Should cable shields be connected to earth ground, or to power supply negative?

Question

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.

Answer 1

What you have here is this:

^{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.

Answer 2

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.

Answer 3

You should connect 0V of the DC supply to ground to prevent it floating. The cable shields should also be connected to ground.

Answer 4

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.

Answer 5

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.