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I'm working on a PLC cabinet, and I'm wondering whether I should hook cable shields to earth ground, or to the power supply negative.

I checked and the power supply DC output is isolated. There's no conductivity between earth ground and the power supply negative.

Would the answer change if the supply were not isolated?

The cables go out to sensors, and they carry 24v power, 4-20ma signals, and 0-10v signals.

Side question: Is there anything wrong with leaving the dc supply isolated? The chassis is grounded to earth.

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  • \$\begingroup\$ is the shield there for safety, emi ? \$\endgroup\$
    – user16222
    Commented Jun 13, 2018 at 21:00
  • \$\begingroup\$ Is it coax or shielded pairs? How long? \$\endgroup\$ Commented Jun 13, 2018 at 21:16
  • \$\begingroup\$ The shield is there for EMI. The cables are probably about 10 meters max with 2-8 conductors per cable. \$\endgroup\$
    – Drew
    Commented Jun 13, 2018 at 21:23

5 Answers 5

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What you have here is this:

schematic

simulate this circuit – Schematic created using CircuitLab

It may be most advantageous to tie the shield to the case, the goal of a shielded cable is to take electric fields that might capacitively couple into the signals you wish to shield and shunt them someplace else. Since the isolated supply is most likely capacitively coupled (isolated) to the box tying the shield to the isolated supply is not going to provide a good pathway to shunt currents away from your signal, it could also create common mode problems if a large current is moving down the shield. If you tie the shield to the chassis ground, the currents that impinge across the will flow down to ground away from the signals you want to protect.

It really depends on which is the lesser of two evils, there are circumstances where one would be better than the other, I'm talking about the general case. Cable shielding is about controlling currents on the shield. Typically in my experience tying the shield to the case is best, but I've also seen a few cases where it isn't.

If your requirement is also having the shield isolated then tie it to the powersupply ground.

A second thing to note is what is tied on the other end of the shield, because if the shield is tied on both ends, there can be currents that flow from ground loops and if you get that, the current can inductively couple and create noise in your signals inside of the cable. In a general sense, tying the shield on both ends to ground is bad for this reason.

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  • \$\begingroup\$ "create common mode problems if a large current is moving down the shield." is true for one example but not true in general to say CM current is high. CM noise can be and most often due to a high CM impedance (floating DC) with poor balance eg stray C2 or saturating input CM voltage creating a low SNR. (FYI) \$\endgroup\$ Commented Jun 14, 2018 at 14:39
  • \$\begingroup\$ Thats what will happen if you tie the shield to the isolated supply, and a large current will bump everything up if the supply really is floating. So in general, tie the shield to the case. Since I don't know the values of C1 and C2, I can't speak for which case will be better \$\endgroup\$
    – Voltage Spike
    Commented Jun 14, 2018 at 15:13
  • \$\begingroup\$ Ok but the ungrounded shield is the worst case . Ethernet often uses 1nF to receiver case 0V then puts it thru a CM choke then an isolation transformer then if EMI on an external mic is bad with charger connected , an earth ground or VGA monitor (which is earth bonded) is needed. \$\endgroup\$ Commented Jun 14, 2018 at 16:58
  • \$\begingroup\$ An ethernet cable doesn't have a shield and if it does, the shield is not connected to the transformer. The shield is most often connected to the PCB ground, which is usually tied to ground in some way. \$\endgroup\$
    – Voltage Spike
    Commented Jun 14, 2018 at 17:42
  • \$\begingroup\$ I should have said CAT6 Ethernet and yes they do use a cap from shield to case \$\endgroup\$ Commented Jun 14, 2018 at 19:29
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Isolated supplies and cables is the worst case for CM suppression on unbalanced sensors.

In order to divert high impedance stray E and H field noise on signals you want to attenuate CM noise from DM signal to avoid the nonlinear conversion of CM noise currents into mismatched input impedance or exceeding the CM input range of high impedance inputs.

Classical EME solutions or EMI reduction methods include ;

  • lowering differential mode (DM) impedance of link end-to-end while raising CM impedance ( useful in telephony , ethernet , HDMI and commutation noise in motors , etc ) or using CLLC line filters.

    • Useful for line noise to microwave
  • Lowering impedance of Rx to HF noise with shunt RF caps (like Y caps on a line CM choke). Or both of above (best case)

  • Raising impedance of series RF with CM choke and ferrite beads and absorbing loss into lower impedance load ( useful for RS-422 , and HDD low impedance signals etc)
  • The path of diverted ground currents should be the shield ground path and not the signal differential path.
  • terminated shield with low impedance earth bonded ground at source ( if possible) or load ( if not) but not both if risk of ground difference noise voltages
  • back drive shield from load with low impedance CM signal derived from Instrument Amp IC (common for EEG uV signals)
    • this shunts external stray E and B field noise from reaching the internally shielded pair and satisfies medical specs for high SNR and low AC line leakage current

However 20mA current sources have high impedance to cable losses and improved immunity to voltage drop in cables they may not be perfectly balanced to Shane the Rx bandwidth or diode protection rectifiers If CM range is exceeded by RF modulation from say nearby mobile phones. Thus common mode LLC line filters may be needed.

Anecdotal

One cannot predict performance without knowing the EMC spectral noise. I had a couple career embarrassing moments when high power Radios or arc welders nearby caused untimely errors during operation.

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    \$\begingroup\$ Who is the -1 W5V0? DT!? \$\endgroup\$ Commented Jun 13, 2018 at 21:12
  • 1
    \$\begingroup\$ I don't know who but I am seeing really somebody is frustrating you for sure.. With downvotes.. Why don't you ask in meta if possible to track the silent protestor? \$\endgroup\$
    – User323693
    Commented Jun 15, 2018 at 16:58
  • 2
    \$\begingroup\$ This answer has a lot of information, but I do not see a clear answer to any of the questions asked. \$\endgroup\$
    – Joe S
    Commented Jun 15, 2018 at 17:17
  • \$\begingroup\$ OK I agree but the question lacks even more details on EMI impedance spectrum nearby , impedance of cable and length. available earth bonds and spectral difference in current if shunted. etc etc. Any speculation on other answers is purely by guesswork. with many assumptions. If anyone disagrees they have alot to prove and even more to learn. \$\endgroup\$ Commented Jun 15, 2018 at 18:34
  • \$\begingroup\$ The 1st line is correct. Floating shields are the worst case as they induce ground shift noise if connected to signal ground. \$\endgroup\$ Commented Jun 15, 2018 at 18:46
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You should connect 0V of the DC supply to ground to prevent it floating. The cable shields should also be connected to ground.

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  • \$\begingroup\$ I've heard this before, but what's the advantage? As far as I can tell it just makes ground loops more likely. \$\endgroup\$
    – Drew
    Commented Jun 14, 2018 at 2:26
  • \$\begingroup\$ How do airplanes do? At the end it’s about a proper return path and how signals are referenced to that path. \$\endgroup\$
    – RemyHx
    Commented Oct 7, 2022 at 19:24
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The DC -Ve is not connected to earth ground internally in any power supply as far as I know. This is because you can use multiple power supplies to create different voltages, which would not be possible if they were connected.

Examples:

  1. use two 12 VDC power supplies to get 24 VDC and 0 V.
  2. use two 12 VDC power supplies to get -12 VDC and +12 VDC (by connecting the shared cable to earth or any other reference for 0 V)
  3. use three 15 VDC power supplies to get 45 VDC and 0 V (or 30 VDC and -15 VDC, or 15 VDC and -30 VDC)

I have seen 0 V being connected to earth via a ground disconnect terminal block. In this type of terminal, the 0 V is connected to earth via a removable metallic link.

I'm not fully sure of the reason, but I think it is because you can have 0 V tied to the earth instead of floating, or if you want it floating, then you can disconnect the link.

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Look up 2017 NECGrounding and Bonding.

On the side of the equiptment, if the device is insulated from Earth ground, you can just tie back and tape the shield, especially if it's control wires, or something with low current.

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  • \$\begingroup\$ While you may be correct that it can be left unconnected, it won't shield the sensitive control and data signals, and the floating shield can act as an antenna which picks up or radiates electromagnetic interference. So that does not really answer the question which is a better way to connect the shield, and not connecting it at all is even worse. \$\endgroup\$
    – Justme
    Commented Feb 15, 2023 at 21:13

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