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Join Date: Feb 2011
Location: UK
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AC Voltage Interference Over DC Control Line

04/09/2013 4:42 AM

Hello fellow CR4 members,

I have a disturbing situation with a control cable of about 200m long in star topology. It is used to control some luminaires with a digital protocol (DALI-like) and differential DC voltage 13V-22V. A 2-core screened Belden cable is used though I am not sure the screen is grounded on all branches. The problem is that I get 2-4Volts AC over the cable which is messing up the commands sent to the system causing instability, and probably putting more strain on the components on the electronic ballasts. I think that a filter could do the job but it is more than 20 years I have been dealing with electronic design last time and feeling a bit rusty now. Any ideas and suggestions would be highly appreciated.

P.S. The installation is in a aircraft maintenance installation, ultra clean environement but loaded with high tech equipment probably adding harmonics to make it worse.

Thank you

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#1

Re: AC Voltage Interference Over DC Control Line

04/09/2013 10:47 AM

First off, make sure your shield (screens) are grounded at only ONE point, preferably at the source if it is a star topology. The biggest problem in these sorts of installations is that multiple people are making connections and are not sure if they are the ones who have to ground the shields, so they ALL do, which negates the benefits of having it! Either that or they confuse the task with the grounding of POWER cable shields, where you MUST ground both ends to prevent them from radiating interference. In control signals, you want to DRAIN any induced voltages with the shield to ground. But if you ground at multiple points, you can create ground loops that can then induce their own interference into the signals. So start by assessing the shield connection points and remove any that are not at the source.

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#2
In reply to #1

Re: AC Voltage Interference Over DC Control Line

04/09/2013 11:00 AM

I am sure that some of the shields are not grounded at all but I can not be sure for ground loops until I check them myself, I have really bad experience with many of subcontractors crews. I need to check all of these connections which is not quite easy task, height of 12+ meters, and high precision equipment that needs to be moved and recalibrated. But the point is that i get the AC voltage on the 2-core part and not on the screen. I agree that the shielding could help or even eliminate EMC voltage but I would like to investigate every possible option before I start messing the whole installation there. The site is operational 24/7 and every work has to be done in a very restricted time-frame.

Thanks for the suggestions!

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#4
In reply to #2

Re: AC Voltage Interference Over DC Control Line

04/09/2013 3:40 PM

If you have access to the star point can't you just measure from there?

Check the earth continuity (screen to earth) of each of the branches at the start point (there should be a few ohms or less depending on the cable run, meter accuracy, etc), then disconnect the screen earth connection and measure the earth continuity (screen to earth) again. If you have an open circuit then good, your star point is the only earth point, if you are still measuring a few ohms or less then the screen is earthed inside the light.

Simple and no ladder needed, you may (I say may) even get away with checking the screen earthing (not safety earthing obviously) with the equipment running.

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#5
In reply to #4

Re: AC Voltage Interference Over DC Control Line

04/10/2013 5:44 AM

Unfortunately this cannot work since in some branches of the control network there are more than 2-3 luminaires in series. That is the reason for high accessing. But since we all agree that this is probably earthing problem I will have to double check I guess.

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#3

Re: AC Voltage Interference Over DC Control Line

04/09/2013 1:43 PM

Like JRaef said, ground the control line shield at one end only for low-freq interference (50/60 Hz). Twisted pair wire is also best for this type of installation. Since your wiring is already installed and you are looking for a solution for the installed system, you could try to raise your ground potential of the control signals. By raising it above the reference utilized by the AC circuits it would put the AC peak voltages under your control signals.

The chassis bonding scheme of the controllers and power supplies internal to those controllers with reference to the hotel bonding scheme (star or not) could also induce/reduce the interference seen by the control circuits.

I had a similar situation and the "control chassis" utilized a VME backplane which could have been bonded or floating (compared to reference ground). I changed the existing bonding scheme by isolating the power supply from the chassis and bonded the DC common/reference to the chassis ground. This brought the AC interference down below my analog (DC) signals.

The (problem with most) CAT cables today (is that they) are manufactured with bonds on both ends, which is fine for high speed/frequency communications, but older control schemes may be negatively affected, especially when used in concert with AC lighting.

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#6
In reply to #3

Re: AC Voltage Interference Over DC Control Line

04/10/2013 5:47 AM

This is quite interesting, I didn't think of it as an option. Thanks for the suggestion, I may give it a try if everything else fails.

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#7

Re: AC Voltage Interference Over DC Control Line

04/10/2013 10:34 AM

The presentation was fundamentally incomplete. That indicates , that the OP has his work cut out for him. Worse, he might encounter resistance from some equipment makers to his findings. With that said, let's start.

1,. Proving, and measuring single point grounding of shielding is always a bitch.

1.1,. Maintaining it with generally uncooperative equipment suppliers is an absolute necessity, and a bitch of a problem on the square.

Having these out of the way, here are my concerns:

2,. TRUE differential signalling without shield has a 30-60dB dampening from an external aggressor signal, depending on the quality of the balance. Meaning, that outside of a powerful radio transmitter, you should be ok. But...

2.1,. If you take the twisted pair as one, the Common Mode signal is as big, as it would with a single wire.

2.2,. At this point, you need to consult the manuals of all connected equipments, to see which are suspect both on generating and handling badly on the receiving side the unwanted Common Mode signal. Common Mode Suppression factor on both sides chips is a good starting point.

2.3,. The impedance for the Differential Mode wanted signal is normally 50, 75, 150 Ohms. The Common Mode one is normally (if you are lucky) much higher.

2.4,. That means, placing a few hundred Ohms resistor to the ground from BOTH terminals drops common mode, without dampening differential mode significantly. It may drop it below the threshold the receiver cannot handle.

Let's stop here. As I said, you have your work really cut out for you. I do not, nor do you know, where the problem's origin will be found. But, I bet, you will be quite vigilant in the future about the introduction anything new.

Talk with you later.

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#8
In reply to #7

Re: AC Voltage Interference Over DC Control Line

04/10/2013 12:03 PM

Ah, yes...the future...

It's always best to look toward the new installs with glittering eyes...

...then have them turn dull and dazed when they look back and wonder...why the Hell did they do that!

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