industrial signalling and noise immunity

I deal with alot of industrial 4-20mA. the cabling from tranducer/ transmitter to output is is always twisted pair, shield c/w a drain
wire. 1. twisted pair is so induced current signals (from magnetic fields) cancel, correct? 2. shielding (foil etc), acts a faraday cage to the wiring within essentially 'shielding' the charges in the signal wire copper from outside e-fields influence, correct? 3. what is the drain wire for? 4. would there be negative outcome with using an overall shielded triad cable c/w drain wire, isolating one wire, in the stead of the twisted pair (ie if the contractor accidently pulled triad and not twisted pair)?
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Sorry for 1) i should say it is differential and cancels
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The signal is differential, the noise is common mode and cancels.
Using a twisted pair (fed with a balanced circuit) improves noise reduction compared to non-twisted pair due to greater common mode rejection.
If the contractor "accidentally" pulled non-twisted when twisted was specified, the contractor should be required to correct the problem by pulling twisted pair. If that is not what was specified, consider a new contract to get the job done correctly. There is no substitute.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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