Electronics > Beginners
Locating a failed buried wire using a TDR
Doctorandus_P:
It's common to have splits in underground cables.
They don't run separate cables from each house to the power company.
Usually these splits (or repairs) involve epoxy.
Whether a 25 year old cable should be replaced is another question.
I'm certain though that undergound cables in the city where I live do not all get replaced every 25 years.
Locating the fault seems a logic first step. If the fault is caused by some external event or by simple rot of the cable could make clear what the next step should be.
Just curious: Does anybody know what the life expectancy is of underground power cables?
Gyro:
--- Quote ---Just curious: Does anybody know what the life expectancy is of underground power cables?
--- End quote ---
In our street, about 15 years! ::)
The cable is plastic outer coated, with Aluminium sheath inside and 3 phase Paper insulated cores inside that. The house builders weren't careful about handling and bend radius when they installed it. This lead to cracking and water ingress. We've been suffering power outages once or twice a year since the first failures (at about 15 years).
The electricity company comes out each time, replaces the substation fuse, then, when the fault gets more permanent, fit an automatic re-closer/recorder until it becomes a hard fault. Then they use a combination of TDR and sniff test to locate the position (our pavements now have rows of little holes in the tarmac). Then they dig down, splice in a new length of a few feet with epoxy jointing boxes at each end, leave it for a week, and then refill to hole and re-tarmac.
The whole thing should have been replaced years ago but that would involve massive disruption and reconnection of the tails (presumably also paper insulated) to each house!
P.S. We had two faults outside our house alone - they flexed one of the ends of the old cable a bit too much when splicing and it failed a metre further along about a year later. At least we have a very neat section of tarmac pavement outside.
soldar:
You might spend a lot of time and effort to patch here and a short time later it fails again elsewhere. With a little labor and not much money you can do a really good job which will allow repair and upgrades.
What I would do is bury conduit which is larger than needed. That allows for additional cables, repairs etc.
No bends. Straight and ending in a register well which can be brick, masonry or just a plastic box like used in sprinkler systems. From there another conduit to wherever. That way you will never have a cable stuck. And I speak from experience. In Spain they always put conduit which is too narrow and has too many curves and most of the time you cannot introduce another cable or even get out the original one. It is really stupid.
If you bury conduit with upwards bends at the ends chances are water will accumulate in there.
While you are at it you might want to install two conduits so you can separate power from data. This might be required by code but it is a good idea anyway.
It is not complicated and if you do it right you will save yourself a lot of aggravation in the future.
chris_leyson:
Over a few weeks some gas contractors have been digging up a street near me. All over the pavement on both sides of the street they've marked LV and HV. LV markings are for the most part along the pavement within a few feet of property boundaries, whereas HV markings are all over the shop, they're under the pavement or sometimes in the street. They're just saying to the contractors "don't dig here" maybe they got that from local council plans or maybe they got it from a guy sweeping the area with a detector.
Stray Electron:
--- Quote from: David Hess on April 02, 2019, 10:21:00 am ---
I agree with Muttley that simply finding the fault will not be helpful because the damage is likely to require complete replacement.
--- End quote ---
I agree. If the circuit has failed twice already, the rest wouldn't be far behind. In addition, code does not allow you to splice wires outside of a junction box, it has to be one continues cable. And trust me, I've had to replace underground wiring that was spliced underground without a sealed box, it doesn't last long!
I know it's painful but I think your best bet is to just replace the entire cable. And I wouldn't use aluminium wire underground.
Youtube has a couple of good videos on using a TDR (a simple scope and a home made pulse generator) and you can find the electrical velocity by measuring the remaining good conductors. You can also measure the broken conductors from each end to get a more accurate measurement of where the open circuits are. It might not give you the exact distance but it will be a lot closer than just having to dig up the entire cable.
FYI one of my buddies uses an AM radio to locate broken underground cables and it works for him. He doesn't even connect any kind of source to the cable, he just listens to the static as he walks the length of the cable and he can hear the difference when he gets to the break. He tells me that he's even located broken water pipes that way.
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