# Time domain reflectometry on medium voltage cable



## muelleroni (Aug 10, 2017)

Hi, 

right now I'm working with an Time-Domain-Reflectometer from Megger (Teleflex VX). The examinee is a medium voltage cable for 30kV which has a lenght of 87 meters . While I'm Measuring the Cable it's not under voltage.
My Question/Problem now is that the propagation velocity, which is measured by the Teleflex VX, is too low. Depending on the material of the cable and the dimensions it should be around 195,56 m/µs but the Teleflex VX show 150m/µs.
Is their someone who worked with TDR and experienced something similiar?

The Examinee:
630mm² stranded Al conductor 
Inner SC 
Insulation (CTM ) (​ɛ​r=2,35) 
Outer SC: bonded 
Swelling tape 
Al tape + copo 
HDPE outer sheath 



Sorry for my poor enlisch.

Best regards
Alexander M.


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## splatz (May 23, 2015)

It's my understanding that the TDR doesn't MEASURE the velocity of propagation, it USES the VoP to extrapolate length / distance from the time measured for reflections to return to the device. 

So you usually have to set up the TDR for the manufacturer's specifications for the VoP of the cable you're using. 

If you have a cable of a known length you can see if the actual VoP matches the spec VoP and adjust if necessary. 

Without the correct VoP the results should be off by a set factor. For example if the actual VoP is 200 and the setting on the device is 150, you should see distances that are only 3/4 actual (150/200) if I am doing the math right in my head. 

But short answer, you need to set the VoP in the device setup and if possible verify against a known length conductor. Megger support should be able to help if it's not clear in the manual, I'd hope?


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## muelleroni (Aug 10, 2017)

Thank you for the quick answer.
We know the length of the cable. So we set up the TDR so we can see the Reflection of the open end of the cable at the right distance. This way we set the VoP, but like I already said, it is an unrealistic VoP for this type of cable. 
I already tried to cantact the support, but they couldn't help me with my problem.


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## splatz (May 23, 2015)

OK, now I think I see - based on the fixed length cable, 
either the instrument is not calibrated correctly, 
or the actual VoP does not match your calculation. 

Would it be possible to make a test reading on another cable or wire where you know the VoP for reference? 

If another wire is off by the same ratio, 
that indicates the device is not calibrated correctly; 

if the measurement is OK on another wire, 
either the original cable is defective, 
or your calculation was wrong.


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## muelleroni (Aug 10, 2017)

Thank you very much. I will try your suggest either tomorrow or the next week. 
I will tell you my experiences.

Best regards
Alexander M.


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