Interference

4-20mA is an industry standard signal that is largely immune to interference. It was chosen as a standard as a result of its immunity to interference.

I am assuming you are using shielded pair for the instrument signal cable and multi-conductor (not single conductor) for power cable.

NEC doesn't have restriction of using same tray for multi-conductor power cable and instrument cable (see NEC 392.9 A), but with having a solid fixed barrier between lower voltage rated cable (< 600V) and higher voltage rated cable (> 600V) (see NEC 392.6 F).

However, in the specifications of many projects, power cable and instrument cables are suggested to run in separate trays. NFPA 79 has a guideline to have 15" - 30" distance between power cable and instrument cable at different criteria (I don't have much detail at this moment).

I would suggest you to look into the project specification. If the specification doesn't have anything restriction about the minimum distance for the electrical and instrument trays, yours 100mm distance should be fine.

- MS

One of the sources of interference is cross coupling between cables. Cable separtation will reduce this considerably. 100mm will certainly have a major effect, but it depends on the magnitude of the interfering signal and the sensitivity of the equipment.

There is another source of interference, especially when VFDs are used. This is conducted noise which will spread through all the metalwork that is bonded together and to earth.

The best way to correct this, is to install the VFD correctly. This requires as low a high frequency impedance between the motor frame and the VFD frame.

Typically, this is achieved by the use of a screened cable correctly bonded to the frame of the VFD and to the frame of the motor.

For more information, see http://www.vfd-emc.co.nz