Individually Tapped Pairs

We are taking production trials for Individually shielded 10 pair cable. This cable is having twisted pair with Copper Conductor and Foamed PE Insulation having diameters of 0.5mm/1.3mm. The pair is wrapped hellically with Polyester tape then with Polyester backed Al Tape. A Cu drain wire is provided.
After laying up of all the 10 pairs , the Shield of all the pairs alongwith drain wires are observed short with each other. We have observed that the edge of the Polyester backed Al tape is causing the same.

I want to know whether this will effect the electrical parameters of the cable.

Regards

Sheks

Some words of caution are required here. I discussed your question with a good friend of mine and a very qualified professional electrical design engineer (PE) with decades of design experience in the wire and cable industry. This is what he had to say:

"The shorted screens would only be a problem if the drain wires were connected to individual grounds where there was a significant ground rise potential between them. Assuming the pairs are all terminated locally, then this should not be an issue. Also, I believe that drain wires are usually grounded on one end only."

Ground looping could indeed be problem and therefore I recommend you tape each shielded pair with a third and final overall polyester tape to prevent shorting and possible ground looping. Remember, you have no idea how the cable will ultimately be used

Another way is to purchase the composite tape with the polyester width extended on both sides beyond the aluminum width. Thus, when it is helically taped for 100% shielding, the polyester component would always prevent shorting.

Sincerely,

Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

Dear Peter,
Thanks a lot for the valuable inputs.
So, the deviations observed in some electrical parameters like Attenuation, Mutual Capacitance and Characteristics Impedance are probably not due to the shorting of all the drain wires & composite Al tape.
Therefore, the question is now that why the properties are being effected to a large extend after taping. The indicative observed values before and after taping for some of the parameters are as follows:

(1) Mutual Capacitance: 34 nf/Km (before taping), 46 nf/Km (after taping)
(2) Attenuation at 1 MHz: 17 dB/Km (before taping), 25 dB/Km (after taping)
(3) Characteristic Impedance at 1 MHz: 115 Ohms (before taping), 105 Ohms(after taping)

The Composite Tape dimension is 0.032mm Al + 0.018mm PET.
The dimension of the first Polyester tape over the pair is 0.036mm

Waiting for your valuable advise once again.

Shekhar

Sheks

I expect you crushed the foam during the twinning and taping processes. Your pair tapes were probably applied with too high a tension. You may have also stretched the conductors. A mutual capacitance increase is generally due to over compaction of the core. That could mean that your DODs may be slightly too low and that the core is too sensitive to overall core taping. Perhaps you crushed the core with the overall tape. Forensic statistical analysis is required.

Shekhar, you must have development and process engineers with appropriate training and experience on staff. You must also run cable experiments. (For instance, why are you asking us about pair shielding shorting and the effects on the conductors when you could have easily run the experiment yourself?) You have certainly asked a lot of technical questions about your overall development project since September 11, 2006 but please be clear that we are here to give wire and cable people general advice, not to provide a free consulting engineering service to your company from the other side of the world. We are all volunteers here and nobody is obliged to answer your questions. Please understand this.

Thank you.

Sincerely,

Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com