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Electronics & Test

An Update on Shielding

NO RATINGS
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tekochip
User Rank
Platinum
Ground Lift
tekochip   9/30/2013 11:14:31 AM
NO RATINGS
In audio, nearly every Direct Box I've ever seen has included a ground lift switch, but on stage it's nearly impossible to get clean power, and I always carried a ground lift adapter for my amp power too.

L. West
User Rank
Iron
cable shield grounding
L. West   9/30/2013 12:52:38 AM
NO RATINGS
The myth of actually shielding with the "shield" grounded at one end only (SPG) has spread throughout the EE design community.  Pseudo-technical argument instills fear of ground loops formed by cable shields grounded at both ends.  See Analog Devices, Analog Dialog 17-1, 1983, for one of the most asinine drawn out of rationales.  The ultimate insanity is when coaxial signal line shields are grounded at one end only.

A cable shield grounded at one end (SPG) behaves like a low pass filter to magnetic fields and a high pass filter to electric fieldsA shield it is not.  The SPG shield actually amplifies EMI at the ¼-wave resonance of the shield with Q ≤ 10.

In fifty years' experience, this author has trouble-shot too many failed systems where the coax signal line shields were left open at one end.  Grounding the coax shield to the line driver and receiver signal return pins with no break in continuity in between killed the noise problem every time.

Two points: (1) A shield installed for shieldingpurposes only must be grounded at both ends preferably to the box connectors; (2) A coax signal line uses the shield for signal return therefore must be grounded to the line driver and receiver signal ground/common pins with no breaks in continuity.  If there is an EMI noise problem with this, the cable should be replaced with a triaxial cable with the outer shield grounded to chassis at both ends or a twinax.

There is a real problem grounding any conductor including shields between two units operating off of two power mains in or between buildings.  Ground potential differences can be kilovolts.  Ethernet has common mode in-line isolation rated at 1.5kV rms per IEEE 802.3.  The shield is not the issue; engineers' knowledge of ground potentials, isolation, and shielding is the issue.  Floating one end of the shield only makes the problem worse.  Isolation units can be inserted to solve most problems.  Fiber optics solves the most difficult problems

With respect to the ground loop induced noise, the ratio of the induced load current to the current in the shield ground loop is Iload/Ignd loop = Z'T·l/2·Zload, exactly what a shield with transfer impedance, Z'T·l, is supposed to do, reduce the induced current below the current on the shield.  The ratio of the induced load voltage to what it would be without the shield in a magnetic field, ω·B·A, is Vload/ω·B·A = Z'T·l/(Z'T·l+ Zgnd) which boils down to L'T/L'gnd.  The shield's transfer inductance, L'T, ranges from 200pH/m to 1nH/m, over an order of magnitude lower than any external cable inductance, L'cable ≈ 200-400nH/m in the shield-ground loop (shielded cable ~3" off of the groundplane). [L'cable = (μ/2·π)·ln(h/2·r)]  Without the shield or with the shield floating at one end, the loads are in the ground loop and are excited directly.  The grounded shield only reduces the induced noise.

Now to the real problems with coax signal lines properly grounded at both ends: unless they are routed close to a structure/chassis ground/cable tray, they emit and receive more EMI than we like, particularly in the broadband mode.  Floating one end of the shield makes it worse.  Routing cables along grounded cable trays helps reduce radiated EMI problems.  If noise problems persist with coax signal lines, they should be replaced with triax or a twinax.

Cable shields should be grounded to the boxes their wires are connected.  No separate connections should be made to the telecommunications grounding busbar (TGB) or telecommunications main grounding busbar (TMGB).  "Grounding" is a misnomer here when we really mean ensure the continuity of the box shields through the cable shield.  ANSI-J-STD-607-A-2002 guides box grounding. 

When shielding cables connected to floating loads like sensors, ground the shield to the sensor case.

Bottom line: Disassociate ground loops from cable shielding.  It is a false argument devoid of correct physics and engineering of shielding and of ground loops, themselves.  If a cable shield doesn't form a ground loop, it is not a shield!!  Ground loop problems and high ground potentials need to be solved by other means like isolation.  Ground loops involve signal and power return connections.

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