Question about tails.

sjm · Jun 9, 2012

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.

DurhamSparky · Jun 9, 2012

that talks about reduction of CSA of supply cable... not to exceed 3m before O/C protection is installed...

supply cable terminates at Service Head/Meter this is protected by O/C protection by DNO... normally 16mm Tails.. if we are installing 16mm Tails or above it negated the 3m rule..

spark 68 · Jun 9, 2012

DurhamSparky said:
that talks about reduction of CSA of supply cable... not to exceed 3m before O/C protection is installed...

supply cable terminates at Service Head/Meter this is protected by O/C protection by DNO... normally 16mm Tails.. if we are installing 16mm Tails or above it negated the 3m rule..

433.2.1 Note underneath reg says, A reduction in current capacity may be due to a change in csa, method of installation, type of cable or conductor or in environmental conditions,.

So not just a change in CSA then, it also points to other regs in that group, and also 434.3 (iv) mentions the distributor placing one or more OCPD's from the origin and the main distribution point.

It is very convoluted, and bouncing between regs to reduce the risk of a fault to a minimum and to reduce the risk of fire.

As I said earlier in this thread it is more of a DNO rule, and as we know they are a law unto themselves, and can refuse to supply power to an installation that does not conform to their 'wishes'.

Guitarist · Jun 9, 2012

Basically, the DNO think that you could compromise the CCC of the cable, even if it is the correct size, by extending it too far (possibly through insulating materials). By putting a limit of 3 metres on it before fusing becomes "your responsibility", they have effectively said that any problems are no longer their concern.

spark 68 · Jun 9, 2012

Hi guitarist,

It could also be for fault protection of the tails and co-ordination between OCPD's, ie. between the DNO's fuse and the fault withstand of the cable length in between the OCPD's in the CU for the final circuits, aka the forward fusing rules.

I don't pretend to know where the 3m rule comes from, presumably the powers that be have done extensive testing and calculations to arrive at that figure.

As I also said, it is above my payscale, I just follow the recommendations.