Detecting splices

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eHunter said:
Yes, using a time-domain reflectometer (TDR) splices, taps and cable defects can be detected electronically.
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Provided, of course, that the splicing method used changes the characteristic impedance of the cable at the splice enough to create a detectable reflection. The better the splice, the harder it is to detect it.
For example, a good connection between two coax cables using approved connectors should not show up as a splice using TDR.
 
The method is not approved, they just made the splice by twisting cables.
Is it detectable?
The connection was tight that the previous voltage check passed, but then it just suddenly dropped this morning after 2 years of operation.

Thanks guys.
 
GoldDigger said:
Provided, of course, that the splicing method used changes the characteristic impedance of the cable at the splice enough to create a detectable reflection. The better the splice, the harder it is to detect it.
For example, a good connection between two coax cables using approved connectors should not show up as a splice using TDR.
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A good TDR in the hands of a capable and experienced operator will be able to detect just about any splice including a Cadweld.
 
Cloud said:
The method is not approved, they just made the splice by twisting cables.
Is it detectable?
The connection was tight that the previous voltage check passed, but then it just suddenly dropped this morning after 2 years of operation.

Thanks guys.
Click to expand...

A TDR will pinpoint an open or short location with amazing accuracy.
Since it is a relatively expensive instrument, renting the instrument is an option.
 
I have never used a TDR and don't know much about using them, but I would suspect you may need to have a somewhat accurate idea of how much conductor you are attempting to measure to best understand the results.
 
eHunter said:
A good TDR in the hands of a capable and experienced operator will be able to detect just about any splice including a Cadweld.
Click to expand...

eHunter said:
A TDR will pinpoint an open or short location with amazing accuracy.
Since it is a relatively expensive instrument, renting the instrument is an option.
Click to expand...

Renting one pretty much means "capable and experienced" is no longer in the equation.
I have a TDR and would not trust it alone to determine where I was going to open up a finished wall. Owning one does not make me capable, just somewhat experienced. Newer ones may be easier to use.
 
kwired said:
I have never used a TDR and don't know much about using them, but I would suspect you may need to have a somewhat accurate idea of how much conductor you are attempting to measure to best understand the results.
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Most of the better TDR instruments have a builtin lookup table for the cable type and cross section that sets the expected test pulse propagation speed for that given conductor. This facilitates a fairly accurate time to distance calculation resulting in a fairly accurate pinpointing of any reflected anomoly on that conductor.
A TDR is sensitive enough to detect when a conductor has been abused or mishandled during its life or where a conductor is bent, kinked or the bend radius is too small.
 
eHunter said:
Most of the better TDR instruments have a builtin lookup table for the cable type and cross section that sets the expected test pulse propagation speed for that given conductor. This facilitates a fairly accurate time to distance calculation resulting in a fairly accurate pinpointing of any reflected anomoly on that conductor.
A TDR is sensitive enough to detect when a conductor has been abused or mishandled during its life or where a conductor is bent, kinked or the bend radius is too small.
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My older units do not have this and I have had to make my own. Yea, fun. Do you happen to know of a reference for these?
 
VOP can be determined by dialing the TDR in on a known length of cable of the same type being tested. I have went to the supply house and connected my TDR to a 1000' roll of cable to get VOP. For this instance it was 12/2 UF-B, don't remember the VOP though...
 
eHunter said:
A TDR will pinpoint an open or short location with amazing accuracy.
Since it is a relatively expensive instrument, renting the instrument is an option.
Click to expand...

I mean, the voltage dropped. The connection is still holding but loose. We did manual checking by visual to locate the splice.
I want to include a gate check during QAQC using this equipment, workers here cannot be trusted. :)
 
Cloud said:
I mean, the voltage dropped. The connection is still holding but loose.
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Fortunately, a sudden high resistance along an otherwise uniform cable will definitely create a reflection for the TDR to discover. A bad splice is a lot easier to find, even without a lot of experience, than a good splice.
 
Cloud said:
I mean, the voltage dropped. The connection is still holding but loose. We did manual checking by visual to locate the splice.
I want to include a gate check during QAQC using this equipment, workers here cannot be trusted. :)
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If the cable is exposed, the Wilcom T124 instrument would be of value in assuring the shield splice integrity before coverup. The T124 requires physical contact with the spliced area of the cable to perform the testing.
3M Dynatel also has several models that will test for sheath/shield integrity/faults.
A TDR would would be very useful if there is not physical access to the cable splice or if you needed to locate splices, test for faults or other anomolies on buried or otherwise inaccessible conductors after installation.
If the connection is "still holding but loose" it will fail and most likely fail under load.
What splicing kit are your workers using?
All that being said, if your workers are not trustworthy will procuring a new test instrument for their use make them trustworthy?

GoldDigger is correct about a poor splice being easier to spot than a good splice on a TDR, but most splices are visible on the display, even to the inexperienced.
 
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eHunter said:
What splicing kit are your workers using?
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Twist and shout. :eek:hmy: They already know that it is not allowed in Client code (even wire nuts), I don't know about NEC. Cables are pre-measured, so my guess is that it was damaged during pull in and the worker was afraid or just too stubborn to change the entire length.

eHunter said:
All that being said, if your workers are not trustworthy will procuring a new test instrument for their use make them trustworthy?
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If I will let them do the test, of course will be useless. I will assign one from my team to do if not so witness the testing, we're a third party.

Thanks a lot.
 
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Cloud said:
Twist and shout. :eek:hmy: They already know that it is not allowed in Client code (even wire nuts), I don't know about NEC. Cables are pre-measured, so my guess is that it was damaged during pull in and the worker was afraid or just too stubborn to change the entire length.
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Or some other trade damaged it and thought they fixed it and told nobody.
 
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