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Shielded Wire

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flagstone(at)cox.net
Guest

Posted: Wed Jan 09, 2008 1:17 am Post subject: Shielded Wire

Bob:

  

 I've read thru the various sections of the book on  grounding, wire selection, noise etc, as well as the Blue Mountain article and  the FAQ.  

  

 I already done some wiring in my wing and I'm  concerned now that I may have done it incorrectly.

  

 Electrically, my wing contains:

  

     Strobe light, power supply  mounted at wing tip

     Nav Light

     Landing light

     Taxi light     

     Pitot heat power

     Pitot heat  controller

     Fuel pump

     Fuel flow  sensor    

     Remote compass

     Potentiometer

  

 I used twisted shielded wire for all wires with  return wires for all power supplies.  It was intended that  all the return wires would terminate at what you call a "single point"  ground.  All power leads/returns are routed thru the nose ribs,  all control/sensor wires are routed along the rear spar.  They don't  come together until the panel.  The wing is all metal, fuselage is truss  tube.

  

 For each power lead, I connected the shielding and  the return wire together at the load location and grounded them locally as well  as running the return wire back to the "single point" ground.  Then,   I  was going to connect the return wire and the shielding  at the "single point" and ground again there.

  

 In reading Blue Mountain's comment, "Shields are  connected at the source end, and cut off flush at the load end" and your comment  on pg 18-14, "I'll further suggest that there's is greater risk that shielding  improperly terminated at both ends is 100x more likely to be the root cause of a  noise problem.....", indicates that what I have done may cause problems.   It was my understanding at the time that that was the correct way to do  it.  Now I can't find what led me to that understanding. 

  

 For better or worse, I'm stuck using the shielded  wire so what should I do to properly install it.  Does it cause problems to  ground the shield at both ends.  Does it cause problems to ground the  return wire locally and back at the "single point ground."

 Your comments and explanation would be  appreciated.

  

 Thanks

  

 Mark

  

  

  

  

  

  

  

  

  

    

  

   [quote][b]

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nuckolls.bob(at)cox.net
Guest

Posted: Wed Jan 09, 2008 8:34 am Post subject: Shielded Wire

At 08:31 AM 1/9/2008 -0800, you wrote:



 	  Quote: 	
		  Bob:



I've read thru the various sections of the book on grounding, wire 

selection, noise etc, as well as the Blue Mountain article and the FAQ.



I already done some wiring in my wing and I'm concerned now that I may 

have done it incorrectly.



Electrically, my wing contains:



     Strobe light, power supply mounted at wing tip

     Nav Light

     Landing light

     Taxi light

     Pitot heat power

     Pitot heat controller

     Fuel pump

     Fuel flow sensor

     Remote compass

     Potentiometer



I used twisted shielded wire for all wires with return wires for all power 

supplies.  It was intended that all the return wires would terminate at 

what you call a "single point" ground.  All power leads/returns are routed 

thru the nose ribs, all control/sensor wires are routed along the rear 

spar.  They don't come together until the panel.  The wing is all metal, 

fuselage is truss tube.


	


   It's not necessary to separate these bundles . . .

   but it doesn't hurt.



 	  Quote: 	
		  

For each power lead, I connected the shielding and the return wire 

together at the load location and grounded them locally as well as running 

the return wire back to the "single point" ground.  Then,  I  was going to 

connect the return wire and the shielding at the "single point" and ground 

again there.


	


    One and only one ground . . . local is probably fine

    but not both places . . .



 	  Quote: 	
		  

In reading Blue Mountain's comment, "Shields are connected at the source 

end, and cut off flush at the load end" and your comment on pg 18-14, 

"I'll further suggest that there's is greater risk that shielding 

improperly terminated at both ends is 100x more likely to be the root 

cause of a noise problem.....", indicates that what I have done may cause 

problems.  It was my understanding at the time that that was the correct 

way to do it.  Now I can't find what led me to that understanding.


	


   How many shielded wires do you have? Generally

   speaking, p-leads, strobe-head leads and the occasional

   avionics signal leads are shielded and the installation

   instructions for those systems should be followed for

   how to handle shield grounds. In some cases, a shield may

   be PART of a signal or power distribution path and is CONNECTED

   at both ends . . . but only one of those ends will be 'ground'.



 	  Quote: 	
		  

For better or worse, I'm stuck using the shielded wire so what should I do 

to properly install it.  Does it cause problems to ground the shield at 

both ends.  Does it cause problems to ground the return wire locally and 

back at the "single point ground."

Your comments and explanation would be appreciated.


	


   Where, if any place have you arbitrarily added

   shields where they were not called for by

   manufacturer's installation instructions?



   I am reminded of a telephone call I received from

   a LongEz builder about 15 years ago. He recited

   the fact that he had wired his airplane with shielded

   wire throughout and added a litany of devices on which

   he had added "noise filters". He ended the recitation

   with, "What else do I need?" I was somewhat taken aback

   and had to inquire, "What kind of noise problems are

   you experiencing?"



   "Oh, none yet. The airplane hasn't flown. I'm just

   trying to make sure that I don't have a noise

   problem when I'm finished."



   I had to inform him that he'd added a lot of unnecessary

   pounds and much $time$ to the effort of assembling

   his project. I had to advise him to rip it all out

   and start over but taking his cues from (1) manufacturer's

   installation instructions and (2) observations of

   the recipes for success by others - get the service

   manual for a C-172 and see what wires are shielded.



   I'm curious as to your statement about "stuck using

   shielded wire" . . . let's first explore what wires

   are shielded and why . . . and then sort out the

   best methodology for handling the shields.



   Bob . . .

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flagstone(at)cox.net
Guest

Posted: Wed Jan 09, 2008 6:14 pm Post subject: Shielded Wire

Bob:



Maybe this is a little clearer:



All the wires in the wing are shielded.



The wire used is MIL-DTL-27500TG2T14.   That particular one is 12AWG, two

conductor, twisted pair, shielded and sheathed.  The unshielded version was

not available without special order and long lead time.  The weight

difference between shielded and unshielded was 2.75lbs for 1,000 feet so I

got the shielded version.



There are no manufacturer's recommendations per se.  Sizing for loads was

done as per AC43-13-2A based on the power consumption rating for each load.



All power loads have a power lead and a return lead.  The sensors have how

ever many wires are required by the sensor.



Ignoring connectors, fuses, switches, ground buss etc, all power loads were

to be  routed as follows:



        Power lead:  Bat(+) -to- Load.

        Return lead:  Load -to- Bat (-) and,  Load -to- Airframe (at Local

Ground) -to- Bat (-)

        Shielding: - Bat(-) -to- Airframe (at Local Ground)



I can't find where I read it but something led me to believe that if you

were going to use shielded wire it was OK to do it the way I did.



So, given that I am going to use the wire I have, that I am going to run a

return lead for all power loads and that I want to hook up the shielding on

the wire, what is the correct way to do it?



Regarding the return lead, I'm pretty clear that the return lead should not

be grounded at the Bat (-) and at a Local ground.  Quote from your response:



" One and only one ground . . . local is probably fine

    but not both places . . ."



No problem changing it but why is it such a bad thing to do it that way.

Using the landing light as an example.  What harm does it cause to ground it

locally and also run a lead back to the bat (-).  Why is that worse than

only grounding locally and the corollary,  why is that not better than only

running the lead back to the bat (-).  Is the same applicable to the

strobe's power unit and the fuel pump.  Would it be the same for something

like a flap actuator motor.



Regarding the shielding:  I'm less clear on this. Removing the local ground

from the return lead will also take the shielding out of the ground path

from the Load to the Bat(-).  But what about grounding the shielding itself.

Again using the landing light as an example, what is the difference between

grounding at both ends and grounding only at the Bat(-).  Is the same

applicable to the strobe's power unit, fuel pump and flap motor.



Looking forward to your explanations.



Thanks



Mark

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nuckolls.bob(at)cox.net
Guest

Posted: Thu Jan 10, 2008 1:02 pm Post subject: Shielded Wire

At 06:12 AM 1/10/2008 -0800, you wrote:



 	  Quote: 	
		  



Bob:



Maybe this is a little clearer:



All the wires in the wing are shielded.


	




  ?Why?

 	  Quote: 	
		  The wire used is MIL-DTL-27500TG2T14.   That particular one is 12AWG, two

conductor, twisted pair, shielded and sheathed.  The unshielded version was

not available without special order and long lead time.  The weight

difference between shielded and unshielded was 2.75lbs for 1,000 feet so I

got the shielded version.



There are no manufacturer's recommendations per se.  Sizing for loads was

done as per AC43-13-2A based on the power consumption rating for each load.



All power loads have a power lead and a return lead.  The sensors have how

ever many wires are required by the sensor.



Ignoring connectors, fuses, switches, ground buss etc, all power loads were

to be  routed as follows:



         Power lead:  Bat(+) -to- Load.

         Return lead:  Load -to- Bat (-) and,  Load -to- Airframe (at Local

Ground) -to- Bat (-)

         Shielding: - Bat(-) -to- Airframe (at Local Ground)



I can't find where I read it but something led me to believe that if you

were going to use shielded wire it was OK to do it the way I did.


	


   I'm not sure I'm interpreting you words into an

   accurate schematic of your architecture but since

   the use of shielding where shielding adds no value,

   then exactly how you wire it has no significance

   with respect to performance.

 	  Quote: 	
		  So, given that I am going to use the wire I have, that I am going to run a

return lead for all power loads and that I want to hook up the shielding on

the wire, what is the correct way to do it?



Regarding the return lead, I'm pretty clear that the return lead should not

be grounded at the Bat (-) and at a Local ground.  Quote from your response:



" One and only one ground . . . local is probably fine

     but not both places . . ."



No problem changing it but why is it such a bad thing to do it that way.

Using the landing light as an example.  What harm does it cause to ground it

locally and also run a lead back to the bat (-).  Why is that worse than

only grounding locally and the corollary,  why is that not better than only

running the lead back to the bat (-).  Is the same applicable to the

strobe's power unit and the fuel pump.  Would it be the same for something

like a flap actuator motor.


	


   My personal goals for system design are to achieve

   the desired level of performance with a minimum of

   parts and minimized $time$ to acquire/install/maintain.



   You appear to be asking advice for doing things that

   I would not choose to do because they do not add

   value under the design goals stated. Doing doesn't

   necessarily degrade performance but it's certain

   not to enhance it. Are there risks for venturing outside

   the simple ideas that support conventional wisdom?

   Difficult to predict but probably not.



 	  Quote: 	
		  Regarding the shielding:  I'm less clear on this. Removing the local ground

from the return lead will also take the shielding out of the ground path

from the Load to the Bat(-).  But what about grounding the shielding itself.

Again using the landing light as an example, what is the difference between

grounding at both ends and grounding only at the Bat(-).  Is the same

applicable to the strobe's power unit, fuel pump and flap motor.



Looking forward to your explanations.


	


   A detailed trek through the physics from which

   conventional wisdom and common practices evolved

   is beyond the scope of what I can do here on

   the List. I did a short-course on noise propagation

   issues and solutions a few years ago at Hawker-

   Beech . . . and that was a 4-hour, Power-Point

   presentation.



   Suffice it to say that if my mental image

   for what you've described is correct,

   then there is no reason to expect degradation

   of performance and no reason to expect an

   enhancement of performance. I.e, shielding

   added without specific noise propagation

   problems to be solved only adds to weight

   of the airplane and cost of ownership. It

   follows then that I have no advice based

   on physics to offer for "doing it right".



   Bob . . .

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Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Thu Jan 10, 2008 2:54 pm Post subject: Re: Shielded Wire

Okay, cut this message out and punch three holes into it and stick it into your book. This is the cat's meow on coaxial cable shielding and grounding from Kimmel Gerke Associates. I hope they balance my liberal use of their stuff with the free advertising given here. 



http://www.emiguru.com/kgb/sum1996.htm     Kimmel Gerke Bullets.



"For many designers, cables and connectors are an afterthought. After all, they just connect things together, right? But cables and connectors are major entry and exit points for EMI into and out of electronic systems. They act as "hidden antennas" for radiated energy, and "hidden conduits" for conducted energy. 



"In fact, cables and connectors are so important, they are the often the first things we check out when troubleshooting EMI problems in a system. Here are a few thoughts on cable and connector design for EMI. 

Trade off shielding for filtering. If you don't filter I/O interfaces, then you must assume that the highest frequencies inside the unit will end up on the cable wires. (Or you must assume the cables will intercept the highest frequencies and bring them into the unit.)

 

"Just because that RS-232 interface is only operating in the kilobaud range doesn't stop higher frequencies from hitching a "free ride" on the cables. We've solved many radiated problems (emissions and immunity) by adding simple filters to interfaces that were only supposed to carry "slow" or "low frequency" signals.

 

"The alternate to filtering the cables is shielding the cables. We generally prefer to filter slower cables, since it's usually more cost effective and easier to maintain EMI integrity. 



"Ground the cable shield. If you do decide to use shielded cables, then the immediate question becomes how best to ground the cable shield. The correct method depends on both threat frequencies and circuit impedance levels. 



"For problem frequencies below 10 kHz [audio], the preferred approach is to ground one only one end of the shield. This is to prevent "ground loop" coupling from the cable shield to the inner wires. This is particularly important for preventing 60 Hz coupling into low level/high impedance circuits used in audio or instrumentation systems.

 

"Unfortunately, this is exactly the wrong thing to do for high frequency threats. For most RF situations [above 10 kHz], the preferred approach is to ground both ends of the cable shield, to minimize coupling at the open end due to standing wave effects. So what do you do if both threats exist? You can ground one end with a small capacitor (1,000- 10,000 pf typical) or you can use two shields - one grounded on one end, and the other grounded on both ends. In the latter case, the shields must be insulated from each other. 



"One more case where both ends must be grounded is the low frequency/low impedance case, where magnetic fields are the major culprit. In this case, the strategy is to minimize "loop size" by providing an adjacent return path for noise currents. This is often necessary in electrical sub-stations, where large 60 Hz magnetic fields exist due to the high current levels. Most of the time, however, single point grounding at low frequencies is the preferred approach. 



"Use Care With Connector Terminations. This is extremely important for shielded cables with EMI threats above about 1 MHz. Poor joints (or worse, "pigtail" connections from the shield to the connector) are a leading cause EMI-induced failures on shielded cables. We've seen 20 dB increases in emissions, and 10 fold reductions in ESD immunity due to poor connections. (Remember, with a 1 nsec edge rate, ESD is a 300 MHz EMI problem.)

 

"To maintain high frequency cable shield integrity, you need a circumferential bond between the cable shield and the connector shell. In addition you need full metal-to-metal between the mating connectors. 

Finally, you need a solid connector-to-chassis connection. One problem we often see here is with chassis connectors that overlap connector cutouts. The imprecise fit creates a slot that very effectively couples high frequency energy directly to/from the cable shield. Think in terms of a garden hose -you need a very tight connection at the faucet, or else it leaks.

 

"What About The Cable Shield Itself? Yes, this is a valid concern, but only after you have addressed the connector termination issues. For frequencies below about 10 MHz, most cable shielding materials behave about the same, and provide very respectable amounts of shielding. For frequencies above about 10 MHz, however, the cable construction becomes important. Loose single braids become increasingly leaky, so above 10 MHz, you may need high density braid shields. Double braids work very well here, as do braid over Mylar cable shields. For high frequencies, both shields should be connected together. "

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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

nuckolls.bob(at)cox.net
Guest

Posted: Fri Jan 11, 2008 8:25 am Post subject: Shielded Wire

Before folks get too obsessed with this exercise, know that

Bill Kimmel and Daryl Gerke are the acknowledged gurus of

EMI prevention and control. I've taken Bill's two-day

course and companies I work for have used their assistance

many times for solving protracted problems. I have the

course notebook on shelf above my desk.



MOST of what's addressed in this posting deals with situations

that are not found in your light aircraft electrical system and

only a few issues will be a part of your avionics installation.



Further, these "bullets" are all true but lifted out of context

of the several-to-many minutes of discussion that preceded or

followed it in the course of understanding the physics behind

the idea. "Bullets" or "rules of thumb" are exceedingly poor

substitutes for understanding, especially this set which deal

with situations that the astute system designer should have already

considered the issues before you bought the product.



This illustrates the rationale for my suggestion that the

manufacturer's installation instructions should be your golden

source for how shields should be treated and whether or not

they are even necessary.  THOSE are the guys who should have

taken Bill's course (or equal) and they've already done the

homework (EMC lab or extensive field trials where the effects

of EMI were the focus of attention).



For the general population of OBAM aircraft builders to clutch

onto these "twigs of wisdom" in the flood of potential EMI problems

is not a good use of $time$.  While the data are good, the likelihood

that the neophyte wire slinger in an RV is going to extract a

project-saving solution to a problem is small and the expenditure

of $time$ will be significant.



If anyone is motivated enough to acquire some skills in this

arena, then it needs to be supported by the sum total of books

and articles from which these gems of wisdom were plucked and

probably the support of a capable teacher to help assemble the

simple-ideas into good inventions. If you do encounter some form

of EMI problem in your project, the likelihood of cherry-picking a

solution from the list below is small.



Bob . . .

At 02:54 PM 1/10/2008 -0800, you wrote:



 	  Quote: 	
		  



Okay, cut this message out and punch three holes into it and stick it into 

your book. This is the cat's meow on coaxial cable shielding and grounding 

from Kimmel Gerke Associates. I hope they balance my liberal use of their 

stuff with the free advertising given here.



http://www.emiguru.com/kgb/sum1996.htm     Kimmel Gerke Bullets.



"For many designers, cables and connectors are an afterthought. After all, 

they just connect things together, right? But cables and connectors are 

major entry and exit points for EMI into and out of electronic systems. 

They act as "hidden antennas" for radiated energy, and "hidden conduits" 

for conducted energy.



"In fact, cables and connectors are so important, they are the often the 

first things we check out when troubleshooting EMI problems in a system. 

Here are a few thoughts on cable and connector design for EMI.

Trade off shielding for filtering. If you don't filter I/O interfaces, 

then you must assume that the highest frequencies inside the unit will end 

up on the cable wires. (Or you must assume the cables will intercept the 

highest frequencies and bring them into the unit.)



"Just because that RS-232 interface is only operating in the kilobaud 

range doesn't stop higher frequencies from hitching a "free ride" on the 

cables. We've solved many radiated problems (emissions and immunity) by 

adding simple filters to interfaces that were only supposed to carry 

"slow" or "low frequency" signals.



"The alternate to filtering the cables is shielding the cables. We 

generally prefer to filter slower cables, since it's usually more cost 

effective and easier to maintain EMI integrity.



"Ground the cable shield. If you do decide to use shielded cables, then 

the immediate question becomes how best to ground the cable shield. The 

correct method depends on both threat frequencies and circuit impedance 

levels.



"For problem frequencies below 10 kHz [audio], the preferred approach is 

to ground one only one end of the shield. This is to prevent "ground loop" 

coupling from the cable shield to the inner wires. This is particularly 

important for preventing 60 Hz coupling into low level/high impedance 

circuits used in audio or instrumentation systems.



"Unfortunately, this is exactly the wrong thing to do for high frequency 

threats. For most RF situations [above 10 kHz], the preferred approach is 

to ground both ends of the cable shield, to minimize coupling at the open 

end due to standing wave effects. So what do you do if both threats exist? 

You can ground one end with a small capacitor (1,000- 10,000 pf typical) 

or you can use two shields - one grounded on one end, and the other 

grounded on both ends. In the latter case, the shields must be insulated 

from each other.



"One more case where both ends must be grounded is the low frequency/low 

impedance case, where magnetic fields are the major culprit. In this case, 

the strategy is to minimize "loop size" by providing an adjacent return 

path for noise currents. This is often necessary in electrical 

sub-stations, where large 60 Hz magnetic fields exist due to the high 

current levels. Most of the time, however, single point grounding at low 

frequencies is the preferred approach.



"Use Care With Connector Terminations. This is extremely important for 

shielded cables with EMI threats above about 1 MHz. Poor joints (or worse, 

"pigtail" connections from the shield to the connector) are a leading 

cause EMI-induced failures on shielded cables. We've seen 20 dB increases 

in emissions, and 10 fold reductions in ESD immunity due to poor 

connections. (Remember, with a 1 nsec edge rate, ESD is a 300 MHz EMI problem.)



"To maintain high frequency cable shield integrity, you need a 

circumferential bond between the cable shield and the connector shell. In 

addition you need full metal-to-metal between the mating connectors.

Finally, you need a solid connector-to-chassis connection. One problem we 

often see here is with chassis connectors that overlap connector cutouts. 

The imprecise fit creates a slot that very effectively couples high 

frequency energy directly to/from the cable shield. Think in terms of a 

garden hose -you need a very tight connection at the faucet, or else it leaks.



"What About The Cable Shield Itself? Yes, this is a valid concern, but 

only after you have addressed the connector termination issues. For 

frequencies below about 10 MHz, most cable shielding materials behave 

about the same, and provide very respectable amounts of shielding. For 

frequencies above about 10 MHz, however, the cable construction becomes 

important. Loose single braids become increasingly leaky, so above 10 MHz, 

you may need high density braid shields. Double braids work very well 

here, as do braid over Mylar cable shields. For high frequencies, both 

shields should be connected together. "



--------

Eric M. Jones

www.PerihelionDesign.com

113 Brentwood Drive

Southbridge, MA 01550

(508) 764-2072

emjones(at)charter.net

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