Shielding and grounding of Profibus

What measures do you take on your Profibus networks? Do you take special measures as grounding the shield of the profibus cable to mounting plate of the cabinet? If so, is the shield only connected to ground on the entrance and exit of the cabinet, or is it grounded at each profibus node? Do you install eqiupotential ground cables between the cabinets with a large cross-section (f.i. 10 mm2 or more)?
Or do you take other measures?
Did you ever have profibus problems? What was the cause, and what was the solution?

Thanks

bump

im interested in opinions too

Hello guys;
Since no one else is reacting, let me start the discussion.
Ever since I found this little Profibus Cabling Guide (edited by the PIC)I have been using it to teach my automation team the basics of Profibus-DP copper installation. It is a great review of the techniques required for simple and correct installation of Profibus wiring.
It can be downloaded (amongst other interesting documents) here:
http://www2.sea.siemens.com/PIC/Support/PROFIBUSandPROFINET-Downloads.htm
It discusses shielding and grounding, simply stating that shields must be well connected to good Profibus connectors, and that the grounding of the node (PLC or Slave) should be done so: "
To prevent EMC interference from entering the device, the cable shield should be connected to the functional ground of the device (generally the electrically conductive case)."

Basically, most of the issues I have ever had with profibus networks, I would say 95% of them, have come from faulty cable installation, selection and engineering design. You will save plenty of time and grief if you simply:
-respect the maximum length of a segment (limited by the bus speed selected) and the number of nodes allowed on a single segment before a repeater is required (this is a design issue)
-select high-quality Profibus-certified cables and connectors (this is a selection issue)
-ensure shields are properly connected to the grounding plates of the Profibus connectors (installation issue)
-make sure the color code of the Profibus internal wires is repected throughout the network installation
-keep a Profibus diagnostics tool handy when checking a site installation; we use Siemens' BT200 portable tool, handy, light and good for most basic problems. We have used ProfiCoach for more complex issues, a lot more expensive but nothing is left to imagination: oscilloscope reports, software analysis, unmatched results.
Let's see what others have to add...
Hope this helps,
Daniel Chartier

Plc_User said:

What measures do you take on your Profibus networks? Do you take special measures as grounding the shield of the profibus cable to mounting plate of the cabinet? If so, is the shield only connected to ground on the entrance and exit of the cabinet, or is it grounded at each profibus node? Do you install eqiupotential ground cables between the cabinets with a large cross-section (f.i. 10 mm2 or more)?
Or do you take other measures?
Did you ever have profibus problems? What was the cause, and what was the solution?

Thanks

Click to expand...

Profibus: Yes, we ground the shield of incoming and outgoing profibus cable to the mounting plate of the cabinet.
Only at these points, even if there are more than one node in the cabinet. Anyway .. the profibus cable being pulled inside cabinet are off course properly separated from other cables, and only placed within cable trays with max 24Volt cables.

Eqiupotential: Yes and No. If I do the hardware design/cable sheet I always specify a 16mm2 cable to be pulled paralell with all prfibus cables.
Unfortunately not of all my colleagues are doing it.

Problems: Ohh yeah .. lots of
Some examples:
1.Poor connection of the shield of the profibus within the profibus connectors.
2.Profibus connectors of too bad quality, I prefer to use Siemens "Fast Connect" connectors whenever possible, never had any problems with these connectors.
3.Profibus cable not being separated properly from power cables along cable tray, the noise from the power cables (fed by a VFD) disturbed the profibus.
4.Profibus cable are pulled a too long distance, have to install repeaters or decrease baud rate to have a stable communication.
5.Missing equipotential between nodes caused alot of problem in a project a few years ago, we had the ground cable in our cable list .. but for some reason the electricians hadn´t pulled it. During first week of production we had ALOT of problems with nodes that for short time were not present. Electricians work day and night pulling the ground cable .. after connecting it to all cabinets/boxes .. TADAA .. no problems since that!
In this project we were using some older Siemens nodes (ET200U) also and I think that the earth connection of the backplane were not so good as of the newer nodes and that caused the big problems together with a not good enough separation of profibus cables from power cables (difficult since it was an existing machine line).

So my experiences is that you can never be too careful when working with design and commissioning of profibus networks.

Guys I don't want to divert this thread.... but having said that... my plant is planning a fiber ring for profibus now.

Does fiber eliminate many of these issues and/or create others?

Anyone know of good info links? It would be appreciated.

TIA...Von

Von said:

Guys I don't want to divert this thread.... but having said that... my plant is planning a fiber ring for profibus now.

Does fiber eliminate many of these issues and/or create others?

Anyone know of good info links? It would be appreciated.

TIA...Von

Click to expand...

Off course the fiber will eliminate the "noise" from other surroundings cables and equipment that is a very common problem.

I have only small experience with fiber, working with Siemens OLM a few times and they´ve done a good job = no problem just working as supposed.

Yes, my experience is the same, you have to ground Profibus at a number of locations, not just one. But an incoming cable into a cabinet is sufficient to be earthed once.

I remember a Hannover fair booth, where we did a temporary Profibus installation, you know one of the kind, that people don't see and just need sto work for one week. It didn't. Pulling anumber of 16 mm2 earth cables connected to common ground made it work .

The experience we made with fibre is positive looking at the noise reduction. But once people have to repair it, after installing it years before, they fail. You need to be experienced in connecting fibre. In my experience, it much easier for the electrician to handle copper.

2 Issues I know abut Fiber and Copper Profibus

First thing is about fiber. Unless the fiber is glass it will deteriate over time. I do not mean just the connection points. The plastic itself degrades over time. You get about 10 years from the plastic fiber. After 10 years the whole cable should be replaced.

FYI...I know this to be true because I have measured the light throughput on a 10+ year fiber compared to a brand new one.

Second thing is about Copper Profibus. If you are not using a Profibus Connector (the Sub D 9 Pin thingy). Ferrules are your friend. I unweave the sheilding and then twist into a wire, insert the end into a ferrule and now I have a solid connection that won't wiggle apart. Soldering can owrk also but then you need an iron and it is real time consuming.

If I am using the Profibus connector from Siemens then I use the fast connecting stripping tool Part# 6GK19056AA00. This tool strips the sheild weave to just the right length so I know it hits the ground plate in the connector and no where else.


One other thing to note. Sheild grounding is only as good as the grounding network itself. If you are creating noise on your ground network (example a motor VFD driven being grounded wrong) this can effect your Profibus signal.

FYI... DeviceNet is just as touchy.

powering PLC and repeater from different PSUs

What about powering PLC and repeater from different PSUs? Should I expect some problems (in case they are isolated, in case they are not isolated, one is and another is not)?
I'm in situation when I need to connect more than 32 profibus nodes, so I need repeaters. Repeaters will be in eletro switchgear room and they will be powered from local 24VDC PSU. I do not know, whether M terminal is grounded or not.
Grounding networks of DCS and Electro are separate to the point where they are connected together into plant grounding network (in the basement of the building or between two buildings I suppose).
Thank You for comments.

Hi
I am not an expert but I would make sure the grounding is tied toether, and link the 0V to ground at all your panels. Maybe check with your equipment supplier regarding installation techniques

Cheers

I have had two occasions where the Siemens Profibus connector failed giving me no signal, what a pain to trouble shoot. Both of these were the metal style straight connectors, I never had a problem with the plastic "piggy back" style.

cjd1965 said:

Hi
I am not an expert but I would make sure the grounding is tied toether, and link the 0V to ground at all your panels. Maybe check with your equipment supplier regarding installation techniques
Cheers

Click to expand...

I would create ground loop in case the grounding of electro part and DCS is connected by connecting them with one more wire to equalize potentials, would I not?
What to do this when the grounding of DCS or electro part is interconnected in the building and there is still potential difference (badly made grouding grid)?

Does each building have a dedicated earthing point in your country?

It would be upto your end user to make sure their 'earth' points are effectively earth.

You can do earth loop impednce tets with meters and check it is ok if you need to

earthing

There is usually plantwide earthing network, when building is part of wider plant (when it is newer one). I think best solution will be just to measure potential between grouded shield (grounded on DCS side) and earthing of the target cabinet. When there 0V, there will be no problems. Am I right?

yes, you want the potential difference to be 0V, and make sure the earthing is all linked, and ground the profibus cable shield and profibus devices inside each contol panel ( the grounds must be 0V to earth also )

recently in the last few days a thread was added with a link to installing profibus, search for it (bloke504 i think)

cheers
Cheers