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Topic Title: Can we use TDR for detecting Screen Faults?
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Created On: 26 December 2012 09:21 AM
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 26 December 2012 09:21 AM
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Joined: 26 December 2012

Q.1 Can we use TDR (time domain reflectometry) to DETECT (pre-locate) DEFECTS (breakage of Shield/Screen wires of XLPE Cables and/or grounds through weak jacket) for Cable Shield?

NOTE: It is already established that the screen wires/shields are defective. The applied voltage in the Shield wires/Screen of the XLPE cable did not rise, even when the far end (other end) of the shield was open. Therefore, it is certain that shield wires are somewhere partially and/or completely broken AND GROUNDED. Please note that the likelihood of ingression of moisture in the shield wires and possibility of being corroded is also high. However, it is not sure that what is the extent of damage and at how manay places is the screen/schield wires affected.

CAN WE USE TDR IN THE ABOVE MENTIONED CASE TO ASSESS THE GENERAL HEALTH OF THE SCREEN, by injecting pulse in the screen and grounding both ends of the conductor, simultaneously connecting the ground wire of the test equipment with the conductor at the testing end.

What will be the difference (if any) IF Conductor is OPEN from both ends AND low voltage high rise time pulse is injected in the screen/shield wires with other end of the screen open while using substation ground for test equipment grounding and grounding of the shield wire of test lead.
 26 December 2012 09:11 PM
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Possibly, it will depend upon the nature of the fault you have within the cable.

A TDR will work best when you have a a complete short circuit fault i.e. zero ohms. The effectiveness of the of the TDR is reduced as more resistance is introduced into the fault. If the fault is purely down to moisture then it may struggle. A TDR also doesn't have any real output capability in terms of current or voltage so cannot break down a higher resistance fault like an insulation tester would.

To get an idea of the the location of the fault you would also need to know the velocity factor of the shield / screen. I have plenty of data for copper conductors of various sizes but don't have anything for screens, so if you do get a successful reflection you may not get accurate distance data.

The output will be dependant upon the setup of the TDR and what type it is, but generally open circuits are seen as a stepup and short circuits are seen as a stepdown.

If it is possible to isolate both ends of the shield then alternatively it may be possible to use a Genny to inject a signal into the screen and then trace with the CAT, a break in the screen would either divert the Genny signal or stop it all together.

Both methods take quite a lot of experience in setting up and utilising so it may be best to call in contractors specialising in fault location. It may save you time, money and frustration in the long run.

Kind regards

Donald Lane
 26 December 2012 10:54 PM
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My favourite solution for screen faults, particularly where there is a screen to earth fault, would be to make a wheatstone bridge, using the screen to earth fault and an earth rod as one of your resistances. Assuming that you have a three phase cable, you should be able to use two cores to determine the end-to-end resistance of a single core, which you can then use as one of your "long test leads" to get to the far end of the screen. Depending on the length of your cable, you may need to use something like 600v dc or more to get sufficient current flowing in the earth connection. This method will give you a percentage of total cable length to the fault. The cable length should ideally be measured from the installation records, but can also be measured using a TDR if records are not available.



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