Devicenet grounding

Hi everyone,
I have a question concerning devicenet grounding. The
recommended pratice is to only ground your network at
one point. Has anyone had any experience with not
grounding it. I don't see a problem by leaving it
floating but would appreciate any feedback on possible
problems. The client has a ungrounded 24Vdc supply philosopy.

Thanks.

The client has a ungrounded 24Vdc supply philosopy.

Click to expand...

So do I.

I have just finished commissioning a job today with 3 Device Net loops. The first loop is only short, about 50 metres. 16 I/O blocks in the loop all Omron units. No grounding of power supply (common between all loops).

The second loop runs down 45 floors in the building with a tap off to an AB 8 in/8 out module on each floor.

The third loop also travels down the 45 floors to 3 main switch boards and a fire control panel. There are 34 Omron Device Net I/O blocks on this loop and the thick Device Net cable is fire retardent. Do not even go near what that cost but it suited the spec.

Have also run a separate 24VDC supply to operate the outputs.

Had no problems whatsoever during commissioning.

The PLC system is Omron CJ1 (3 off networked). The system runs on 24VDC batteries and chargers (emergency power generators). I always use a 24VDC/24VDC switchmode power supply between the batteries and the PLCs. Good isolation. Grounding the output of the switchmode kills the isolation. The batteries are not grounded either. Have never had problems using this philosophy.

We don't have to many devicenet systems but I believe the they are all grounded. The shortest one that we have has about 10 devices and maybe 30 feet of cabling, all in the same enclosure.

That system was fine for about 1 1/2 yrs then out of the blue that network had issues, no hardware changes at all. Grounded the supply and the network was happy once again! Of course it happened at night but thats when I work.

If I recall the specs for devicenet are grounded and would try to convince the customer that this 24vdc is for communications not the typical I/O. Therefore the customer might agree to different specs based on the use and recommendations of devicenet.

Drewcrew6

We have Contrologix processors with DNB Devicenet scanner modules. Before we grounded them we would ocasionally get Devicenet errors that didnt really do a whole lot of damage. Our panelview + 1000 would record the Devicenet failure in the Alarm history but the line would never stop running. Very anoying. Problem completly went away when we grounded the Devicenet at one point.

I mainly use ControlLogix processors, I sometimes use Devicenet to talk to ancilliary equipment (usually torque drivers) and we ALWAYS ground the devicenet. If we don't we get spurious errors as other have described here, usually not affecting the system any more than a hiccup but annoying all the same. I would recommend grounding.

Most often the power supply is not the issue,
But the comms cable shield/screen - which should only be grounded at one point.

I have used grounded and ungrounded supplies in many instalations and not noticed much difference - unless the building main earth was of very poor quality.

But the comms cable shield/screen - which should only be grounded at one point.

Click to expand...

YES!!!!