Sheilded wiring

[glenn1340]

Hi guys,

Looking for some advice about earthing shielded communication wiring on a VRF system. Do I ground the earth shielding at the header outdoor unit only, leaving the indoor unit shlelded not grounded; only connected in series, or do I ground all the shield on the indoor units.
.
I hope that makes sense. The Toshiba manual isn`t to clear on it and I`ve looked up on the net but I`m still scratching my head.


Thanks in advance guys,
Glenn

[install monkey]

as you say- ground it at the condenser and on the indoors join the braiding together but not to the indoor units earth- terminal block it

[brunstar]

you only ground one end of the cable and it makes sense to just do it at the outdoor unit so then you know where it is grounded. If you ground it to more than one point then the electrical noise will jump from unit to unit and not ground it causing more problems than not having a ground.

[chilliwilly]

You just need to ground it at one end, on coms/voice and data you earth it at the hub/closet end. In this case you would just earth it where the invertor or main board is housed as the main board or inverter is the source of communication requesting data from ancilliaries.

[glenn1340]

Thanks very much guys. Easy to understand answers.
Regards,
Glenn

[Tayters]

Thanks very much guys. Easy to understand answers.
Regards,
Glenn

Hi Glenn,

The thinking behind grounding at one end is that if you grounded at both ends and there was a potential difference between the two points then a current would flow and possibly mess up the transmission signal.

This all made perfect sense to me until I saw a Fujitsu VRF which indicated in the install manual to earth at every point. Made me wonder if in this case it would be earthed at so many points then the previous scenario wouldn't make a difference.

So really it just left me a bit confused. I suppose my answer would be to earth at one end or if you fancy taking a leaf out of Fujitsu's book (unit in question was a V-Series and is also the same on the newer VII and JII) wire at every point.

Cheers,
Andy.

[hyperion]

If you earth the screening at more than one point, the screen can act as an "aerial" and induce interference into the data signal. Standard electronics theory from way back in my early days of playing with audio etc.

[stufus]

The reason some manufacturers say to terminate it at all indoors is just for continuity of practice.If you look closely at the terminal marked for the shield connection it is a dummy terminal (nothing on the other side) and therefore just a connection point for ease of maintaining continuity on the shield.
Cheers
Stu

[install monkey]

most observant!-no flies on you!!

The reason some manufacturers say to terminate it at all indoors is just for continuity of practice.If you look closely at the terminal marked for the shield connection it is a dummy terminal (nothing on the other side) and therefore just a connection point for ease of maintaining continuity on the shield.
Cheers
Stu

[chilliwilly]

If you earth the screening at more than one point, the screen can act as an "aerial" and induce interference into the data signal. Standard electronics theory from way back in my early days of playing with audio etc.

Are you sure your not getting confused with earth loops? Earthing both ends will still drain away induced emfs but will cause different potentials to cause a loop current on a long run or high impedance circuit.

[hyperion]

Are you sure your not getting confused with earth loops? Earthing both ends will still drain away induced emfs but will cause different potentials to cause a loop current on a long run or high impedance circuit.

Not sure about about your earth loops theory etc, but it sounds plausible, particulrly for long cable runs.
Just know that if you earth both ends of a screened cable in audio applications, this can induce mains "hum" into the signal. Normally audio signal cables are relatively short.

[soleowner]

Earth Loops are Grounding Loops which are bad for primary voltage circuits. This is a different discussion altogether.

Comm Buss shielding (signal ground) is key for preserving the printed circuit boards (PCB) from premature degradation. The termination point (signal ground)) should be on the chassis of the outdoor unit.


Electrical-Symbol-Labels-On-A-Roll-94784-ba.gif


If the system has a centralized controller, that too may have a shielded drain wire termination. Always refer to the manufacturer's installation manuals for guidance.

The indoor units are continuous has mentioned in earlier posts. The dummy terminal is essentially a wire nut.

Why do I need a shield (signal grounding):

Electromagnetic Fields (noise suppression)
Radio Frequency Interference
Shielding aka Drain wire

EMF/RFI is naturally occurring all round us and artificially generated by electrical components. The effects of EMF/RFI on the PCB is well documented. The shield wire captures, collects then drains the EMF/RFI to the termination point where it is displaced into the earth. The PCB themselves are also grounded at numerous locations. It is important to note that the building's Ground system must be of high quality. Properly shielding the comm buss is only half the work. Without a good Ground circuit, the drain wire looses potential. If the building power systems has no ground, then the PCB's will become an amplifier for receiving EMF/RFI and begin to degrade rapidly.

Warning: Do not add a Ground rods or other grounding means without engineering the affects first.

I was on a project that was having comm issues. The four year old VRF system was having a comm buss error code that would not disappear. The installing contractor was "dealing" with it for one year. I was called out and within 30 minutes found that the building's power system ground was connected to a hot water pipe. Ten faulty indoor unit boards were corroded and crusty with white salt looking residue. We changed all the IDU PCB"s and the electrical system properly grounded and now everything is okay. It has been over a year now with every still okay.

I had to write a report so I researched grounding, bonding and shielding. I learned that one way to qualify an electrical engineer or contractor is ask them what are the differences and why do you need bonding, grounding and shielding. Most do not know the correct answers.

[glenn1340]

Thanks again for all the very helpfull answers. If only manufacturers manuals could be as clear.


Regards,
Glenn

[chilliwilly]

Not sure about about your earth loops theory etc, but it sounds plausible, particulrly for long cable runs.
Just know that if you earth both ends of a screened cable in audio applications, this can induce mains "hum" into the signal. Normally audio signal cables are relatively short.

Yes I know where your coming from I'm not trying to undermine your statement, I'm just adding my experience. Radio matching circuits tend to be very long in hospitals and have many signal isolators (humbugs) installed within the circuits. This is a solution to a problem that eventually will arise due to different electrical engineers having diffferent opinions at which end should be screened, or whether or not they should actually use screened or unscreened circuits. So that when an extension of the hospital building is being carried out any earth/ground loops will be eliminated no matter which end is terminated on the extra circuits.

An indirect earth/ground connection can be picked up at the termination point at the bed head trunking, especially if its metalic segregation trunking due the nature of the band 2 circuits supplying socket outlets. This can include RS 232 "D" type data outlets or any other data outlet chasis. Some clarks of works recommend that every metalic outlet is bonded or proof that class 2 insulation has been incorporated into the design, and can be verified on a random visual inspection.

Installing the humbugs on original multiple or complex circuit installations elimiates any problems due to conflicting specifications. Especially regarding earth/ground loops and whether or not the source or ancilliary is the choice of termination.

Audio and video... ah yes I remember that from years back, its amazing that you can spend years learning about technology. And then eventually the microprocessor becomes the foundation for everything we use that needs electricity and in turn makes everything a disposable item. I bet you you feel the same way?