Cables for instrumentation

I know that for 4-20ma transmitters twisted pair cable should be used with the screen grounded at one end only. However for digital signals ( limit switches , push buttons ) can 2 core/ part of a multi core cable be used ?

You could say Yes, but you could also say No........

Would you run 2 core for 500mtrs then tell me there is 90volts on a 24vDC digital input ?
Would you run this 24vDC digital signal in a 27core cable which has other prewired cores with 240vAC digital signals.
Would you use 2 core armoured or unamoured ?

As for 40-20ma signals, why do we use twisted pairs and what circumstances would ground the screen at both ends. The art of joining devices together by copper (or aluminium or steel) could be regarded as a science in itself.
There are more than 1.000 references related to mixed cable-types, application-voltage, code restrictions, etc, in Control.Com archives !

First and foremost are those related to Electrical Codes, regardless of country. It's quite simple really... insulation of any cable (whether single-core or multi-core, where core means the current or voltage carrying conductor) should be equal to the insulation of the insulation rating of that cable having the highest voltage.

Most problems occur when some conductors carry currents that are transitory in nature, often called EMI for Electro-Magnetic-Induction. For example consider a 12 Vdc 2-wire cable to a small relay. It will seldom produce a transitory current when 1st energized, but could likely do so when it's de-energized, The same is true for solenoid circuits or control devices to motor starter coils. So, be wary of any inductive circuit !

And to make things worse, these problems occur after the plant has been in operation a while ! Following is what I used to recommend to my clients:
1) All inductive devices should be provided with snubbers, i.e., Diodes across the coil if DC, MOV's if AC.
2) All instrument cable wire-ways should have at least one dedicated pair to be connected to a recording oscilloscope or transient-disturbance detector, especially during commissioning or start-up of the plant or facility.

Following below is a small list of EMI topic retrieved using Control.Com Archive Number coding:
1026157812 Pwr Cable vs TC Cable Interference.
1026211569 480 Vac vs 24 Vdc Interference.
1249659232 24 Vdc vs RS-485 interferance.
1387511994 RTD Interference Problem.

Regards, Phil Corso