Cable Fault

[VoltzElectrical]

Today's first job.

Householder is building a retaining wall and we have lost supply on one of our street lighting cables. Inspector has done cut and test and got voltage to one point but lost it further on. No visible damage to exposed section of cable.

My job was to cut out section of cable and put in a new section. I did this and restored supply. I was determined to identify the fault in the section of cable I had removed so I kept cutting it in half and testing end to end until I had a 2 foot section left within no continuity. Then I started stripping it.

This was 6mm concentric cable and shortly after I had the live core separated . There was no visible damage, it looked perfect except about 4 inches from the end, the cable was floppy. There is a clean break in the solid aluminium conductor.

I'm assuming this is a manufacturing fault? I still have the piece in my van as evidence of what can happen in a piece of visually acceptable cable.

Any thoughts or similar experiences?

 

[Richard Burns]

I could well be wrong but is this not where the manufacturer starts a new length of aluminium core in a cable and should cut out the joint, but has not in this case?

 

[VoltzElectrical]

I've been in touch with a more knowledgeable member who seems to think its an inherent fault during the manufacturing process associated with the annealing or softening of the aluminium causing stress of the metal that failed at the stress point when the cable was moved, but not apparently damaged externally. A bit like bending and re-bending a metal part to cause it to intentionally fail.

I've had it only once before on a length of 1.5mm2 twin and earth cable that was open circuit, but I didn't investigate it as I have done in this instance.

This is the second concentric cable fault open circuit I've had this week so I was keen to probe further. The other is a farm sub main that I have on a temporary supply till the cable comes in next week. I may or may not have the same opportunity to find the exact point of failure on that cable depending on whether the Farmer pulls up the old one, or just excavates for the new one.

 

[Deleted member 26818]

Could the cable have been stretched either during installation or when the building work was conducted?

 

[VoltzElectrical]

Well I really couldn't say even when it was installed, maybe many years ago. I guess we'll just put it down to a manufacture's fault that has now been rectified and from our point of view we'll just move on.

 

[Knobhead]

Aluminium doesn’t like vibration or being stretched. Without seeing a magnification of the break point it’s pure speculation as to what’s happened. 6mm² Al concentric ground vibration would be my bet.

I doubt it will be a manufacturing fault.

 

[VoltzElectrical]

That's an interesting point.

 

[Engineer54]

Never use aluminium cables of any kind, far too many problems associated with aluminum cables. The only good thing is the price, but as far as i'm concerned, even that is false economy both initially and over the long term!! I know from an ex DNO area manager friend that they had no end of problems with ally conductor cables....

 

[VoltzElectrical]

I agree. That is the second Aluminium concentric that has suddenly gone open circuit this week. the other was 25mm sub main on a farm.

 

[VoltzElectrical]

I pulled out the snapped core from the insulation at the short end, (about 3-4 inches) and the aluminium conductor has been pulled uniformly down to a point about 1mm across. The insulation is perfect. I'm not convinced that this 'stretch' has happened on site and still believe it happened when the cable was being manufactured. I think the 'final' break happened when the cable was 'disturbed' during the excavations only because of the visible structural integrity of the external of the cable, and the minimal ground disturbance of the shallow and careful excavations.

I can post a picture of the cable if you want, but it will be pain to do so!

 

[Silly Sausage]

I can post a picture of the cable if you want, but it will be pain to do so!

Then pain it is V!

 

[VoltzElectrical]

Then pain it is V!

OK Archy, the offending piece is tucked into my sun visor in the van. I will take the piccys then download them then up load them on the weekend. Is that ok lol?