How can I verify proper grounding on my weighing system?

The importance and design of a proper grounding system is critical for your system operations, and an improper grounding system can cause intermittent outages, drifting and unexplained inaccuracies.  There are static energies developed in a scale system when product moves from station to station.  This static energy needs to have a clear and precise path to ground, away from the load cells.  Static high voltage discharges will seek the simplest and easiest paths to ground.  Depending on the path one path will receive more discharge than another, but they will all receive some of the discharge.  Making paths, other than one through the load cells, better discharge paths will minimize the amount of energy the weight controller will need to reject.  That path could be into the load cell and follow the load cell cable to the A/D converter and lock-up the system.

A weighing system is measuring a small millivolt signal from the load cell. Any Electromagnetic Interference (EMI) and/or Radio Frequency Interference (RFI) noise, static discharge or ground loops can cause the weight readings to fluctuate.  Large EMI/RFI emanations will lock-up your system. A properly designed weighing system will have a single point for earth ground to collect any energy build-up before it reaches a level that affects the A/D circuits. This means that all grounds are connected to the same ground point and therefore have the same potential.

The vessel or silo's path to ground should NOT be through the load cells.  Damage to your load cells or weight controller can occur. Ground straps should be used to bypass the load point by connecting the vessel to earth ground. This will allow any potential charge to discharge to ground and not travel through the load cells. Vessels bolted to or sitting on a concrete floor does not mean you have a common ground point. Only by measuring resistance levels can you determine if your ground system is adequate.

The shield and vessel ground should be at the same ground potential as the PLC, DCS, and weight controller. The shield connection should be verified at the summing junction box.  Perform the shield test step listed below.

Verify the shield wires are connected in the summing junction box. This is not a grounding point, only an intermediate termination. The shield terminals are only provided as a connection point to provide a through connection to a single ground point.   Verify the summing card is installed inside a metal box and that box is at ground potential. This is to provide protection from Electromagnetic Interference (EMI) and/or Radio Frequency Interference (RFI).  

Use Hardy C2® certified shielded cable from the summing junction box to the weight controller. Verify proper connections with an ohmmeter. The shield wires should not make contact with the load cell body. The shield should act as a drain for EMI/RFI noise and SD events. This shield "drain" should terminate through the summing junction box and on to the weight controller chassis ground.

SHIELD TEST

1.     At the summing junction box, measure any shield to earth ground. This reading should read less than one Ohm.

2.     If the reading is more than one ohm investigate for an open.  All the load cell shields should be terminated on the proper terminal block.  The shield for the cable run to the controller is properly terminated at the TB 7 connector and the other end tied to the controller earth ground.

3.      Return to the controller and remove the shield wire connection.

4.     Return to the summing junction box and (with all connectors installed and shield wires connec