Cables/wires buried under ground

[ians]

A couple years ago I was called out to site to check a CCTV installation ... just random cameras around the property not working ... on investigation we found that there was no voltage at the camera ... found a junction box and cut the camera cable and there was 12.8 VDC ...so replaced the cables ... that’s where it got interesting ... all the cables which were buried in conduits were damaged (no continuity) ... but no visible damage ...cuts etc.

Today I found the same issue ... RG59 cables buried in conduits ... no power at the camera ... pulled the cable out the conduit and cut it ... I could not see any visible damage on the cable ... did a continuity test ... it was open circuit ... cut the cable in half again ... still no continuity ... cut it again and Walla we had continuity.

I stripped the piece which had no continuity and found the red cable had turned from red to pink and the core was like a powder ...you can pull it and it breaks into pieces.

These RG59 cables were in the ground for 3-5 years.

So what causes this problem ... acid in the ground?

Will this happen to all cables buried in conduits ... house wires ... twnin+E ... surfix ... RG59 ... Cat 5/6/7?

I have always thought cables exposed to UV light were the problem ... but it seems a cable buried in the ground is also at risk.

Something else I have learnt ...no matter how well you glue and seal the PVC conduit there will always be water in the conduit ... I was pumping the draw string into the conduits ... every single pipe we installed (I personally made sure that every single coupling was glued properly) and I personally backfilled to make sure none of the pipes were covered with stones or any other sharp objects.

[AndyD]

Is the cable you refer to the coaxial with the additional red and black power wires? We used to call it Shotgun Cable and as far as I know the sheath and insulation are both PVC so it should be stable even if it's in wet conditions. Was it just the insulation on the red wire that deteriorated or did the entire cable suffer?

If it was just the power wires that were the problem I'd guess that prolonged current overloading would be high on my list of suspects. I've never seen buried PVC sheathed cables damaged by acidic soil conditions or other underground environmental conditions for that matter.