Problem with noise on data cables

Hello koch.jn

It is important to ground the screen of data cables at one end only.

This is to prevent that screen taking mains power or other current in case of a fault to ground in Mains or Low Voltage, Medium Voltage wiring.

In all Data and IP installations I have ever been involved in, those screen grounding wires arrive back to a central point, all insulated from all other metal.

From that central point, an insulated wire runs to a SEPARATE ground electrode, driven at least 30 feet from any power mains grounding rod.

That way, no problems can develop.

Trust you feel assisted.

Kind Regards....

I have found that if you cannot get a good ground by knocking in stakes or using a ground mat, eg in sand, then a seperate low resistance cable connected to the star point of the supply transformer will suffice, same as IS earthing,

Which end of the cable should be grounded, I have a Gateway at one end that connects to many Multilin IED's.

Thanks.

Hello cdelareza

The grounding should always be back at the central point.

There should not be any way that a stray Mains Power fault current could actually flow in that earthed screen, ever.

So if you have a central Logger, Computer etc we shall call A, and a junction Box called B, and a further subjunction Box run from Box B, we call that sub-junction Box C, then an instrument or probe D, the screen circuit of each Data or Instrumentation cable goes outwards, connecting A -> B -> C -> D.

The metal case of the Central Logger or Computer, should be connected to power Mains Earth, in the normal way.

The metal cases of B, C, D, may be separately connected to that central Logger/Computer Mains Earth, at the Logger/Computer only.

The screen earths must always be not connected anywhere to Mains Earth, unless back at the Central Logger or Computer (Depending on local Regulations).

Refer my earlier Post:http://cr4.globalspec.com/comment/253476/Re-Problem-with-noise-on-data-cables

The reason for total isolation of Mains Power Earthing (Grounding) and screen cabling, (Except at that single Central Point), is that in case of any Mains Power, or Power Transformer fault, no fault current could pass through the screen wiring, causing it to burn out, damage equipment, cause a fire, or electrocute persons.

If you're still unsure what I mean, I shall draw you a (rough) diagram, and Post it here for you.

Reply here, advising you now understand, or otherwise, with

Kind Regards....

Hi all,

as sparkstation says correctly it`s important to have only ONE end grounded. But unfortunately it`s not always the same side - I`ve had applications where both variants had to be tested and that one with the best results was chosen.

If you have VERY sensible data line receivers, especially with analog transmission, and a VERY noisy industrial surrounding it may be necessary to use double-shielded cables, which may be twisted (for differential signals) or triaxial (for single sided signals). It`s a more expensive way but will give you (mostly) excellent results.

Double shieldings have preferably to be connected as follows:

1. The outer shield has to be connected on BOTH SIDES TO GROUND and is intended to form a ground loop in which the induced currents are shortened.

2. The inner shield has to be connected on ONE SIDE TO GROUND (test out which) and is intended to form a shield against the induced voltage of the outer current loop plus the external in-coupled voltages.

Regards Uwe

The wiring regulations/codes in many countries require that any technical earth or functional earth is ultimately bonded with the mains earth/ building structure and other incoming services at the point of entry so a totally separate earthing/grounding system for data would often be illegal. In the past totally separate earth systems were often specified for audio/broadcast systems but these raised a whole host of safety and functional issues and are now illegal in most areas.

As Sparkstation has said grounding the screen at one end only is the usual procedure. There are some environments where the other end may need to be connected via a capacitor because of RF issues but it would need an expert on site to determine if special measures are needed.

Depending on the sources of the disturbing signals ( voltages and frequency) the best procedure would be:

1. use only one point for grounding

2. eventually put the signal cables into metal pipes, which are grounded

3. why do you not consider to use glass fibre cables ?