Should I shield digital signal cables in contact with 120 VAC or 480 3-phase

[mace]

I will be adding contact relays and current switches to a control panel that is majority 120VAC and some 480 3-phase. The coil voltage is 24 VDC and the current switches also connect to 24 VDC. The DC wires will be going to another panel. The contact relays will come from the output module and the current switches will go to the input module. Should I be concerned about EMI in my 24 VDC wires? If so do you recommend using shielded cables? Thank you for any assistance!

 

[Robb B]

DC signals are generally immune to induced signals, I regularly have 120v AC, 24v AC, 24v DC signal wires in close (touching sometimes!) proximity to VFD and motor starter
power wires and have never seen instances of induced signals. Shielded cables are good practice, but not necessarily required for functionality. Definitely required for some code applications, though. I see a lot of European VFD (and some motor starters) call for screened power cables, I suppose the intent is to stop the EMI at the source, rather than protecting other wires, though they generally recommend shielding everything.

TL, DR: Shielding is nice, but probably not necessary anywhere for anything, until you reach 100's of amps.

 

[Bolt]

I was revisiting a PLC install just yesterday. The PLC was receiving an intermittent/pulsing signal from a device, causing one of the three similar 24 VDC PLC input to flicker. I couldn't measure more than 2 volts AC or DC on the wires with my meter. I went to the originating panel, found the control cable's ground wire wrapped around the cable and the shield cut off. I unwrapped the ground, used a magnet to hold it to the back panel, and the flickering input quit instantly. I believe the electricians ran the control cable ran in a conduit with 480 VAC input power to a VFD, but not entirely sure if it was or wasn't in close proximity to to the VFD's output wires. I simply reterminated the shield and did the other 2 VFD's control signals for good measure and it's been working fine now.

 

[JTCat]

DC can pickup interference. It's not as common with low voltage AC, but I've seen it mainly in wet locations.

I agree with Robb B that you should be looking at shielding the higher voltage wiring to prevent the signal from propagating outward.

 

[ganutenator]

Solid state 24VDC power supplies can cause a lot of noise.
I was able to prove this.
We shield most all of our 24VDC signal wires.