How to test cable for broken strands?

[artakami@yahoo.com]

Hello.
I had a problem with cable with broken strands. It took me a week to find that out.
Is there any way of finding broken strands in cable before it will break completely?

[Kiriakos-GR]

What is the diameter of the cable ?

[artakami@yahoo.com]

1.5mm²

[Kiriakos-GR]

Ok

1.5mm2 + strands = a vehicle is near by

Well if you tell us the all story it could be best, plus the length of the cable ?  plus the model and brand of the multimeter that you have near by ? 

[PetrosA]

Kiriakos is right, we need more information and it would also be helpful to know what kind of damage is likely - i.e. was the previous damage a manufacturing defect, at the end or in the middle of the wire, corrosion, abrasion, stress, overload condition, at a crimped joint, etc. and do you have access to the cable to do a visual inspection?

[Jimmy]

If you need to test it properly you will need to load the cable up to near maximum current then use a thermal imaging camera to see if there is a hot spot or use laser temp gun.

[robrenz]

If you have, or have access to a mili/micro Ohm meter or a LCR meter you can try this.
If you have more of the same cable cut a piece the exact same length and measure its resistance and compare it to the suspect cable. If not calculate the nominal resistance that your length of cable should be using a table that has the resistance for the exact construction or your cable (number of strands of what size).  Measure the resistance of your cable and if possible bend the cable along the entire length while measuring to see if the resistance changes significantly (attempting to get broken strands to separate and loose contact) If the actual cable resistance is a lot lower than the calculated nominal you know there is probably a problem.  All that said, making consistent repeatable sub Ohm matching measurements  (not accuracy) requires very good technique.

[artakami@yahoo.com]

The cable I had problem with is 1.8m long multicore control cable (semiliar to http://uk.rs-online.com/web/p/actuator-sensor-cable/0144894/ ) has been used for PLC signalling and 2 of cores used to activate solenoids. the problems were with one of solenoid cores. Cant say its design error because there is 20 identical capsules in ride with identical solenoids and identical cables and that was only problem with that cable. BTW its been running over 10 years. the environment is close, air conditioned (+20C) no direct sunlight or any other negative effect.
The cable has little movements in some places, and it was broken on one of them.
It could be a manufacturing defect, but because it is more then 10 years old and smooth running of ride is critical I would like to make sure that it will not happen again.

If any additional information needed, will be happy to share.

[artakami@yahoo.com]

If you have, or have access to a mili/micro Ohm meter or a LCR meter you can try this.
If you have more of the same cable cut a piece the exact same length and measure its resistance and compare it to the suspect cable. If not calculate the nominal resistance that your length of cable should be using a table that has the resistance for the exact construction or your cable (number of strands of what size).  Measure the resistance of your cable and if possible bend the cable along the entire length while measuring to see if the resistance changes significantly (attempting to get broken strands to separate and loose contact) If the actual cable resistance is a lot lower than the calculated nominal you know there is probably a problem.  All that said, making consistent repeatable sub Ohm matching measurements  (not accuracy) requires very good technique.

Unfortunately I dont have a LCR meter. The only measuring device I have is Fluke 117 and old oscope. But have loads of cable to cut to same size and play with. Dont know can I do something with multimeter oscope and long cable?

[Uncle Vernon]

If the break is substantial a cable tracer (growler) may locate your fault, but you'd have to disconnect all the cores to test first.

A cable that length it's going to be cheaper to order a new piece and fit it than the time taken to test and try and affect a partial repair.

[Kiriakos-GR]

Hi artakami I think that I have the solution for your problem,
a) Use your multimeter to measure the current that this solenoid cores need.
b) Measure the voltage on the contacts of a properly working solenoid , when is armed. 
c) Get a more advance DMM for such troubleshooting with Data logging.

And when ever you face again the same issue or suspect  that you have the same issue,
you will have at hand the electrical data about Volts and Ampere that those solenoid cores need.
And all that you need to do, is to have the data logging multimeter connected in parallel with the solenoid,
so to record any voltage instability, by having it to take a sample at the rate of 1/Second.

You can also use the "Min Max Average"  function, so to detect voltage instability, by paying more attention to the min values that would cause instability at the  solenoid core.

The multicore control cable its an difficult subject,  for example if you get bigger diameter you loose mechanical flexibility, if you have small diameter nothing will work.
Good luck.


 

         

[PetrosA]

Did you do an autopsy of the break? It sounds like the environment isn't harsh enough to have contributed, so I would look for other causes - namely a manufacturing defect. If you slice open the cable jacket where the break is, you should get a clearer picture of how the break happened. I wouldn't be surprised if you find that the wire was spliced near the point it broke and that the stiffness of the splice stressed the strands on either side of it during normal motion leading to failure. It could also have been a kink in that wire which lead to a stress break. Either way, you should be able to see the cause after cutting the wire open. If your budget allows, the best thing would be to check the remaining cables on a regular basis with an IR camera since it will show you a hot spot as the fault develops.

[firewalker]

I believe I had read a thesis about this issue. They was using a high frequency pulse (signals love to travel on the surface of the conductor, that why the antenna guides on tv sets are usually hollow) through the cable. When the pulse met an "anomaly" of the cable there was an echo back to the source. Measuring times, bla bla bla, they could spot the location of the problem.

They was testing long distance cables though.

[alm]

What you're describing is TDR, time domain reflectometry, but I'm not sure if some broken strands will show up as a large enough change in impedance, and you need more than a DMM and an old scope for it. At least a pulse generator with a fast edge rate.

[Neilm]

I know that there are several commercial devices that can allow you to measure the impedance of cables looking for changes along the length, but the ones I have seen tend to be for network cables in long runs (>10m)

A TDR instrument would have problems working over only a couple of meters as the edge of the pulse would have to be very sharp and the pulse very short in order for the electronics to be able detect the break - and that is before you take into account the change in impedance due to the connection from the instrument to the cable.

Neil

[Mechatrommer]

how much is this thermal imaging camera? or that  cable tracer cost? how about replacing the 1.8m long cable with a brand new good brand one? rated for many years.

[IanB]

'Tis true. One of the best ways to prevent failure of critical components is a program of automatic replacement at regular intervals. This will be very familiar to car owners as "scheduled maintenance", wherein such components as spark plugs, timing belts, fluids, oil filters, air filters and the like are replaced as indicated, whether they look like they need it or not.

[sonicj]

what kind of ride is it?

[Time]

I made a device that did just this when I was in gradschool.  It was a tool to be used in the lab.  Kind of sad someone made a thesis out of it.

I believe I had read a thesis about this issue. They was using a high frequency pulse (signals love to travel on the surface of the conductor, that why the antenna guides on tv sets are usually hollow) through the cable. When the pulse met an "anomaly" of the cable there was an echo back to the source. Measuring times, bla bla bla, they could spot the location of the problem.

They was testing long distance cables though.

[ejeffrey]

how much is this thermal imaging camera? or that  cable tracer cost? how about replacing the 1.8m long cable with a brand new good brand one? rated for many years.

I regularly use 2 meter cable assemblies that cost > EUR 200, and in some cases quite a bit more.

Even if the cable is cheap, it all depends on what the cost and source of failure is.  If the failure is due to slow degradation with time, then periodic replacement may be the most economical option.  However, if the failure is due to a single catastrophic event such as running over the cable with a caster wheel, mishandling during installation, or factory defects then replacement may not provide as much benefit as you hope.  Preventative maintenance has its own costs above and beyond the cost of materials -- most notably, maintenance itself is a big cause of failure, so even with a program of regular replacement it is quite useful to have something like a thermal camera that can verify correct operation after doing the replacement.

[Alex]

maintenance itself is a big cause of failure

I second that. Isn't it in aviation where some parts are not touched unless they fail?

[Neilm]

maintenance itself is a big cause of failure

I second that. Isn't it in aviation where some parts are not touched unless they fail?

Also the F1 scene. Many years ago, the cars were taken to bits between qualifying and the race and "inspected". Usually about 25% of the field still failed to finish. Now, they can't touch the car between qualifying and the race and you often get all cars finishing.

Neil

[ejeffrey]

maintenance itself is a big cause of failure

I second that. Isn't it in aviation where some parts are not touched unless they fail?

"If it ain't broke, don't fix it" is more than a clever saying.  A completely anecdotal story: At some point I had the oil in my car changed (at an oil change place -- not DIY).  I then proceeded to drive 400 miles before noticing at a gas station that my hood was not quite closed.  I opened it up to find that the reason was that the oil cap was wedged between the engine and the hood, and there was oil spray everywhere from the uncapped fill spout.  Luckily for me the cap was still there and the design of this particular engine was such that I didn't loose too much oil.  I topped it up and was on my way.  It could easily have been a lot more annoying.

Obviously this is a fluke occurrence, and the majority of oil changes are completely problem free.  Likewise, some periodic maintenance is necessary.  There is still always a risk that if you do something 99 times you will screw up the 100th time.  Zero maintenance is the ideal situation.  Maintenance that can be done by non-invasive monitoring is second best, and anything that requires disassembly should be your last choice.  Of course if you have to disassemble something anyway replacing any parts that might be subject to wear may be a good idea -- as long as the factory defect rate is negligible.  If there is no expectation of degradation over time or you expect a certain level of DOA parts you may still prefer a known good part to a new replacement.

[vk6zgo]

I doubt if anything short of an x-ray is going to find broken strands if there are still some conducting strands left,
The thing is,even if you only have one strand left,the resistance change from a few millimetres of this single strand is going to be so small,as to be swamped by normal manufacturing tolerances.
All the other methods,apart from the temperature sensor idea (& I doubt that will work,either),only work if you have a complete break in the cable.
The only methods that will work for cables with only a few strands left, are destructive methods,such as wiggling the cable till it goes open,or the method used by power cable earth tests ,which discharge a large capacitor between the two ends of the earth lead.
If only one strand is left,for instance,the current pulse with cause it to become open circuit.

VK6ZGO

[Jimmy]

Thermal cameras will pick up the cable break if the cable is under enough load and can be done while it is operating

http://www.goinfrared.com/success/ir_image/1097/industry_id/1026/

[vk6zgo]

Thermal cameras will pick up the cable break if the cable is under enough load and can be done while it is operating

http://www.goinfrared.com/success/ir_image/1097/industry_id/1026/

If the circuit continuously draws enough current,as you noted above.
Many cables only have current flowing when a function is performed,& are idle at other times.
If the Thermal camera method works in this case,it ,& x-ray are the only non-destructive methods that will.

VK6ZGO

[vk6zgo]

maintenance itself is a big cause of failure

I second that. Isn't it in aviation where some parts are not touched unless they fail?

I'm more than a little doubtful about this.
If you wait for a part to fail in aviation,it might be hard to fix it when the aircraft is in a thousand little pieces spread over some farmer's back paddock,or at the bottom of the ocean.

VK6ZGO

[Kiriakos-GR]

the problems were with one of solenoid cores

I would expect to read more practical approaches, about the specific problem.
That is a solenoid core who operates at 12V or 24V, and needs almost 1A ampere so to arm it self.

One primitive and brave electrician    , he would replace the solenoid in a such of a problem,
and if this had no effect, he would replace the cable.

There is no actual need to ever test a cable for broken strands.

It is the fate of the control cables to die eventually due the stress conditions that they face.
Another example of where such control cables gets stressed at it max, are in the control boxes of huge movable cranes inside of a factory.
The worker gets the control box of the movable crane at hand  ( the cable is 8-9 meters long and is connected  with an crane of  11 meters high),
The in-door crane moves on metal rails that are on the side wall of the large building.

In order to protect those control cables from the adequate pulling, that causes the distraction of them, we use " steel wire " that connects the crane with the portable control box, in parallel with the control cable, so to be stressed the " steel wire " instead of the control cable.

The steel wire haves the tendency to extend it length, and need few readjustments until to stabilize.
No one does those fine adjustments,  the workers usually do not report anything at the maintenance department until something really stops working,  and eventually the control cable stays unprotected, and dies. 

In the last factory that I was working, he had  14 of those .
 



   

[phil_jp1]

I believe I had read a thesis about this issue. They was using a high frequency pulse (signals love to travel on the surface of the conductor, that why the antenna guides on tv sets are usually hollow) through the cable. When the pulse met an "anomaly" of the cable there was an echo back to the source. Measuring times, bla bla bla, they could spot the location of the problem.

They was testing long distance cables though.

Yeah, when I was studying in university, we used that technique to localize place where cable was broken. But for such a short cables like this it must be very expensive equipment to measure such a short time between pulses.

It's just my theory, but why don't you try to measure cables resistance? If it would be some broken strands, resistance of that cable should increase.
In order to measure resistance of that cable you can use 4-wire resistance measurement method. And on top of that you don't need to measure an absolute resistance. Instead you need to measure relative cables resistance - which makes it much easier.

You would need some, let's say 12V(5V, 24V, etc.) power supply and power resistor (let's say 10 Ohm 20W for 12V power supply).
Connect all that stuff in series with cable and then measure voltage drop across that cable. That way you will get relative cable resistance readings, using pretty much any multimeter, that has mV scale.

More voltage drop across the cable - the more resistance it has. Big voltage drop = big resistance = broken strands is found!

[alm]

I'm not sure if the extra resistance from the very short distance with less strands is going to be significant compared to variations due to cable length and connectors. The problem with broken strands is that the increase in resistance is very local, so the dissipation heats up a very small part of the cable. Not sure how to test this without a large amount of current and a thermal imager.

[phil_jp1]

I'm not sure if the extra resistance from the very short distance with less strands is going to be significant compared to variations due to cable length and connectors. The problem with broken strands is that the increase in resistance is very local, so the dissipation heats up a very small part of the cable. Not sure how to test this without a large amount of current and a thermal imager.

That's why I said, that it's just a theory.

[artakami@yahoo.com]

what kind of ride is it?

Its slow moving ride 200mm/s, nothing like a roller coaster. But smooth and continues running is critical as in any attraction business.

[artakami@yahoo.com]

Sorry for leaving abandoned the topic. Ive been away.

Ive found this page http://www.cirris.com/testing/resistance/broken_strands-2.html
It looks they have done lab research on subject.
There is described another method as well (http://www.cirris.com/testing/resistance/broken_strands.html)  but, as they say, not very reliable results.

Now I need to persuade management to buy a Thermal Imaging Camera (for around £2.500 (almost unimaginable))

[amspire]

I think the most likely simple method would be to set up a machine to measure the elasticity of the copper wire in the cable. So you clamp the probe at one end and the banana plug at the other somehow, apply a force and measure the stretch. For this to work, it would need the cables to be made in a specific way. If a solder termination at one end was done at a slight angle, that give some extra stretch and confuse the measurement. This method would probably require a force that can easily break strands, as the stress may not be equal on all strands. It wouldn't work on any Kevlar reinforced cable.

Other then that, the only other likely methods I can think of that might work would be to put some RF or fast rise time pulses through the cable, and somehow probe the cable for anomalies like small localized amounts of ringing. Sounds like something that could take a lot of research.

If a good enough test jig can be made, it may be possible to send a 10 to 50 picosecond pulse down the cable and look at reflected signals. A broken strand would show up as a small anomaly in the reflected waveform, and from the delay, you can identify the position. You would be needing a scope with a bandwidth of over 20GHz but it doesn't have to be a real time scope. A Sampling Scope that has an equivalent sampling rate of over 50Ghz would probably be OK, but even better may be needed. If it worked, it would be very expensive. A big problem is the more strands, the harder it will be to see any broken wire reflection on top of the background noise and good quality multimeter cable has lots of strands. An extremely difficult test rig to build and to make work.

Detecting broken strands is a common problem in steel cables, but they are able to use Magnetic Flux Leakage detection which unfortunately only works with materials that can be magnetized. If they made special wires for instrument leads that need to be tested where each strand was a copper coated steel wire, then this method could be used.

Overall, it would probably be much cheaper to replace probes every couple of years then it would be to test for broken strands.

Richard