I am terminating a bunch of IS instrument cables in Remote I/O cabinet. The cabinet is 2m by 2m with depth of about 0.3m. Cable runs from entry to terminals for about 1.5m before terminating at separate terminal blocks for conductors and screens. Should I keep the screen with the conductor runs(sleeve over the screen and cores) within the cabinet or its acceptable for the screens to be separated at entry? What is the acceptable length conductor runs without screens before signal interference in deemed significant?

 

Best is to use a real control panel with EMC contacts on the plates. If you don't have this contact each plate of the panel well with the earthing with thick cabling.
https://www.rittal.com/com-en/products/PG0002SCHRANK1/PGRP5103SCHRANK1/PG0139SCHRANK1

Best is to use EMC cable glands and connect the braid of the cable inside this cable gland.
https://www.rittal.com/com-en/products/PG0900ZUBEHOER1/PG0918ZUBEHOER1/PG1159ZUBEHOER1/PRO14265

Depending on the frequency of the signal that must be screened:
<10kHz: only earth screen at side of the device that is reading the signal (so no screening on the sensor side, leave that open)
>10kHz: earth screen on both sides (both at device reading signal as well as sensor side).

Don't make the mistake of stripping the screen of the cable and then turn it up into a cable, connect a cable shoe to it and connect that to earth.
You now made a coil and a coil does not want to conduct the high frequency noise signal.

Best is always to connect the screening with a EMC cable gland to the panel.

 

Hi Patrick,
Thank you for the insight on the signal frequency ranges for screen termination purposes.
I am afraid the Panel is client-supplied and already here(on site) and comes with mounted MCT frames and no option for EMC glands or EMC MCT inserts.

 

I am terminating a bunch of IS instrument cables in Remote I/O cabinet. The cabinet is 2m by 2m with depth of about 0.3m. Cable runs from entry to terminals for about 1.5m before terminating at separate terminal blocks for conductors and screens. Should I keep the screen with the conductor runs(sleeve over the screen and cores) within the cabinet or its acceptable for the screens to be separated at entry? What is the acceptable length conductor runs without screens before signal interference in deemed significant?

Please compare field instrument Cable A and B, 1 pair 0.75, screened, Braid armoured cable.
1- pair core termination on the IS terminals
2-screen termination to IE bar.
3-run of heat shrink sleeved pair cores + screen
4-Brain armour termination to PE bar.
In option A, cores are maintained close to screen by common heat shrink sleeve(point X to Y) for the 1.5m run through the cabinet/panel.
In Option B, cores and screens and separated at point X, leading to 1.5m run of cable without screen.

Cabinet rating 230Vac, 50Hz.

A or B?

 

O O, I see Siemens and know from experience that Siemens electronics are very sensitive to noise and ground loops.

I would drill holes and install EMC cable glands for the screening. If you don't want to do this make sure you don't turn the screening in coils that resist the noise going through like I said. Don't be that guy that just solders a earth wire to the screen or turns the screen into a new cable: this is a coil that resists the flow of the noise.

Remember that because of skin-effect high frequency signals (and high frequency noise) run on the outside of the cable (therefore we use the screening around the conductors of the signals).

But, before we continue discussing further, what kind of signals are going into the panel? When it is 4-20mA it is so robust you will not have problems. But with thermocouples that are inside a kiln operating >1200°C you must watch out, especially with Siemens electronics.

So, what signals are you measuring. Voltage? Current?
And what frequency are those signals? DC? Just plain 24VDC digital input/output signals?

gr. Patrick

 

Patrick,
I am afraid this is a certified cabinet and no option for drilling a/ or any alterations other than field cable termination.

These are field instruments, intrinsically safe 4-20mA signals.

 

with 4-20mA DC signals you don't have to worry so much. They are stable and robust. It is DC so connect the screen only on the measurement side (cabinet) and not the sensor.