Correct Shield Installation

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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110711-0755 EDT

Smart $:

Probably all high quality, meaning wide dynamic range, low output high impedance microphones will require electrostatic shielding and maybe magnetic shielding of the microphone and its cable. Almost always this will end up meaning the microphone is grounded at the amplifier input. In times past, before the 3 prong plug, ground might have meant simply the amplifier chassis.

Some microphones, like carbon button, are high voltage level devices in a low impedance circuit with low dynamic range and do not need shielding.

But back to instrumentation. Whether electrostatic shielding is required for an instrumentation cable is a function of many factors. Some of these are signal level, impedance, type of signal, redundancy of information, error correction, and sources of noise. Thus, one needs an understanding of the equipment and environment to make a decision on whether or not shielding or grounding are required.

In the case of a 20 mA current loop assume resolution is 0.1% and that noise should be limited to something on that order of magnitude. Then noise currents in the loop must be less than 0.02/1000 amperes or 20 microamperes. Suppose the loop resistance is 1000 ohms, then to produce 20 microamps requires an induced voltage in the loop of 20 mV. The twisted wire pair most likely will prevent this level from magnetic coupling and electrostatic shielding would not have prevented magnetic coupling. At 100 Hz to get a capacitive reactance of 1000 ohms requires 2.6 microfarads. If the voltage source to the capacitor is 200 V and this needs to be reduced to 20 mV, then approximately a coupling capacitance of 260 pfd would produce this signal level.

The geometry to producing this coupling would be very complex. Probably easily achieved if the current loop wire was in the same duct with a 480 V supply. But that is bad practice. If the signal wire was just strung across a room, then very unlikely much coupling. Shielding of the twisted pair does somewhat reduce then need to be concerned with the signal wire position relative to high power wiring.

But my guess is still that Ken9876's problem is not how the cable shielding has been done so long as the entire cable was shielded.

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GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
Does a microphone carried around by a performer have to be grounded at the point of its use, vs grounded thru the microphone cable at the amplifier input?
gar ... audio and other AC shield connections are not necessarily a ground ... but they are, IME, at a zero AC potential.

Back to the slightly off OP topic ... what I've been taught is that the best connection is USUALLY at one end only, and that that is the "sensitive" end ... thus the amplifier end with a transducer (microphone?).

I've further been taught that "always" and "never" should be removed from a control engineers' vocabulary; I've found cases where the opposite end worked better, and <gasp> where both ends worked better.

What has given consistent problems when I've been called to troubleshoot others analog wiring (usually not 4-20mA, rather +/-10 and LVDT type signals) is shield connections at a j-box. When a shielded cable comes in, has its shield connected to the box at the conduit fitting, has its signal leads connected to the outgoing cable signal leads at a terminal strip, with the outgoing cable shield connected to the box at ITS cable gland fitting ... problems arise. As mentioned by others, a shield terminal on the strip for each shield in the cable, not connected to the panel, is usually wise. I also push for maintained twisting out of the cable and for minimum practical wiring outside of the shield.

Do we agree? I find your information correct 99.44% <g> of the time, at least!

George
 

Smart $

Esteemed Member
Location
Ohio
110711-0755 EDT

Smart $:

Probably all high quality, meaning wide dynamic range, low output high impedance microphones will require electrostatic shielding and maybe magnetic shielding of the microphone and its cable. Almost always this will end up meaning the microphone is grounded at the amplifier input. In times past, before the 3 prong plug, ground might have meant simply the amplifier chassis.

Some microphones, like carbon button, are high voltage level devices in a low impedance circuit with low dynamic range and do not need shielding.

But back to instrumentation. Whether electrostatic shielding is required for an instrumentation cable is a function of many factors. Some of these are signal level, impedance, type of signal, redundancy of information, error correction, and sources of noise. Thus, one needs an understanding of the equipment and environment to make a decision on whether or not shielding or grounding are required.

In the case of a 20 mA current loop assume resolution is 0.1% and that noise should be limited to something on that order of magnitude. Then noise currents in the loop must be less than 0.02/1000 amperes or 20 microamperes. Suppose the loop resistance is 1000 ohms, then to produce 20 microamps requires an induced voltage in the loop of 20 mV. The twisted wire pair most likely will prevent this level from magnetic coupling and electrostatic shielding would not have prevented magnetic coupling. At 100 Hz to get a capacitive reactance of 1000 ohms requires 2.6 microfarads. If the voltage source to the capacitor is 200 V and this needs to be reduced to 20 mV, then approximately a coupling capacitance of 260 pfd would produce this signal level.

The geometry to producing this coupling would be very complex. Probably easily achieved if the current loop wire was in the same duct with a 480 V supply. But that is bad practice. If the signal wire was just strung across a room, then very unlikely much coupling. Shielding of the twisted pair does somewhat reduce then need to be concerned with the signal wire position relative to high power wiring.

But my guess is still that Ken9876's problem is not how the cable shielding has been done so long as the entire cable was shielded.

.
When I said the microphone doesn't need to be grounded, I meant NEC EGC type grounding. The NEC does not cover magnetic or electrostatic shielding of microphones. Nonetheless, the shield grounding of the microphone-amplifier circuit is single ended, so I don't see how you are proving anything using this analogy.

Yes, many factors are involved in whether or not shielded cables are necessary. It seems the instrumentation industry has pretty much made shielded twisted pairs the standard for 4-20mA signaling, whether necessary or not.

Also, you discuss the effects of coupling on shielded twisted pair signal wire. You have not mentioned anything about the twisting of the pairs, i.e. the number of twists per inch or such. It is my understanding this affects "noise" rejection. From what I've seen, most signaling cable used for 4-20mA instrumentation has only about one-quarter to one-half twist per inch.

I am also getting the impression you are discussing the coupling effect when the source is outside the cable. When the shield is grounded on both ends, the shield itself becomes the source of noise.

And FWIW, resolution of 4-20mA instrumentation is given as a percentage of the span, 16mA... so the comparative numbers are 20% less than yours.

Then your last sentence seems to contradict everything you have been saying, which in essence from my perspective is that grounded shielding serves little to no purpose on a 4-20mA signal circuit.
 

RichB

Senior Member
Location
Tacoma, Wa
Occupation
Electrician/Electrical Inspector
Having the shields connected at more than one point "may" cause a problem---Have seen where we needed to connect ONLY at the end in the cabinet to the ground buss and again where we needed to float both ends--Generally I was taught to try and float at the equipment and bond at the cabinet to the ground buss
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110715-1134 EDT

My DSL line has been and still is not working most of the time. So I have not responded. Need to hear from Ken9876 on what he has found.

.
 

Ken9876

Senior Member
Location
Jersey Shore
Did not look yet gentlemen, thank you for all the input. I'm going there on wed, first I'll swap the cables(connect cable 1 to resolver and cable 2 to resolver 1) and see if the problem moved or stayed with the same card and cable. I'll go from there. Is there a good source to find standards of control wiring and low voltage wiring, I really would like to get into this aspect of the trade. I really wanted info on the shields and got some good info here, this a draw bridge so there is alot of RF near by, and a cell tower. Little more info, the resolver is bonded to the shield through the connector, mil style. The resolver itself is mounted to a metal frame. There is no bonding jumper from the resolver to the mounting frame. The frame is bolted to the steel of the structure. The plc end of the cable is not bonded. The power supply is not grounded, is this what you mean by floating?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110717-1456 EDT

Ken9876:

Your swapping is a good idea. But keep in mind that moving the cables may change an existing problem if the problem is in the cable.

My definition for a floating power supply is one where the output is not conductively connected to the input. This would mean a very high resistance from any input terminal to any output terminal, and that the breakdown voltage of the high resistance isolation path would be many times the expected maximum voltage between input and output.

You have indicated that the cable shield is connected to the resolver. I will further assume that resolver mounting plate is conductive and is effectively connected to the cable shield via a low resistance path. This means that the shield will be at a potential relative to any reference point you choose defined by the potential of the mounting point to your reference point. In other words the voltage of the mounting point on the bridge to your choosen reference point.

Pick the EGC point at the PLC as your reference point. The voltage difference between the PLC EGC and the shield at the PLC will be the potential difference between the PLC EGC and the mounting point on the bridge. This voltage could be large and noisy if there is current flowing thru the bridge structure, and/or the earth and other grounding conductors between the bridge structure and the EGC point at the PLC. So both an AC and DC voltage check between the shield and the PLC EGC would be useful information.

.
 

Ken9876

Senior Member
Location
Jersey Shore
Took a closer look at this job today, found several things. One they connected the signal ref(common 24vdc -) to the shield. After reading the instructions and talking with the manufactuer I found out this was no good. Two I measured .4 vdc from egc to shield at the plc. Three, I found some shady connections at various terminals, which I corrected. Measured for voltage ac and dc between resolver and frame/mounting plate, found no voltage. I'm going to talk to the system intergrator and my engineering dept. about the shield being connected to the signal referance. Observed the operation of resolver from the plc software and viewed excatly where the fault was happening. Seems this unit may be out of cal. a bit, this device seems to increase the current faster than the unit that works correctly which is causing a fault in a compare statement in the program.
 
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