Access the metal armature of a sensor cable for conecting to other conducor... Realistic?

[DanielFuente]

Hi everybody, I have a cable for sensors in certain system. I would like to connect the metallic armour of the cable to other main conductor just for beeing sure that there will not be voltage differential in some circunstances, like lightning.

Is there some easy way for doing that?
Which elements do I need for manage that?
Would it be difficult to cut manually the cable to access the metal reinforcement?
What is the best way to access the armor to connect to other drivers?

Some manufacturer recommended?

I am open to any idea...

Thanks a lot!

 

[VEBill]

In general, one would separately earth ground that outermost part of the cable. Connecting it to any of the signal conductors is generally a bad idea.

Disclaimer - your description isn't entirely clear, so I'm guessing at the details of your situation.

 

[DRWeig]

Agree with VE1BLL, we don't really know enough about the sensor system to be of much help. I, too, would not ground one of the sensor cables unless it was designed to be grounded.

In general, we ground the shield (armour) at one end only. If lightning is a concern, you would place a lightning arrestor or other heavy surge protector between each signal wire and ground, and between the two signal wires, at the same point that the shield is grounded.

Tell us what is on each end of the signal wires (controller, sensor, valve?) and perhaps we can be more specific. A diagram would be even more helpful, especially if it shows the general routing of the signal cable (outdoor / indoor transitions).

Best to you,

Goober Dave

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[DanielFuente]

Yes, sorry.
It it´s the cable

http://www.hitechcontrols.com/cables/control_cable/pur_shielded/pur_c_pur.html

It is going inside a big wing of a prototype plane for sensors of temperature pt100 and thermostat... and it is close to the conductor for lightning. When the test current from lightning goes through the down conductor are inducted some overvoltages. And that is the reason for needing to fix it to the equipotential system.

Thanks for your answers.

 

[DRWeig]

If the distance between the pt100 and its transmitter is long, I assume you're either using a 4-wire configuration or else the transmitter is at the sensor and your signal in the cable is 4-20 mA?

In the case of 4-20 mA signal and thermostat (on-off I presume), you can apply surge suppressor or lightning arrestor at the controller end. In the case of a 100pt sensor wire, the impedances of a surge suppressor may produce an unacceptable offset -- you'll have to study that one.

In either case, if you prefer to ground one side of each device, I suggest doing so at the controller end only in order to avoid ground loop currents during normal operation.

Best to you,

Goober Dave

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[DanielFuente]

Thanks for your answers guys.
Yes DRWeig, the distance to the transmitter is quite long.

And there are some other cables close to the ones for the sensors, so it is neccesary to prevent the influence on the sensor signals.

Probably if I need to access to the shield not only at the beginning and at the end of the cable, but also in some other middle points, probably just will cut the cable and use some connection stuff, bypassing the shield and connecting also it to the equipotential surface... Do you think there should be some other less invasive solutions?

Thanks again.

 

[DRWeig]

I guess the thing that concerns me most is the effect of grounding of the pt100 wires at multiple points during normal operation. The surface that you call equipotential won't really be equipotential over long distances. I'm afraid of adding small amounts of ground loop current to the RTD wires and thus skewing your transmitter input to one extreme or the other. There will be only microamps in the RTD loop, easily upset by the magnitude of ground current that can be caused by multiple grounding points.

The thermostat conductors should be less of an issue, or no issue at all.

HOWEVER, I'm not familiar with how things are done in aircraft. Perhaps one of the aerospace engineers will come across this thread. If not, you might consider posting a link to this thread here: Aerospace Engineering Other Topics

Best to you,

Goober Dave

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[VEBill]

"...probably just will cut the cable and use some connection stuff, bypassing the shield..."

Your instincts are (again) not aligned with E3 (Electromagnetic Environmental Effects, i.e. EMI/EMC) Best Practice. Continuous unbroken shield is considered to be much superior to a broken-up, intermittent shield as you seem to be proposing. Those little gaps in the shielding are the weak points that will allow increased coupling onto the signal wires.

"...It is going inside a big wing of a prototype plane..."

There must be wiring standards for this prototype aircraft. In general, the same standards would be applied (within reason) to temporary wiring. Those standards should call up applicable E3 wiring practices.

 

[IRstuff]

What are your accuracy requirements? Are you trying to prevent damage, or trying to operate through a lightning strike? Why, if the latter?

Why not convert to fiber?

TTFN
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[DanielFuente]

You are right, it is not the best idea to cut the cable.

What I really want is to prevent the overvoltages inducted when there is a testing lightning stroke in the system... that is the reason for trying to connect the shielding of the sensor cable to one equipotential system...

Probably it is not a tipical situation. And I am thinking about the easiest way of doing it. Maybe just cutting the plastic outside of the cable and use a ring over the nude shield for fixing the connection cable to the eq. potential system.

Thanks for your opinions,
I apreciate them a lot.

 

[IRstuff]

Better shielding, better grounding; ideally find a solid local ground to suction off the induced currents. If signal integrity is that bad, then some sort of buffer or even digital conversion would be preferred.

TTFN
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[DRWeig]

I think the OP is at this time trying to pass a qualification test -- if lighting test current is going through the nearby ground conductor, what voltage relative to ground is induced on the signal cable shield?

I think a grounded metal separation might be in order (if possible) between the lighting down conductor and the signal cable... In other words, shield your shield. The separation can be grounded all along its path without worrying about making multiple grounds along the cable shield and thus making it susceptible to ground currents during normal operation, not to mention a lightning stroke.

So, can you install a metal barrier between the lighting down conductor and your cable? Can your cable pass through a metal tube that is grounded in multiple places?

Just my thoughts.

Best to you,

Goober Dave

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[VEBill]

I've seen double-shielded cables overbraided with a third shield (!) just to make sure.

 

[zappedagain]

Intermark makes a series of Frame Grounding Clamps (FGC) that they designed for use in the middle of cables. I tried it once as a shielding clamp at the ends of a shielded cable and was disappointed in the RF shielding that it gave. A full shield works much, much better. The FGC may be reasonable for lower frequency applications.

Z

 

[DanielFuente]

Thanks a lot.

I found also other solutions like the SHIELD-KON by LappKabel, and also the SKINTOP series, but they don´t adjust perfectly to the range of temperatures that I would like.

The Intermark is really a simple and effective idea but in my situation as it is made of plastic does not fit all rigth. I will think about even making my own Clamps, as it is not a so difficult dessing.

If you know some other manufacturers, I would be pleased to know it.

Thanks a lot,
David.