Individual or overall and individual screened cable

[Lc85]

When is it best to use overall and individually screened cable. If we have a signal and power in the one cable then this should be overall and individual, say for a pressure transmitter? Does anyone know any rules of thumb for determining how to choose?

Thanks

 

[VEBill]

You just have to think about the purpose of the shield(s) and it becomes somewhat obvious. If the power wires are carrying noisy power, and the signal is a relatively weak signal, then the signal wires should be shielded from the noise on the power wires.

Some power might be clean and pure DC without any noise at all. It can happen, but it's often a bad assumption.

Some signals might be robust and highly immune to noise. Not as common as more susceptible signals.

'Twisted pairs' is helpful too.

 

[danw2]

2 wire loop powered field transmitters typically use shielded twisted pair (STP), with the shield on the outside, individual conductors are not shielded.

Cable with an internal shielded conductor or pair is typically used with something like magmeters where the electronics module is mounted remotely from the tube so that the excitation signal and unconditioned raw measurment signal need to be kept as noise free as possible on transit between the tube and the converter/transmitter.

Same concept thing with industrial flame scanners where power runs with the signal.

The vendor is usually pretty clear about using either vendor supplied cable or will provide specs for determining an equivalent.

In the realm of processs field instrumentation, 'special' cable with internal shielded conductor or pair is the exception not the rule.

 

[zappedagain]

I get better than 80 dB SNR with unshielded twisted pairs for power and signal in a cable with just an outer shield. YMMV. I originally started with individually shielded twisted pairs and testing showed that it wasn't required. Keeping a faraday cage including the outer shield was key for my application to shield cell phone interference.

Z