Why do the tails need a SF if over 3m?

[HappyHippyDad]

I was wondering why the tails need to have a switch fuse if longer than 3m?

I have hunted through the threads and most say that the DNO only allows their cut out fuse to protect the tails up to 3m but why is this? In theory we could just put the switch fuse 1m into the 3m run.. how is that helping having another fuse 1m from the cut out. Also if the cut out is 60A you will just be putting another 60A KMF??

Regulation 433.2.2 states:

"The device protecting a conductor against overload may be installed along the run of that conductor if the part of the run between the point where a change occurs (in cross-sectional area, method of installation, type of cable or conductor, or in environmental conditions) and the position of the protective device has neither branch circuits nor outlets for connection of current-using equipment and fulfils at least one of the following conditions:


i It is protected against fault current in accordance with the requirements stated in in Section 434.

ii Its length does not exceed 3m, it is installed in such a manner as to reduce the risk of fault to a minimum....."


So is that saying that if we meet the requirements in the above paragraph AND point (i) then we don't need a additional switch fuse?

Cheers all, looking forward to the responses on this one..

 

[davesparks]

The only answer I have ever known is 'because the DNO said so'
No matter how daft it may seem they have their rules and we have to follow them.

 

[telectrix]

the longer the cable ru, the higher the risk of damage causing a fault condition. the 3m seems to have been picked out a a hat of numbers by chance.

 

[Silly Sausage]

2m is too short and 4m is too long, so 3m is a good compromise.

 

[telectrix]

3 miles. that's a bloody long cable.

 

[Richard Burns]

The 3m limitation is only there as a general catch all situation, the DNO can decide to change this distance if they deem that it would be appropriate to do so, however it is not practical to contact the DNO and get an individual assessment for each job so the general ruling that the 3m distance is not exceeded can be applied and you can be confident of reasonable compliance.

Overall the DNO do not want their fuses to blow as it causes extra work for them so they prefer you to provide protection for their fuse! In a general situation the design of the installation should be such that downstream protection works first and there is never an overload at origin and any fault current should be interrupted by downstream devices before the fuse at origin goes.

The situation with the tails from the fuse to the CU is that the only protection is the DNO fuse and if you have a long run of cable then the risk of a fault occurring is increased, so the cable should be kept as short as possible; 3m is a good compromise as it allows an easy run from ground level outside a wall to anywhere on the inside of the wall of a domestic property.

When you start to consider the regulations of BS7671 regarding fault and over current protection then fault and over current protection should be provided at the point of change of CCC, etc. unless it is already protected earlier in the circuit, (or for overload it is not possible to overload).
There are also exclusions, one of which is where the supplier says it is OK to use their fuse which is what I said initially (and the DNO say max 3m in general).
(there are also other specifics that are irrelevant here).
In all cases where omission of, or introduction of the protective device later in the circuit, is used then the cable should be protected so that it is safe without that protective device.
In general you cannot easily omit both overload and fault current protection (unless the DNO allows it, as above again) on a cable that exceeds 3m in length.

Therefore the easiest and most straightforward way to manage this is to apply both fault and over current protection within 3m of the origin, job done.

Trying to get around this sort of situation, unless for a good design reason, is not worth the bother.

 

[driverman]

For a great many years it was always only 2m tails allowed which on some jobs was very tight to comply. 3m seems to be adequate. Mind you I have seen tails much longer and run in loft area!

 

[HappyHippyDad]

Thanks all. Its bugged me for a while this 3m! Now I know.. and shall just do it when the need arises!

 

[GMES]

I for one think it is time that Admin gave Sir Richard Burns a Forum Title, due to the fact that he is always willing to give members a sensible and in depth answer to the questions they need answering, Keep up the good work Richard it is appreciated on here.

 

[Richard Burns]

I am just here and wasting time I should be devoting to the business on reading lots of posts and maybe answering a few, in, hopefully, some reasonable way.
Thank you for the appreciation, not sure it is really deserved but thanks anyway. I think the forum has done away with the majority of titles and probably not necessary for me.

 

[HappyHippyDad]

I for one think it is time that Admin gave Sir Richard Burns a Forum Title, due to the fact that he is always willing to give members a sensible and in depth answer to the questions they need answering, Keep up the good work Richard it is appreciated on here.

I definitely concur Glenn!

Although I suspect Richard probably isn't too bothered by titles, he seems to be one of those very rare types that just seems happy to give help.

 

[GMES]

He is the Gift that just keeps on Giving!!!

 

[uksparks]

3m is fine for the vast majority or jobs, but on yen very odd occasion it would be nice if they were say 5m.