Calculating cable length from resistance

[oxocube]

I've noticed a number of threads regarding exam questions asking to calculate the length of a cable by measuring resistance. But I can't think of a practical situation where you would ever need to do this and seriously doubt the accuracy of the method. I know there are specialist cable fault finding companies that use specialist equipment, but apart from them using standard meters is pointless.

I can't see that its a test of understanding electrical theory either because varying temperatures could make it a pointless and inaccurate calculation. I think a better test of someones electrical knowledge would be "Explain why measuring cable resistance is an inaccurate method of determining cable length"

Or have I missed something?

 

[Deleted member 26818]

One practical use would be as a rule of thumb reference to indicate whether there were any anomolies with testing results.
For instance if you installed a 50m run of 2.5mm² T&E, you would expect the R1+R2 measurement to be about 0.9755Ω.
As such if your measured value was for instance 1.9755Ω, you would know straight away that something was wrong, that perhaps the cable had been damaged, or that there was a loose connection.
Conversly, if the measured value was 0.4755Ω then you might consider that there could be parallel earth paths, or that again the cable may have been damaged perhaps by a screw causing a short.
You would then be able to determine the approximate position of the short, which would be about 24.37m along the run of the cable from where the measurement was taken.

 

[ezzzekiel]

Voltage drop

 

[Engineer54]

We have had to use that precise method to locate faults on more than one occasion on direct buried cables when there wasn't a cable fault locator available. It works quite well!! .....So long as you know the actual route of the cable. If you don't, it's a real pain without a good cable finder...

 

[Deleted member 26818]

I should imagine that it's not that good in a house either.

 

[Knobhead]

Try doing your tests with a "Ductor", then you'll pin point it.

 

[Knobhead]

PS with the pulse reflection we were only a ½mile out!

 

[Deleted member 26818]

I've always found that the easiest way to find a cable in a field, is to get a man with a JBC and tell him to dig a trench for a water pipe.
Let him decide where to start.
Guaranteed, that whithin 20 feet of wherever he starts, he'll wreck the cable.

 

[Engineer54]

Try doing your tests with a "Ductor", then you'll pin point it.

You've lost me there!! How will a Ductor tester locate the fault in a cable??

have used a Megger ''Wheatstone bridge'' tester a good few times , ...many years ago. Wonder if they still make them??

 

[Knobhead]

I was referring to finding a fault in a house, but it would have to be a dead short. Wheatstone bridge, that brings back memories.

 

[Knobhead]

The most positive location of a fault I’ve seen other than the thumper was done by a totally lunatic electrician. An old .2”² PILC cable had a Ph to Ph high resistance fault. So our resident loony removes the 450A fuses and puts copper links in. I retreated to a safe distance, about 100 yards from the sub while he closed the switch. I was near wetting myself as a tree fell over a couple of 100 yards away. He found the fault, but blacked half the plant out in the process!

 

[oxocube]

One practical use would be as a rule of thumb reference to indicate whether there were any anomolies with testing results.
For instance if you installed a 50m run of 2.5mm² T&E, you would expect the R1+R2 measurement to be about 0.9755Ω.
As such if your measured value was for instance 1.9755Ω, you would know straight away that something was wrong, that perhaps the cable had been damaged, or that there was a loose connection.
Conversly, if the measured value was 0.4755Ω then you might consider that there could be parallel earth paths, or that again the cable may have been damaged perhaps by a screw causing a short.
You would then be able to determine the approximate position of the short, which would be about 24.37m along the run of the cable from where the measurement was taken.

Yeah, I can understand all that, measuring length to calculate resistance and fault location.

But the exam question is the other way around, "measure the resistance to calculate length". Technical exam questions should reflect everyday situations so they make the link between the theory and the practical, and that one does not, why would anyone want to know?

If you want to know how long a cable is use a tape measure.

 

[topquark]

Yeah, I can understand all that, measuring length to calculate resistance and fault location.

But the exam question is the other way around, "calculate the length of a cable by measuring its resistance". Technical exam questions should reflect everyday situations so they make the link between the theory and the practical, and that one does not, why would anyone want to know?

If you want to know how long a cable is use a tape measure.

Yep, it's a bit perverse. I think the idea here is for installation time only. You've done an install and wan't to know how long the cable is. If continuity and IR tests prove OK then it's a valid (if not totally accurate) way of estimating.