other loads.
In reality, there is no benefit from protecting a 25mm tail with a 100A DNO fuse as well as fitting another 100A fuse a few inches further away.

 

[sjm]

Sorry JUD but your last post doesn't make any sense at all.

I ask again why 3 meters. It can't be a rule if it varries from DNO to DNO.

As for saying the designers attitude is somehow incorrect because they take the view that they're the electricians so they'll work it out I don't understand where you're going with that.

As Durham has said the max length of tails is not mentioned in BS7671 so why do we adhere to an arbitary figure when the DNO's probably don't care.

 

[JUD]

Sorry JUD but your last post doesn't make any sense at all.

I ask again why 3 meters. It can't be a rule if it varries from DNO to DNO.

As for saying the designers attitude is somehow incorrect because they take the view that they're the electricians so they'll work it out I don't understand where you're going with that.

As Durham has said the max length of tails is not mentioned in BS7671 so why do we adhere to an arbitary figure when the DNO's probably don't care.

It was meant to be a dig at the DNOs not the designers.

Just a bit of humour

I've edited the post so it's more understandable

 

[spark 68]

Have a quick read of 433.2 /.1/.2 and 434.3 (iv) and also 435.

As guitarist points out, it seems to stem from the effective reduction of CSA and the PFC at various points of the distribution cable, fault protection co-ordination.

When I said it can vary, I meant the 3M is the Max, the DNO might ask for less.

One thing I do know, is this is above my pay scale lol, I just do as recommended.

 

[spark 68]

As Durham has said the max length of tails is not mentioned in BS7671 so why do we adhere to an arbitary figure when the DNO's probably don't care.

It is sort of mentioned, see regs group in previous post.
And they only don't care unless there is a fire/problem/accident (take your pick), and then the 'blame game' begins, it is called covering your arse.

 

[DurhamSparky]

It is sort of mentioned, see regs group in previous post.

enlighten me??

also who cares when the DNO undersized cables melt and burn houses down...!! ????? not the DNO as as they call it..... past the meter is consumer responsibility.....

 

[spark 68]

enlighten me??

also who cares when the DNO undersized cables melt and burn houses down...!! ????? not the DNO as as they call it..... past the meter is consumer responsibility.....

Have a quick read of 433.2 /.1/.2 and 434.3 (iv) and also 435.


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