# Shielding



## MDShunk (Jan 7, 2007)

Someone will post the technical answer, but I know from personal experience that it will appear to the system as if you have no shield if you land both ends, and the results will be less than desirable. It's most typical to land it in the panel that it originates from, and not land it at the field device. I like to fold it back over the jacket and heat shrink over top of that, but other people just cut it off.


----------



## miller_elex (Jan 25, 2008)

BUT!

If you're running comm wire, bond the two shields together, isolated, with heatshring or an empty piece of cable jacket. Whatever you do, don't land the shield twice.

"There can only be one!" Just like the Main Bonding Jumper. Bond the shield at the first controller, or MS/TP router, and so on. You do not want a bunch of COMM wierdness on the job.

Alot of DDC systems like to shield analog inputs as well. It's okay to sink this one under a tech screw in the controller can. Different animal than the green screw and such. So you could twist the drains (leader in the shield,) together for the analog inputs, twist em, and put a fork terminal on them.


----------



## Big John (May 23, 2010)

I also believe it's just for ground loops. The reason it can appear "unshielded" when grounded at both ends is because of current flowing over the shield inducing common-mode noise in the signal conductors. 

My #1 concern has also been with high power ground faults: A shield grounded at both ends may inadvertently become the return path for a faulted power circuit, and I can see that being a serious fire hazard.

-John


----------



## Jlarson (Jun 28, 2009)

I don't know if there really is a more in-depth answer then to prevent ground loops. 


I do know from playing around on installs, it doesn't matter with some equipment and it does matter with other equipment.


----------



## sarness (Sep 14, 2010)

Big John said:


> I also believe it's just for ground loops. The reason it can appear "unshielded" when grounded at both ends is because of current flowing over the shield inducing common-mode noise in the signal conductors.
> 
> -John


As he said, grounding only one end eliminates any kind of current flow which could induce noise.


----------



## Speedskater (Oct 2, 2009)

We are having a similar discussion on a pro-audio forum. Some experts make a strong argument for landing all shielded at both ends, while others argue for only landing the shield at the send end, while yet another group think that the shield should be landed at the send end and a hybrid connect be made at the receive end. The hybrid being a small capacitor from the shield to the terminal, this grounds the high frequency noise current while blocking line frequency shield current flow.

In installations that have different (probably 3 phase) AC power services at each end, large power line frequency neutral currents can flow through the shield if landed at both ends, yet if the receive end is not landed then nearby lightning strikes can arc from the shield to the terminal. 

And the discussion continues....


----------



## Speedskater (Oct 2, 2009)

sarness said:


> As he said, grounding only one end eliminates any kind of current flow which could induce noise.


The noise current is always going to return to it's source.

The trick is to provide a low impedance alternate path or to make your signal cables an unattractive path.


----------



## JRaef (Mar 23, 2009)

It somewhat depends on the nature of the shielding. If discussing signal / comm wires (as I'm sure we are), then you only ground one end because what you are doing is trying to prevent EXTERNAL EM/RF noise from getting expressed onto your signal lines. But if, as in the case of VFD power cables, you are trying to STOP the radiation of EM/RF noise FROM the cables, then you ground both ends, just as a metallic conduit would be. I think because of this issue, some people get confused. So I tell my guys *"If it wants to talk, nail only the tail. If it supplies the Juice, leave no end loose".*


----------



## JohnR (Apr 12, 2010)

Exactly JRaef, If the shield is bonded at both ends, it causes small currents to flow, call it ground loop if you want to, and it acts as an antenna for RF or radio signals that through induction, pump these radio signals into your wiring.
As a multiple of 60Hz, the AM band of radio stations near 1080khz will come through loud and clear. Can drive you crazy trying to find it.


----------



## EJPHI (May 7, 2008)

A lot of good information in this thread.

A couple of things I have learned:

If you have to run a shielded signal cable between two separately derived systems and or a long distance (hundreds of feet) there will be a 60 Hz potential difference between the grounds at both ends. Bonding the shield at both ends will cause 60 Hz current to flow. This can degrade the voltage signal if a differential amplifier is not used on the receiving end. A current loop will have a high degree of immunity to induced voltage caused by 60 Hz shield current.

For rf, bonding the shield at both ends provides the best immunity and lowest emissions. Only bonding at one end for rf makes the cable look like a tuned resonator which can induce high rf currents on the internal wires.

Someone mentioned using a capacitor in series with the shield to bond it on one end while the other end is connected directly to ground. This is a good way to go if you want to prevent ground loops at 60 Hz yet have a more continuous shield at rf where the capacitor has low impedance.

The capacitor method is not fool-proof though. Most capacitors have a self-resonant frequency which depends on their value and the length of the lead. With a bit of hand waving, I'll just say that you should consider the inductance of the shield, the value of the capacitor, and the rf frequency you are trying to reject.

There are classical texts on this subject authored by Morrison and Ott. I will have to dig, but one of these authors strongly supports connecting the shield at the signal source end to reduce the 60 Hz ground loop problem. He also mentions reducing the effect of external rf interference, but his solution is to add yet another shield which is connected to ground at both ends. This second shield is quite expensive and is commonly known in the trade as EMT.


Now if you have wiring for connecting a VFD to a motor, there will be a fair amount of high frequency content at what might not be high enough in frequency to qualify as rf. The shield tied at both ends will help a lot as was mentioned. The thicker the shield the better so again maybe EMT is a good candidate. Twisted pairs/triples are very effective too because of the high currents , but now I digress...


----------



## sparkymcwiresalot (Jan 29, 2011)

If I remember correctly we connected the shield in the ring and only connected the shield on the switch side for devices. Shielded cat 6 is a real pita to terminate. Granted I haven't done much of it but getting those wires staggered just right to go into the connector is tough.


----------



## drsparky (Nov 13, 2008)

It is simple, start with 400' cable, bond the shield on both ends. Now the center (200') of the cable will be above ground potential by the resistance of half the cable length. A charge will build up at this point and any signal passing through this center point will pick up current though induction. 
When running a 4 to 20 millivolt signal this induced current my be high enough to cause any number of problems.
I got called to "fix" a problem that was cased buy a well meaning contractor bonded both ends of a shielded cable, this caused two steam valve from closing 100%. They had spent $35,000 replaced the valves and had the same problem. I opend a splice cabinet and saw the shield bonded. Just lifting the bonding screw caused the valved to shut completely.


----------

