Terminating Shielded Wire Grounds, Solder Sleeves

[Bartman]

Came across the video below which explains how to terminate and join the grounds for a shielded, multi-cable wire using a solder shield. This would be used where a shielded wire is required and where the braided shield needs to be terminated and joined to either the connector for ground or to a ground terminal pin. I found this requirement in reading the installation instructions for the Garmin GSB-15 USB power hub which specifies a three wire shielded cable with the shield being terminated to the case of the device using a ring terminal (See note 3 & 4 in the diagram below the video box). Having never heard of a "solder shield" before, I did a quick google search and happened to find a really good video on the first try!

Note: There are comments under the video (if you go to youtube to watch it) specifying that the shield should be folded back over the insulation before the solder sleeve is put into position and that this is industry standard practice. That step is missing from the video.

 

[Bartman]

Actually, in reading the diagram above, isn't there an error in the diagram? Shouldn't Note 4 be next to the shielding termination symbol closer to the connector instead of the far one? And wouldn't the far end of the shield terminate to the connector on the opposite side of the cable run? I'm assuming the actual "USB Ground" wire is one of the three cables inside the shield and is routed to the USB ground at the other end of the cable where there would be another USB connector.

 

[Hiperbiper]

Ok,
Schematics are not to scale so it doesn't matter where the shield pick off point it drawn, it's all the same potential.
Next; don't confuse "ground" as in power and ground with pegging the shielding of the cable..positive 12vdc and ground (Hi and Low in electronics speak) are what power the radio. The shield (the Bleed Lead) is terminated to the case of the device (one end only) to maintain a common bias with the radio innards and the cabling to keep the signal wires from acting like antennas and mucking up everything around them. Most times the shield is tied to the connector at the device end. The 3 wire USB will have an up, a down and a signal ground running between the cable ends only. The shield is simply an RF "overcoat" to to keep the signals clean.
Chris

 

[Bartman]

Thanks for chiming in Chris. Generally speaking, I don't understand what goes on in wires and pipes so your efforts to clarify and to keep the rest of us pointed in the right direction are appreciated!

 

[Bartman]

Interesting blurb from the Garmin GTR-200 install instructions;

2.7.1 Noise

As audio signals are routed to and from the GTR 200/200B (Headset, Microphone, Music, AUX), care
must be taken to minimize effects from coupled interference and ground loops.
Interference can be coupled into interconnecting cables when they are routed near large AC electric fields,
AC voltage sources, and pulse equipment (strobes, spark plugs, magnetos, EL displays, CRTs, etc).
Interference can also couple into interconnecting cables by magnetic induction when they are routed near
large AC current-carrying conductors or switched DC equipment (heaters, solenoids, fans, autopilot
servos, etc).

Ground loops are created when there is more than one path in which return currents can flow, or when
signal returns share the same path as large currents from other equipment. These large currents create
differences in ground potential between various equipment operating in the aircraft. These differences in
potential can produce an additive effect at audio signal inputs.

The GTR 200/200B audio inputs may detect the desired input signal plus an unwanted component injected
by ground differentials, a common cause of alternator-related noise. This can be minimized by isolating all
audio jacks from ground.

Terminating shields at just one end (single-point grounding) eliminates another potential ground loop
injection point. The single-point grounding method is critical for the installation of various avionics that
produce and process audio signals. Single-point, in this context, means that the various pieces of
equipment share a single common ground connection back to the airframe.

Good aircraft electrical/charging system ground bonding is important.
The wiring diagrams and accompanying notes in this manual should be followed closely to minimize noise

 

[Bartman]

hey guys, the d-sub connector for the av30 only has a couple of wires coming out of it so there's a lot of excess room. can i fill it with a glob of silicone or something to keep the wires from abrading against the connector? maybe hot glue?
thanks

 

[Bob Turner]

I have the same question - would the acetic acid in silicon sealant corrode the solder?
In the olden days there was "potting compound" -used to stabilize wires in a Cannon connector. Current suggestions?

 

[Hiperbiper]

hey guys, the d-sub connector for the av30 only has a couple of wires coming out of it so there's a lot of excess room. can i fill it with a glob of silicone or something to keep the wires from abrading against the connector? maybe hot glue?
thanks

What do you mean by abraiding against the connector?

 

[Bartman]

the hole at the back of the sub-d is maybe a quarter inch but only two 22 ga wires are exiting it. i'd like to fill the gap with something to stabilize and support the wires. even the little strain relief clamp that goes just inside the exit isn't clamping on the two small wires. the connector is made to accommodate a large number of wires and is more suited for when there are many wires exiting it. i'm just using the two power wires but I'd still like for them to be supported exiting the body of the D-sub connector.

 

[Big Ed]

My layman's viewpoint is follow the installation instructions. Certified devices are usually pretty specific and anal about that. Zip tying the wire to something solid a few inches after it exits the D-sub should take care of strain relief.

 

[Bartman]

there isn't anything in the directions that addresses it but if you look at the video at about 1:05, you can see the few wires exiting the canon plug are pretty well pinned in place so there was something employed there to make it like that.

 

[Bartman]

Here's a link where it's mentioned

Wire Termination & Connector Standards

Essential guide to aircraft wire stripping, crimping, and connector installation. Learn about terminal strips, junction boxes, and coaxial cable care.

www.aircraftsystemstech.com

Sealing
Connectors must be of a type that excludes moisture entry through the use of peripheral and interfacial seal that are compressed when the connector is mated. Moisture entry through the rear of the connector must be avoided by correctly matching the wire’s outside diameter with the connector’s rear grommet sealing range. It is recommended that no more than one wire be terminated in any crimp style contact. The use of heat-shrinkable tubing to build up the wire diameter, or the application of potting to the wire entry area as additional means of providing a rear compatibility with the rear grommet is recommended. These extra means have inherent penalties and should be considered only where other means cannot be used. Unwired spare contacts should have a correctly sized plastic plug installed.and

Wire Support
A rear accessory back shell must be used on connectors that are not enclosed. Connectors with very small size wiring, or subject to frequent maintenance activity, or located in high vibration areas must be provided with a strain-relief-type back shell. The wire bundle should be protected from mechanical damage with suitable cushion material where it is secured by the clamp. Connectors that are potted or have molded rear adapters do not normally use a separate strain relief accessory. Strain relief clamps should not impart tension on wires between the clamp and contact.

 

[Bartman]

and from AC43.13

b. Installation Precautions for Small
Wires. As a general practice, wires smaller than size #20 must be provided with additional
clamps, grouped with at least three other wires, and have additional support at terminations,
such as connector grommets, strain-relief clamps, shrinkable sleeving, or telescoping
bushings. They should not be used in applications where they will be subjected to excessive
vibration, repeated bending, or frequent disconnection from screw terminations.

 

[Bob Turner]

In your application I bet a nice chunk of sponge rubber or split rubber hose in the clamp would replace all those missing wires.
On the -200 shell block, there is a relatively large clamp, and I believe the manual suggests some kind of rubber strip to avoid metal clamping down on the wires.

 

[Bartman]

I thought about a short length of silicone tubing but that stuff's slippery where natural rubber won't react with the wire insulation and it stays put better. I agree, a little rubber hose ought to do it.

 

[Hiperbiper]

I've used a trick to make a more solid strain relief when there isn't one inherent in the connector itself...
Put a piece of tubing (I use heat shrink cause I've got tons) around the wire(s) that is slightly larger than the clamshell opening. Now, before you close the connector up fill the tube with hot-melt glue. Now close the shell and tighten the screws! After the hot glue stuff dries you have a neat little strain relief but with the ability to open the connector in the future should you need to.
And as Bob mentioned above; silicone is a no-no around electrical connections...
Chris

 

[Bartman]

it would have just been around the wires but i've put a short length of rubber tubing around the wires where the strain relief clamp is and it seems ok.

good idea with the hot-melt glue.

thanks for the replies guys!