Specialised static grounding systems
28 June 2013
A specialised static grounding system, such as Newson Gale’s Earth-Rite MULTIPOINT, provides the benefits of eight discrete static grounding systems rolled up into a single package. This gives installers the flexibility of providing monitored static grounding protection for multiple components of powder processing assemblies through a discrete wall mounted monitoring unit with eight ground status indicators for each component being monitored.
Earth-Rite MULTIPOINT monitoring unit
In addition, the system will continuously check that all components are connected to a reference earth grounding point, thus ensuring that the ground path resistance between the process equipment and the reference ground never exceeds 10 ohms. A monitored ground path resistance of 10 ohms or less is what is recommended in NFPA 77, “Recommended Practice on Static Electricity” and Cenelec CLCTR: 50404 “Electrostatics - Code of Practice for the Avoidance of Hazards due to Static Electricity”.
If the monitoring unit detects that an assembly component is not grounded, it will send a signal to the controller which, if interlocked with the circuit powering the operation, can halt the process, thereby eliminating the electrostatic charging mechanism and potential charging of un-grounded equipment.
If such an event does occur, the plant’s technicians can rapidly identify which connection needs to be investigated. They can do this by referencing the monitoring unit’s ground status indicator panel which will indicate which channel needs to be checked. Once the connection to the equipment is re-established the controller will provide a permissive condition for the process to start again.
Systems such as these are a flexible solution designed for the powder processing industries. The Monitoring Unit, which consists of 8 green and 8 red panel mounted LED indicators, fitted inside a stainless steel enclosure, can be positioned in a location that is easy to wire to and from, but is also easily accessible for technicians to observe if the LEDs linked to their corresponding monitoring points are green or red. Should any of the red LEDs be indicating a break in continuity with plant equipment the technician can check the corresponding location to investigate if the connection to the equipment is not correctly made or if there is a fault with the monitoring cable.
Connections to plant equipment can either be permanent or temporary. Permanent connections are suitable where the plant equipment remains in place for relatively long periods of time. The most common connection is via two conductor cable with eyelet terminations made by both conductors to the equipment that must be grounded. This ensures the equipment is on a 10 ohm ground path loop back to the Monitoring Unit.
Figure 1 below shows an example of an Earth-Rite MULTIPOINT installation.
If connections are temporary, i.e. where mobile totes and drums are in use, then two pole grounding clamps can be used to ground the totes or drums when they are being charged or discharged with powder.
Figure 1. An example of an Earth-Rite MULTIPOINT installation
The most common set up, from an interlock option perspective, is to halt the entire process via a single relay output from the Earth-Rite MULTIPOINT’s controller so that the isolated equipment can be identified and repaired. Halting the movement of the powder ensures that electrostatic charges are not being generated while a piece of plant equipment does not have grounding protection in place.
However, if the grounding of plant is not required 100% of the time then, depending on the characteristics unique to the process, the number of relays being utilised by the operation may be increased.
Referencing the illustration in Figure 1, channels 1 through to 7 could be connected to one relay, while channel 8 could be connected to a separate relay. Only when the grounding of the tote is required would channel 8 need to be permissive such that its corresponding relay may open a valve that will charge powder into the mobile tote. The relay taking inputs from channels 1 through to 7 could be interlocked with the starter circuit or PLC controlling the conveying system or powder drying system.
The installation of conventional static grounding systems, designed to monitor discrete equipment, like IBCs and road tanker trucks, are unlikely to be a viable solution for typical powder processing equipment like fluid bed dryers or sieve stacks. The quantity of systems required, the lack of available space and installation difficulty would either be too complicated, too expensive, or a combination of both.
A unique static grounding protection system such as the Earth-Rite MULTIPOINT minimises the site’s use of valuable installation “real-estate” while simultaneously bridging the gap between time-consuming checks with multi-meters and the highest levels of grounding protection provided by dedicated static ground monitoring systems.