high resistance in motor feed cable

[fg45]

I have discovered a feed cable (4 core, 3 phase and neutral)that supplies a panel from which several motors are fed from, having a resistance on each core of average 35 ohms. Normally a high resistance would reduce the current, but would it increase it to provide the motors with sufficient power?

 

[Skogsgurra]

What length is the cable? And what gauge?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[fg45]

The cable length is approx 20 meters and the cable size is 25mm^2

 

[Skogsgurra]

Then 0.015 ohms would be a correct value.

With this area, the cables probably carry around 100 A. That would produce a voltage drop equal to 350 volts in a 35 ohm cable. Are you really sure that your measurements are OK?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[fg45]

yes we checked the measurements several times. we thought there could be a poor joint in the cable or it has been damaged, but it seems strange that all of the cores have almost the same resistance.

 

[rbulsara]

There is a lot missing from the story. What prompted you to check the cables in first place? Is this a new installation or an existing?

Have you tried with a different meter? How are you checking the resistance? Are cables disconnected at both ends?

Rafiq Bulsara

http://www.srengineersct.com

 

[edison123]

How exactly did you measure the cable resistance with ends 20 meters apart ?

Muthu

http://www.edison.co.in

 

[waross]

Check your meter and leads. A meter offset and/or high resistance measuring leads would explain ridiculously high resistance values. The odds are much against three simultaneous faults each adding the same amount of resistance to each conductor.
To take Gunnar's example one step further:
100 Amps x 350 Volts drop = 35000Watts or 35 kW of waste heat. The point is, that much resistance (35 Ohms) in a feeder will generate enough heat to be be noticeable and will lead to rapid complete failure.
Metering issues are much more likely.

Bill
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"Why not the best?"
Jimmy Carter

 

[waross]

By the way, that is for each cable. Times three for total losses.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Yes. And add my mistake to that. 35 ohms times 100 A is not just 350 V but 3500 V. So, it is obvious that your measurements are completely wrong. Perhaps you have something like 35 milliohms?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...