# What is the effect on noise immunity of a floating shield on a shielded twisted pair

We have an installed system where resistive sensors are connected to some electronics by shielded cables of between 2 and 10 metres. The cable is a twisted pair sheathed in a stainless steel braid. At the sensor end, the braid is unconnected, and the sensor is wired to each leg of the pair. At the other end (the electronics) the braid is unconnected, one leg of the pair goes to the electronics ground, and one leg is switched at some 100's of kHz between digital supply and ground. The resistance of the sensor is measured by charge-balancing techniques.

My question(s):

How much better is this configuration (a floating shield) better than no shield at all?

How much worse is this configuration than one where the braid is grounded at the electronics end (which was the intended topology)?

Footnote: the issue I am investigating is one of hardware unreliability caused by possible noise injection over a 5 metre run of the above-described cabling. The environment is a laboratory with a likelihood of motors in refrigeration units switching off and on. The Ethernet-connected nodes occasionally become unresponsive and I am wondering what the potential is for larger voltage spikes being introduced to the nodes (or to the PC connected to the same network).

## 1 Answer

Well, I gotta rush but https://www.dataforth.com/catalog/pdf/an507.pdf answers your question qualitatively

Cable shielding is used primarily to minimize or eliminate capacitively coupled interference from electric fields. When properly implemented, it can also be used to minimize inductive coupling from magnetic fields. Shielding is only effective against electric fields if it provides a low impedance path to ground. A floating shield provides no protection against interference. Grounding of shields can be a controversial subject because there are several ways to do it. The correct place to connect an electrostatic shield is at the reference potential of the circuitry contained within the shield. This point will vary depending upon whether the source and receiver are both grounded or whether one or the other is floating.

Quantitatively, i.e. how much worse/better... hopefully someone else will answer.

• I see someone wrote an entire book about cable shielding so there's probably some insightful experimental data in there (or at least in its references).
– Fizz
Oct 23 '15 at 2:33
• A floating shield provides no protection against interference This was the important bit. Thanks. Oct 24 '15 at 22:14
• @rossmcm Well, it is a bit more complicated; a heavy shield of iron even if not grounded provides some protection against eletromagnetic rather than electrostatic/capacitively-coupled noise. But it's hard to get a cable shielded well enough for that because EM fields penetrate materials and there are competing and cheaper ways to achieve protection against the latter, like twisted pairs (on which the induced currents cancel out). More in this Vishay appnote. Alas that doesn't have experimental/quantitative data either.
– Fizz
Oct 25 '15 at 2:15