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TL;DR: Does burying a cable provide shielding? I.e., is a shielded cable in air equivalent to an unshielded cable buried ~ 6 inches in soil?

Long: I'm a technician at a university. I don't have formal training in electrical engineering, but one of my tasks is designing an environmental monitoring system for the field. Analog voltage sensors (thermocouples) will be read by a device in the field, and the signal will be passed ~100 m to a datalogger via shallow buried cables. In selecting these cables, I was concerned about voltage drop (since thermocouple signal is a very low voltage to begin with) and cable shielding (since there are power transmission lines in the area, and AC power supply for the datalogger in the same enclosure box). I asked a more experienced technician for advice, and he advised that in this application he would be concerned about both things.

However, when I approached the project manager about this, I was told not to worry about voltage drop (no reason given), and that burying an unshielded cable was "the same as shielding it". I can order unshielded cable and spend no further time on the issue... but I'm concerned that the manager is just trying to save a buck, and that this could come back to bite us (i.e., I'll be creating more work for myself) in the future.

I'm also generally curious. Is he correct in saying that a shielded cable and an unshielded cable buried less than 6 inches in soil are equivalent?

*Edit to add detail: the site is a pine forest with well-drained (dry), predominantly sandy soil. I don't have much experience to go on, but I would say it's fairly quiet for EM. There's one high-power line ~200 m from the thermocouples, and 120V AC running through outdoor extension cords - the 120V lines are co-located, and will run alongside the TC's in some cases.

**Edit to clarify: I'm not concerned about voltage drop on the thermocouple itself. I'm concerned about drop on the 100 m cable that will carry the signal from the thermocouple voltage-reader (a Campbell Scientific AM16/32B multiplexer) back to the datalogger (a Campbell CR1000).

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    \$\begingroup\$ What kind of EMI environment is your area? To give you some examples: underwater will be fairly quiet, arc welding shop is very noisy. You've mentioned "environmental monitoring system for the field". Is it literally an agricultural field? \$\endgroup\$ – Nick Alexeev May 8 '18 at 18:40
  • \$\begingroup\$ I'm not experienced enough to qualify it well, but I'm going to say quiet. It's a managed forest - 1 hectare plots, each with varying densities of trees. No electrical activity within the forest other than mains AC (120V, 60Hz) supply running into the electronics enslosures (basically a big extension cord running ~200 m into the forest from roadside power lines). \$\endgroup\$ – Keegan Smith May 10 '18 at 14:19
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It's largely down to moisture content.

Put an unshielded cable in a few inches of dry sand with still more inches of dry sand below it and it will look largely like an unshielded cable placed above ground with respect to electric fields it might generate. Maybe dielectric losses will reduce high frequency E-field values.

As for magnetic fields they'll be hardly diminished at all.

Water content makes life difficult for E-fields and salt water is much better at shielding than tap water. This is why it's so darn difficult to use radio waves under water. Penetration is very poor except at profoundly low frequencies.

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  • \$\begingroup\$ I've edited the question to provide more site detail - basically sounds like we have the worst-case scenario for shielding by the soil. Is there a way to test for EM interference on the cable? I assume the simplest would be to look at the signal from a shielded vs unshielded cable... assuming I can only afford an unshielded cable, would EMI just appear as a large amount of noise in the signal? \$\endgroup\$ – Keegan Smith May 15 '18 at 14:19
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    \$\begingroup\$ Can you be clear about what your worries are? Is this EM produced by your cable or EM received by your cable? What bandwidth of signal are you wanting? \$\endgroup\$ – Andy aka May 15 '18 at 15:06
  • \$\begingroup\$ My worry is that several people have told me that thermocouple signals need shielding (e.g. @WhatRoughBeast below), but my boss doesn't want shielded wire for some reason, so I wanted to understand what exactly the risk is, how serious it is, and how to detect the problem if it occurs with unshielded wire. It's a DC signal, so I don't think it has a bandwidth... right? There's no regular wave, though fluctuations in the temperature measured by the thermocouples will probably be on an hourly scale, with a daily cycle. The potential problem is received EM, e.g. from the AC power lines. \$\endgroup\$ – Keegan Smith May 16 '18 at 17:31
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    \$\begingroup\$ If the bandwidth is really low like you say then filtering should remove noise pick-up but make sure your input amplifier has protection in case a biggish glitch comes in. \$\endgroup\$ – Andy aka May 16 '18 at 18:13
  • \$\begingroup\$ Thanks for the deep dive here! One last clarification question: In the electrical eng. lexicon, does the term "signal" typically refer only to cycling current? For example if I had a sensor that outputs a single DC voltage (e.g. 50 mV) and never changes (maybe a useless sensor, but for the sake of argument...) would "signal" still be an appropriate term to use to refer to that voltage? And if so, would that signal still be said to have a "bandwidth"? \$\endgroup\$ – Keegan Smith May 17 '18 at 16:48
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Voltage drop - no problem. While TC voltages are low, they are also low-impedance, and TC conditioners are generally high-impedance, so there should be little effect.

Shielding - Umm, that's a problem. Oh my, yes. Your TC wires will act as an antenna, and with that long a run I'd expect all sorts of pickup. At the least, I'd go for shielded pairs, with the shield tied to ground at the conditioner. I most definitely would not trust the shielding effect of earth, unless perhaps you're working in a swamp. This article suggests converting to a 4-20 mA current loop for distances in excess of 150 feet. Also, if you're using commercial TCs, you should look into ungrounded sensors. These will respond more slowly, but give less grief in the presence of things like ground loops.

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  • \$\begingroup\$ I wasn't familiar with the term "conditioner", but I think our multiplexer/logger assembly fills that role - the TC is read at high-impedance terminals, with a thermistor at those terminals to provide junction temperature compensation. I may misunderstand your answer but I've edited the question to clarify the point: the thermocouple isn't running for ~100 m, the voltage is read by a multiplexer at ~30 m, then the signal is being carried ~100 m on copper cable. Your point about the long cable acting as an antenna is still correct, I think. Our TC's are home-made and ungrounded. \$\endgroup\$ – Keegan Smith May 15 '18 at 14:33
  • \$\begingroup\$ I'll mention the industrial TC conditioners to my boss - depending on their cost, they may be a better way to measure the TCs than our current method (using a CampbellSci multiplexer). \$\endgroup\$ – Keegan Smith May 15 '18 at 14:40

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