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Author Topic: Why is the resistance of 4/O feeder cable so high?z  (Read 3243 times)

Mike Sokol

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Re: Why is the resistance of 4/O feeder cable so high?z
« Reply #10 on: July 25, 2019, 09:18:14 pm »

I have one of these, and added a pair of 75-ft extension leads for a long run. Because it's a true 4-wire Kelvin bridge I've used it measure 100+ feet of buried cable down to a few milli-ohms. The amazing thing is that my 75-ft test lead extensions are simply 18 gauge speaker wire, but because a Kelvin bridge has separate current and voltage paths, the resistance of the test leads have very little effect on the actual ohm measurement. https://www.greenlee.com/us/en/meter-cable-length-clm-1000-clm-1000
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Guy Holt

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Re: Why is the resistance of 4/O feeder cable so high?z
« Reply #11 on: July 26, 2019, 02:57:42 pm »

Guy,
Interesting article!
Curious as to what could cause that much resistance without a rather gross physical appearance.

As Steven rightly points out, internal corrosion and/or broken strands of copper, can increase resistance without a noticeable change in the outward appearance of the cable.

Did you photograph the bad chunk of wire (or Camlok connection) that was responsible for an impedance of 110Ω?

There was nothing out of the ordinary so I didnít take a picture.

Do you think the resistive point would have caused enough heat to be detected with a infrared thermometer.

I would think so.

Among many other tests, this does impedance measurements of hot, neutral, and ground legs: http://www.extech.com/display/?id=14409

No probes, it uses any IEC terminated (load side) power cable that is appropriate to your power system. Measures a single phase only, but I haven't found that to be too big of a problem.

The one I have has saved my equipment and crew more than once.

This option looks more practical than a 4wire Kelvin test under the circumstances. Do you know how the meter reads neutral and ground impedances? I looked at the manual online but it doesnít explain the principles the meter uses to take it measurements.
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Milt Hathaway

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Re: Why is the resistance of 4/O feeder cable so high?z
« Reply #12 on: July 26, 2019, 04:48:18 pm »

As Steven rightly points out, internal corrosion and/or broken strands of copper, can increase resistance without a noticeable change in the outward appearance of the cable.

There was nothing out of the ordinary so I didnít take a picture.

I would think so.

This option looks more practical than a 4wire Kelvin test under the circumstances. Do you know how the meter reads neutral and ground impedances? I looked at the manual online but it doesnít explain the principles the meter uses to take it measurements.

"EXTECH CT70 & CT80 Impedance - How is the impedance measured?"
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Stephen Swaffer

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Re: Why is the resistance of 4/O feeder cable so high?z
« Reply #13 on: July 27, 2019, 11:31:51 pm »

Do you think the resistive point would have caused enough heat to be detected with a infrared thermometer?

I would think so.


It would be interesting-and I think a complex problem to predict-how much heat would be generated.  In a single phase circuit, ohms law would tell us how the voltage drops fell into place and simple power formulas the amout of heat generated in a bad spot.

The neutral of a 3 phase only carries the unbalanced load-and changing/increasing the resistance of the neutral could have an impact on how the load balance out-an open neutral causes all kinds of voltage issues with single phase loads it would stand to reason a "partially open" neutral would as well.
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Steve Swaffer

Mike Sokol

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Re: Why is the resistance of 4/O feeder cable so high?z
« Reply #14 on: July 29, 2019, 09:40:03 am »

Do you think the resistive point would have caused enough heat to be detected with a infrared thermometer?

I would think so.

I've done this with a FLIR camera, and it's easy to see the overheating spots. An infrared camera doesn't have enough resolution to find the exact hot spot.
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Tim McCulloch

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Re: Why is the resistance of 4/O feeder cable so high?z
« Reply #15 on: July 29, 2019, 11:51:12 am »

I've done this with a FLIR camera, and it's easy to see the overheating spots. An infrared camera doesn't have enough resolution to find the exact hot spot.

Story time -

About 12-13 years ago I was electrician for a show that was 100% moving lights except for the 3 amps of draw from FOH console.  The tour electrician said the X phase would have slightly more current but otherwise the load was evenly split between all 3 phases.... so during the show I'm sitting by the Really Big Switch and was bored.  Pulled out the Fluke clamp meter to verify the neutral was carrying only 3 amps... but it measured around 15 amperes IIRC.  So I figured it had to be going somewhere as heat.  Half house show so there was 150 ft of feeder.  I walked the feeder path and ran a hand along the cables.  At the 100ft/50ft connect point I found a CamLoc that was very warm, almost hot, and continued until I reached the LX distro.  Didn't find anything more so I went to the TE and told him what I'd found.  He did the walk back with me, touched the CamLoc and immediately marked it with purple tape.  He indicated that a CamLoc had be caught on a stage leg at a previous show and the local crew just pulled on the 4/0 until it freed up.  "We'll re-terminate that Cam tomorrow."

So yes, damage you can't see can result in conditions you need to be aware of.

/story time
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Re: Why is the resistance of 4/O feeder cable so high?z
¬ę Reply #15 on: July 29, 2019, 11:51:12 am ¬Ľ


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