Resistance in a damaged stranded wire

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Old 03-16-19, 10:04 AM
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Resistance in a damaged stranded wire

Hi,

I always heard that a stranded wire with all but 1 strand left could create issues like having the right voltage and perfect continuity on it but it being unable to carry current to power a light on a car. I tried experimenting, got myself a stranded wire, I cut all but on strand and tried powering a test light with a bulb and later a sealed beam. Both in 12v being powered by 8 AA batteries and respectively consuming .25a and 1.5a. Testing the wire with continuity showed barely any resistance and both lights lit up. I could not reproduce the problem I tried to create.

Have I got this right?
 
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Old 03-16-19, 10:28 AM
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Are you trying to see if you can get a wire so small that it gets hot or melts while trying to power a light? You did not say how large a wire/conductor you were using but if your test didn't work whatever you choose is big enough to carry the load you put on it. If you want to see the wire glow get a smaller wire or turn up the current.

A bare conductor in air can carry more current. It's not encased in insulation and it has good air circulation around it to keep it cool so it can carry more current. Wrap that same wire in insulation and put it in a bundle with other wires and it won't be able to cool and will be more prone to over heating and burning out. That's why on a bench test with the bare wire in the air you might be able to put 40 amps through a 14ga wire but it's only approved for use with 15amps when it might be bundled with other wires and covered with insulation.
 
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Old 03-16-19, 12:09 PM
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You will have to put higher load to see the difference.
If you put 3A or more, you will see the wire heats up and probably will glow red at 5+A
If you were to test that single strand of wire for a longer distance, then you probably will be able to see the difference.
 
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Old 03-16-19, 12:42 PM
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The smallest hair wire will still measure zero resistance.
If you want to see a problem you need try drawing 15 or 20 amps through that one strand that is left. That one strand will heat up and might even melt in half. The rest of the wire will be fine.
 
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Old 03-16-19, 01:37 PM
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The smallest hair wire will still measure zero resistance
It will show resistance, it will just be very low.
 
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Old 03-16-19, 08:00 PM
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It will show resistance, it will just be very low.
Very low as in zero, as opposed to very high or open.
 
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Old 03-16-19, 09:20 PM
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Very low as in zero
It will not be zero. Even copper wire has a resistance. That is part of the formula used to calculate voltage drop. The resistance of 1000' #12 copper is 1.588 ohms. That little strand left in the wire will act as a fuse, get hot, and fail with too much current.
 
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Old 03-17-19, 02:43 AM
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resistance check often may show a good wire but it could be damaged and causing a voltage drop, a poor connection at a terminal is probably even more common than a damaged wire and also could cause the same issue and have a voltage drop and show good continuity.
it may show good voltage if the load is unplugged and just checked with a volt meter for voltage but it would not have good voltage with a significant load on the circuit on a vehicle you would normally use a voltage drop test when the circuit is under load to test each connection and wire individually to see if there is a voltage drop indicating a problem.
 
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Old 03-17-19, 08:33 AM
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Let's say you have a bad or corroded connection, if you would disconnect the wire all the way and check the resistance to both extremities with the connection in between you should should have an abnormally high result?
 
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Old 03-17-19, 08:34 AM
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Understand that any loose strand current test, done with a field of copper around it, is not the same as an individual strand sitting in free air. All the surrounding copper strands, conducting or not, will provide cooling benefit. This then, results in an increased amp capacity for that single strand. Not as high as an undamaged strand set, but higher nonetheless.
 
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Old 03-17-19, 02:17 PM
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, if its still capable of flowing some current you can still read good continuity cause the meter doesn't really put a load on the wire or connection to test continuity, however if it is bad enough it will show high resistance, in your original post you mentioned a light on a car most filament bulbs will atleast light up with low voltage but may be dim, if the light did not function at all it would probably be picked up with a meter showing high resistance.
 
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Old 03-18-19, 06:13 AM
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If you were able to test the damaged wire with the type of meter they have in a science lab you could probably detect a difference. With the meters that are available for home & field use they are not sensitive enough to pick up things like that, nor is it really necessary for them to be. For standard meters, you have to take measurements under load to get the most useful results.
 
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