long runs swa

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went to a cable fault today the run was about 240 meters and looked like 25mm 3 core would that have been made in 1 length or jointed.
Trenching or cost, i assume, was not an issue at the time of install as it was a multi million pound development on waste land
 
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Is there a typo / something missing from this post? Doesn't seem to make much sense (to me, at least)
 
You can order some sizes up to around 1000M in one piece, what size this goes up to I don't know.
 
On construction jobs it was common to joint cables mainly as although you can get cable in long lengths, some one has to pull that cable in, and even with the rollers 25mm cable is heavy so you would need a cable gang to pull that length of cable in. So joints were common.

Also on construction sites cables often get damaged, you would not believe how many cable joints were used when we were building the airport on the Falkland Islands, at one point we ran out, and while waiting for the next ship to keep things running we used Denso tape, once the ship arrived we had to remake those joints, however one was missed, it was found 4 years latter, we were generating more power than was being used so we were looking for some redundant plant which was still connected and using power. Lucky is snowed over night and in the morning we found a patch of ground where the snow had melted. There it was one Denso tape joint.
 
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We once ordered via our materials purchasing department 100 metres of 6.6kv 3ph armoured cable for an offshore instalation. Can't remember the core size but the resultant cable would have made a bull elephant proud. Anyways, the manufacturer phoned us up and said they had 99metres in stock available for immediate shipment - "would that do us". Since we always overestimated length we said "yes". Cable arrives and job was completed. About a year later we received a 1metre length of the exact same cable as our learned materials procurement team insisted on the manufacturer meeting the full length requirement of the order! :)
 
Anyone dealt with these companies that locate underground shorted armoureds, how accurate could a fault be pinpointed, I have been told, that unless the cable is severed through it is hard to locate, but others say where the fault radiates out to earth, that can be located, Ive also heard of "thumpers" that make the ground vibrate, though told they can stress the cable.
I would have thought this day and age there might be some high tech gizmo
 
Anyone dealt with these companies that locate underground shorted armoureds, how accurate could a fault be pinpointed, I have been told, that unless the cable is severed through it is hard to locate, but others say where the fault radiates out to earth, that can be located, Ive also heard of "thumpers" that make the ground vibrate, though told they can stress the cable.
I would have thought this day and age there might be some high tech gizmo

I remember when I was called to a pumping station site which had lost all power, which turned out to be a DNO power failure. The DNO, UKPN, came out and supplied a generator for us. The site was fed from a UKPN 11KV fed into a UKPN substation (on our site) with a 11KV to 415V 800KVA transformer. With the generator hooked up to the transformers outgoing cables (into the pumping station), they then set about fault finding.

Firstly they did the equivalent of "meggering" the 11KV cable and found a short between phases. Then they got a "thumping box" which sent pulses of 11KV down the cables and monitored the cable route for vibrations. They also had a "pulsing" machine which sent an echo pulse and measured any reflections. The intensity and time of the reflection indicated the type and distance of fault. Both of these processes, and they excavated right onto the faulty section of cable.

I was pretty impressed.
 
sounds good, I hope i can get to be on site when they do this one
 
They also had a "pulsing" machine which sent an echo pulse and measured any reflections. The intensity and time of the reflection indicated the type and distance of fault.

Or more technically known as TDR... Time domain Reflectometer. Basically radio frequency signals don't just travel down the cable as simple on/off signals and arrive instantly at the other end, they almost travel down the cable as a wave and if a cable isn't terminated with the correct end of line resister (remember the green terminaters you used to get on 10base2 networking?) then the signal will bounce back from the incorrectly terminated cable and travel back to source and you get horrible effects and the data stops going through. With TDR this is put to use, if the characteristics of a cable are known, then you can inject a pulse and wait for it to come back rather similar to sonar.

It can often work better if what you are looking for is a dead short, or open circuit. That's where fault thumping comes in, basically keep banging fuses in until the fault blows clear and you have an open circuit.

A signal generator and CAT can be used to locate the route the cable takes, if you have this and the distance that the fault is from the end, then you known pretty much where to dig.

... Or you could use dowsing rods!!
 
Using TDR and a skilled operator will even show joints in a cable as the change in electrical characteristic due to the joint will also produce detectable reflections.
 
Using TDR and a skilled operator will even show joints in a cable as the change in electrical characteristic due to the joint will also produce detectable reflections.

It can even show a sharp bend - and there are minimum bending radii for Ethernet cables as a sharp bends cause reflections which can degrade the signal enough to introduce errors.
 
Lengths like that can be pulled in one length, but it could easily have been jointed at some point, either at install or due to damage later down the line. It's rare for a cable to go faulty, more than likely a joint.

TDR's are excellent, but you cannot just hire one - you need to hire SOMEONE! My local DNO use them and train a few lads on them, and they will do private contract work as well. I've used them once to locate a fault on a caravan park - Locating a failed Y joint. We were close, but still ended up with a 4-5 m trench over the cable to find the joint. Better than a 250m one though!

If you look at the cable, are the colours the same shade? No visible markings on the sheath to identify it? Same/different both ends? Look at the core rotation too - If it's got the same rotation both ends when looking down at the gland, it's jointed for sure. If they go different ways, it may or may not be jointed (if that makes sense).

Do you have plans? You know for sure the cable doesn't go elsewhere first? I say this, because a site sparky told us a cable from the plant room labelled sewerage went from the plant room to the sewerage treatment works. Same size both ends etc. When we isolated to swap the board at the other end, we killed a building half way along the run as well. Poor labelling and no plans!
 
Good point about the rotation, never thought of that, though as you say 50/50, there getting a specialist team in now, although the sign is our clients, the centre management own the land, apparently they have located drawings
 
No where near as good as a TDR, but you can get a rough idea of where a fault might be with a 12v lamp, battery and mutimeter.

If you have one core that has faulted to earth through a resistance (say red) and at least one good core (say yellow). Disconnect both cores both ends. Bridge them together at one end, at the other end, wire the long loop you have just made in series with the battery and the lamp. Put the meter in parallel with the long loop to get the volt drop across the whole run, Halve this to find the volt drop along just one of the cores. Now measure the voltage between red and earth, this will tell you how far along the red core the path to earth is. Repeat from the other end of the cable as a bit of a sanity check.

Its not got great accuracy, but it has its uses, if you have a leg of a ring that goes up a wall, above a ceiling and then comes down again, and IR is right down and the likelyhood is that the pesky joiner has screwed it when boarding the walls, you can be pretty certain which of the walls you need to smash holes in... that's of course if you care....
 

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