Any tricks for finding a break in an underground wire? - Page 1
Forum section: electronics / beginners

author: jerryk
author_rank: Regular Contributor
timestamp: September 03, 2021, 03:18:19 am
content: A friend has a buried run of 10/3 UF that is around 350 ft long. The wire worked fine but then he did some recent surface work with a small tracked loader and now the wire does not provide 220 VAC. Circuit at the panel never tripped. I tested the resistance between various pairs of the four wires and there is resistance in the range of 100 K to up to 2 M between all the combinations. I powered these wires with a 12V battery to see if all wires were broken. I found one pair that would send 10V. Sending 110 VAC down that same pair only produced 500mv AC. I suspect the break is not complete or clean.Are there any easy ways to find this kind break without a lot of exploratory digging?Jerry

author: bdunham7
author_rank: Super Contributor
timestamp: September 03, 2021, 03:26:30 am
content: I've not personally used TDR techniques on UF or NM type wires, but here's a tool that you could try:https://www.globaltestsupply.com/product/megger-1002-227-tdr500-3-handheld-time-domain-reflectometerThe only thing is that I don't know the velocity factor of UF cable, nor do we know what has happened at the point of damage, so some experimentation may be needed.How deep is the wire buried?

author: jerryk
author_rank: Regular Contributor
timestamp: September 03, 2021, 03:29:20 am
content: The wire is buried 2' max.Jerry

author: rstofer
author_rank: Super Contributor
timestamp: September 03, 2021, 03:35:30 am
content: The classic way to do it is with a Time Domain Reflectometer (TDR). You stuff a pulse into the wire and measure the round trip time until you see the reflection on the scope. You have to make some assumption for 'velocity of propagation' in you wire. Since you can get to both pieces measure both ways and ratio to get the location of the break. The equipment can be bought but it's very expensive. Instead, Google 'w2aew tdr' and you will turn up a couple of videos on how to build and use one for just a few bucks if you have a scope. You might guess something on the order of 1.2 ns/foot so a break 100' out will have a round trip distance of 200 feet and the reflection will turn up in 240 ns.There are cable locating services that can come out and trace the entire route. They usually do the survey work for utility companies before folks start to dig up the streets. Maybe you can hire one. Google or the Yellow Pages, I suppose.

author: bdunham7
author_rank: Super Contributor
timestamp: September 03, 2021, 03:50:12 am
content: Quote from: jerryk on September 03, 2021, 03:29:20 amIt doesn't matter for TDR methods, but if it is that shallow and you want to use a homegrown method, you might be able to put an RF signal into it and detect it with a wire loop or ferrite core antenna like an AM radio.Also, do any of the wires have any continuity to earth? If so, you can run a cable topside and spike it into the ground at 10 foot intervals, measuring the resistance between that cable and your broken one looking for a minima. One more thing to worry about--is there more than one point of damage? TDR might be most helpful there.

author: alexnoot
author_rank: Contributor
timestamp: September 03, 2021, 04:15:18 am
content: I've worked a lot on damaged underground cables/wires. We use aMegger cable thumper, and they're awesome at finding faults. It's basically a megger on steroids, with a large HV capacitor bank that dumps a lot of energy at once. That gives you a spark at the fault that you often can hear just by listening carefully, otherwise you use the ground microphone unit.It only costs like $10.000I'm probably not helping...

author: bob91343
author_rank: Super Contributor
timestamp: September 03, 2021, 04:33:12 am
content: I was going to suggest using a nanoVNA, a $50 item that has many uses, including TDR. In fact it may be possible to find multiple breaks.If starting from scratch, the velocity factor can be measured while the wire is still intact, and thus finding the break should be simple.

author: james_s
author_rank: Super Contributor
timestamp: September 03, 2021, 04:37:10 am
content: It would probably not be too hard to approximate a tool like that cable thumper. Just find a suitable HV capacitor and connect it to a current limited HV circuit. The voltage will increase until the break in the wire breaks down and discharges the capacitor. A TDR seems safer though, if you have an oscilloscope it's pretty trivial to build.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 03, 2021, 05:08:54 am
content: You are making a hopeful assumption - that the break in all four wires is at the same place. If your friend was all over the place with his equipment that assumption is in doubt. If he hasn't the location of his tracks probably will give you as good a location as you will get with jerry rigged TLDR.Back to the worst case, if four different places in the run have been damaged badly enough to cause an open do you really want to repair those four places and carry on? Your best bet may be to hire a trencher, dig a new trench (or dig along the general route of the old one) and lay down some new cable. Based on the history you probably would want to go deeper than two feet, and/or put in a protective conduit. Don't know what code says in your location, but it might actually be a requirement. In my area you are deep enough, but direct burial is frowned upon.

author: gbaddeley
author_rank: Regular Contributor
timestamp: September 03, 2021, 07:22:55 am
content: You can do minimal explorative digging using binary chop. Dig half way and test to each end. Dig half way along the bad section and do again. Repeat. If you know where the vehicle crossed you can make more intelligent digging choices. The down side is the need to expose the conductors at each dig, and do water proof repairs.It the long run, it will be economic and safer to put the cable in suitable conduit. Eg. 1 inch UG rated pvc pipe.

author: richard.cs
author_rank: Super Contributor
timestamp: September 03, 2021, 08:21:43 am
content: UF cable is this kind of stuff?https://www.wireandcableyourway.com/uf-b-wire-direct-burialSo basically a normal flat cable with a nylon jacket instead of PVC? I'm not exactly surprised that it doesn't survive ground movement from vehicles crossing. It's probably pulled apart in tension somewhere that the ground has sunk, possibly with the jacket just stretching.What's the resistance between the bare ground core and the dirt outside look like with both ends disconnected? How about the other conductors? If the sheath has failed then you could find it by injecting some current and looking for step potentials.In the UK we'd normally use something like this for direct burial:https://www.cef.co.uk/catalogue/categories/cables-and-accessories-swa-xlpe-pvc-armoured-cables

author: RoGeorge
author_rank: Super Contributor
timestamp: September 03, 2021, 08:45:04 am
content: Try this#88: Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger InverterYou can do first from one end of the wires, than the other end, and the ratio of the delays should tell where the defect is.

author: nfmax
author_rank: Super Contributor
timestamp: September 03, 2021, 09:49:24 am
content: If you have a capacitance meter, or even a DVM with a suitable capacitance range, try measuring the capacitance between each core and both other cores shorted together from each end of the cable. The ratio of capacitances from each end should give you the ratio of connected cable lengths. Comparing the ratio measured for all 3 cores may indicate if you have a single break or multiple breaks. This technique can't tell you if you have multiple breaks though.OTOH I wouldn't trust the rest of cable anyway, even if you fix the break - it needs to be replaced!

author: rstofer
author_rank: Super Contributor
timestamp: September 03, 2021, 03:46:18 pm
content: If you can find the break and if there is a way to patch it, you should still plan to megger the individual strands between each other and also to ground. A low resistance reading and the entire project is doubtful.As to the patch, this usually takes two junctions because the strands won't be long enough to rejoin. They better not be! There are epoxy splice kits for doing this kind of thing, 3M makes them. They are suitable for direct burial but the affected area around the splice needs to be clean.https://www.3m.com/3M/en_US/p/d/b00014996/Don't forget the megger test!Here's one approach, the Internet has many pages about this kind of testing:https://professional-electrician.com/technical/insulation-resistance-low-voltage-circuits/

author: james_s
author_rank: Super Contributor
timestamp: September 03, 2021, 05:07:02 pm
content: Quote from: CatalinaWOW on September 03, 2021, 05:08:54 amI think it's not entirely unlikely that the break is in one location, depending on the amount of rocks in the soil. Certainly if it turns out to be multiple breaks then it's not going to be worth trying to repair and even a single break is difficult to properly repair, underground splices are not particularly cheap. I've seen UF cable used quite a few times and for short runs like going from a house to a nearby shed it works fine, but long runs are quite failure prone. If you are in a region where there are hardly any sharp rocks it's a reasonable option, but the soil on my property is chock full of rocks so I would always use conduit here. 18" is the required minimum depth here but I don't think there's anything that says you can't go deeper.

author: jerryk
author_rank: Regular Contributor
timestamp: September 03, 2021, 05:52:48 pm
content: Currently I'm exploring the TDR approach but am falling short on the key component 74AC14. I have plenty of 74HC14 around but am I to assume the HC is too slow on rise time?Things that I have tried are sending 60 HZ down the wire and using an AM radio to locate the wire. This worked amazingly well, however, the expectation was the break would show by a defined change in the radio noise at the point of the break. I suspect we are narrowing down the area of the break but because the depth of burial varies and the fact that I'm certain the break is not a crisp clean one the AM radio method although helpful is too vague and serves as an approximation.Hand digging is not fun in this soil so a precise location is needed.Another thought I had was to send a very high voltage down the wire which seems inline with some of the above suggestions. I have a Cockcroft Walton multiplier circuit I made from an old laser printer that I may hook up and see if I can detect any radio noise at the break.Thanks for all the ideas.Jerry

author: bdunham7
author_rank: Super Contributor
timestamp: September 03, 2021, 06:04:43 pm
content: Quote from: jerryk on September 03, 2021, 05:52:48 pmI seriously doubt rise time is an issue, but power will be. You need as much as possible to get a reflection (I don't know how much) because UF is not coax. The rise time isn't so important because the range and uncertainty about the velocity factor will introduce errors much larger than a few nanoseconds. The reflections will be many, many nanoseconds from the original signal and probably a lot lower in magnitude.QuoteCrisp and clean may not describe the damage! Plain 60Hz would be tough to pinpoint, I would try 1MHz if you have anything with enough power. That may be tough to come by.QuoteUnfortunately, the same reason hand digging is not fun may be why your cable is more mangled than a single fault.

author: james_s
author_rank: Super Contributor
timestamp: September 03, 2021, 06:05:22 pm
content: Well you can get the 74AC14 in a few days for just a few dollars from DigiKey with USPS first class postage, they have no minimum order and postage is cheap for small items. You could also build the Carlson design which uses a rather ordinary transistor. It's a handy tool to have around, I use mine (Carlson) for determining the amount of cable left on a spool/box.I would be tempted to short together all the wires at the far end and then connect something like a neon sign transformer across a couple of the wires at one end and see where it smokes, though the smoke may not rise out of the ground. An AM radio should also pick up the arcing pretty easily though.

author: rstofer
author_rank: Super Contributor
timestamp: September 03, 2021, 06:18:34 pm
content: Quote from: jerryk on September 03, 2021, 05:52:48 pmIf the fault is 290 feet out, the return pulse is back in 696 ns. To get within 3 feet implies being able to resolve about 7 ns on a 700 ns pulse. I don't know how accurately you can place the cursors but I wouldn't bet on getting within a few feet either way. so plan on having to open 10 feet or so. And that assumes the scope horizontal sweep is accurate to those levels. That's why real TDRs use a high speed timer.A DitchWitch will make short work out of opening the dirt. It's customary to put a layer of sand over all buried pipes to warn the digger about buried facilities. Of course, the DitchWitch will make a mess out of buried sprinkler piping and it would pay to know where other buried utilities (like electrical) are located. Gas pipes might be a concern.

author: rstofer
author_rank: Super Contributor
timestamp: September 03, 2021, 06:33:01 pm
content: Is the cable protected by a GFCI circuit breaker? It probably should be. If is isn't, add one. The GFCI will trip if there is any remaining fault after the 'repair'.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 03, 2021, 06:35:41 pm
content: never tried TDR on non-coax, but my guess would be that it should workthe difference in rise time between HC and AC chips is neglectable; to get some kind of power on the wire, I'd bundle more gates together; normally for 50ohms a save bet are 5 gates in parallel joined with 240ohms resistors; at let's say 1MHz I don't see a problem bundling the gates of 2 or 3 74HCs together to get far lower than 50ohms

author: coppercone2
author_rank: Super Contributor
timestamp: September 03, 2021, 07:36:17 pm
content: if you make it slow, that HV thumper is not too hard to make, it would just be harder to usebasically really thick dielectric sealed box with some relays on a timer and a HV power supply

author: BrokenYugo
author_rank: Super Contributor
timestamp: September 03, 2021, 08:01:05 pm
content: While UF is the cheap and nasty way to do it this is still a ~$600 copper cable (plus digging) so I can understand wanting to repair it.I like the idea of measuring the capacitance between wires, that might get quite close if you can get picofarad resolution.

author: sokoloff
author_rank: Super Contributor
timestamp: September 03, 2021, 08:13:06 pm
content: Many mid-range network cable testers will have a TDR built-in that will report (pretty accurate) results to the foot resolution.I have this one and have used it the TDR successfully on sprinkler wires (to help sort out which run was which).https://www.aliexpress.com/item/32801859627.html?spm=a2g0s.9042311.0.0.27424c4dXXeTLg

author: GeoTDR
author_rank: Newbie
timestamp: January 27, 2022, 01:44:23 pm
content: I have been using TDR to interrogate buried cables for over 40 years. Solid aluminum coaxial cables are grouted into boreholes. When the ground moves (landslides, subsidence. etc.) the cables are deformed and it is possible to measure the location and magnitude of cable deformation.

author: CaptDon
author_rank: Super Contributor
timestamp: January 27, 2022, 05:57:09 pm
content: First off, don't go 'thumping' the cable with high voltage capacitive energy. The cable is probably rated at 600vac and you don't want to do arc-over damage to the cable's insulation in unknown or undetected places as you will set up a new failure mode. The next thing is let's start simple, Totally disconnect both ends of the cable, have your friend ground one lead at a time. Start with red for instance. He grounds it at the far end with a piece of wire to any cooper water pipe or ground rod (B.T.W. there should be a ground rod at the sub-panel far end!!!) Then you measure for continuity at your end by placing one of your meter probes on your local ground and the other lead on the red conductor. You should see very low resistance if that conductor is intact. Do this also for the white, black and even the bare ground conductor. At this point you should have determined how many conductors are failed. If you are lucky enough to have at least one good intact conductor you could then measure the 'capacitance' of the bad conductor to a good conductor at each end and look at the ratio. Lets take a really simple example, you say your conductor is 350ft. As example if you measured 1000pf on one end and measured 2500pf on the other end your break would be 'approximately' 100 feet from the 1000pf end or 250 feet from the 2500pf end. If the cable has been ripped open and the conductors are exposed to ground/dirt/dampness/water it will be more difficult. B.T.W., the 74ACXXX series of parts are a pain in the ass and very touchy due to their speed. You can accomplish a makeshift TDR device with a simple function generator producing a squarewave, a bit of series resistance perhaps 100 ohms and a decent triggered sweep scope. The function generator only needs to produce a signal at around 10 to maybe 100hz. Trigger the scope on the rising edge coming out of the generator (use scope ch1) then look for the bump along the waveform on the end of the resistor hooked to the cable (scope ch2). Simple example, if the bump is 100ns from one end of the cable and 250ns from the other end then your break would be 100ft from the 100ns end or 250 feet from the 250ns end. You don't need to know velocity factor if you measure from each end. The break will simply be the ratio of the numbers. Hope this helps!!

author: richnormand
author_rank: Unknown
timestamp: January 27, 2022, 07:42:36 pm
content: Similar issue about 20 years ago between the house and the garage at my parent's place.Made my own TDR with TTL (as suggested previously) and it worked well. However do not forget to compensate for effective velocity in cables.Other issue to be aware of is water ingress. At the break site humidity and water can penetrate a long way in the cable and cause problems later.To get an idea on the path of the broken cable I used a transistor radio on the AM band and injected a weak signal modulated at 2kHz on an unused portion of the band.Alternatively did that from each ends and each conductor. Fortunately the cable was not enclosed in a metallic conduit.Good luck with it.

author: SmallCog
author_rank: Frequent Contributor
timestamp: January 27, 2022, 09:02:12 pm
content: Quote from: GeoTDR on January 27, 2022, 01:44:23 pmAnd I've grouted those sorts of cables in, in a past life with more calloused handsI know this is the expensive option, but it sounds like this power cable is not only damaged but also a bit dodgy in its installation and will be prone to similar damage in the future too.Dig a new trench (hire a chain trencher they're not that expensive) dig it deeper than required and either use sand or conduit to protect the new cable. Personally I'd use both and I'd oversize the conduit to make pulling easier.Subject to local codes/standards/regs, whilst the trench is open think about what else you may need to run between those locations. At the minimum I'd be putting some irrigation pipe in there and capping the ends.

author: jerryk
author_rank: Regular Contributor
timestamp: January 28, 2022, 01:34:46 am
content: I appreciate the additional new replies. I built a TDR using @w2aew's video#88 as a guide. It was a good learning exercise but could not get near the perfect peaks in the wave form for measuring the reflection timing that he demonstrated in his video. It was good for an approximation but since we will be hand digging I'm hoping to get close to the break. I also purchased the TDR mentioned by @sokoloff. It seemed reasonably accurate on all of the test pieces that I measured. The ratio of capacitance, as simple of an idea as it is, also seemed fairly accurate.The sad part was by the time I received the TDR and got my stuff together winter came to Alaska with a vengeance. We are still going to try to locate the break this spring when things thaw. I will sure post what happens as I don't take the help I get here for granted.Thanks - Jerry

author: m3vuv
author_rank: Super Contributor
timestamp: January 28, 2022, 01:46:47 am
content: What is uf cable,is it an american thing?

author: jerryk
author_rank: Regular Contributor
timestamp: January 28, 2022, 02:05:48 am
content: It's a direct burial cable. UF stands for Underground Feeder. It's typically used to bring power indoors to outdoor locations like a remote light, electric gate, small out buildings, etc. The one I'm trying to fix supplies electricity to a remote fueling tank for an airplane.As for American thing, I would not know about that.Jerry

author: cdev
author_rank: Super Contributor
timestamp: January 28, 2022, 02:28:21 am
content: I know you have a TDR, but not knowing how its display works, I thought I would toss this in. With the right transformer connecting it to the transmission line, A cheap nanovna is probably a better TDR than an official TDR unless that TDR is a really expensive device.Is there any reason a nanovna (1 or 2) could not be used? Sure, youre not measuring a coax, but I suspect that if a 1:1 balun was made and used the TDR function would work perfectly with any two conductor power cable.A two ware transmission line made with speaker wire is around 100 ohms. You could probably estimate the impedance using a program called atlc, and build your transformer accordingly. (in terms of turns ratio) I would make a1:1 or 1:2 transformer and try that first. That becomes your calibration plane. Why not?Quote from: bdunham7 on September 03, 2021, 03:26:30 am

author: m3vuv
author_rank: Super Contributor
timestamp: January 28, 2022, 02:29:25 am
content: sounds like a shitty idea to bury a cable if it isnt armoured or in a conduit/ducting,you would get big trouble if you did that here in the uk!!.

author: MarkF
author_rank: Super Contributor
timestamp: January 28, 2022, 04:19:24 am
content: JerryK,Here is another video byw2aewexplaining the TDR measurement technique.Near the end of the video he measures speaker wire (which would be similar to your case) with a different velocity factor and possibly a larger resistor. It will make a big difference in the calculated length to the break.You might want to test the calculations with some extra of the same cable type that you can measure the length to verify your calculations.Also, you probably want to use two adjacent wires when testing since this technique requires two wire pairs or coax.

author: GeoTDR
author_rank: Newbie
timestamp: January 28, 2022, 12:52:24 pm
content: Is it possible for a splice in a coaxial cable to modify the impedance of the cable downstream of the splice

author: coppercone2
author_rank: Super Contributor
timestamp: January 30, 2022, 06:56:48 pm
content: this might be useful for those that want to make more advanced projects for cable locating (commercial interests)§ 15.213 Cable locating equipment.An intentional radiator used as cable locating equipment, as defined in § 15.3(d), may be operated on any frequency within the band 9-490 kHz, subject to the following limits: Within the frequency band 9 kHz, up to, but not including, 45 kHz, the peak output power from the cable locating equipment shall not exceed 10 watts; and, within the frequency band 45 kHz to 490 kHz, the peak output power from the cable locating equipment shall not exceed one watt. If provisions are made for connection of the cable locating equipment to the AC power lines, the conducted limits in § 15.207 also apply to this equipment.