2. Source a earthing solution that provides the “Best Fit” with your objectives.
Your company’s Hazop report will normally identify the risk of static sparks from specific equipment like road tankers, drums, IBCs, etc., and provide an assessment of what impact a fire or explosion caused by an electrostatic ignition could have on the area. It will be your task to determine what the earthing solution needs to look like. Before fully embarking on the search for a static earthing solution, determine the layers of protection you want from an electrostatic ignition hazard. The more layers deployed to protect against an ignition source, the more likely static will controlled in a safe, repeatable and reliable way.
Producing answers to the following questions will help you identify the layers of protection you require from your static earthing solution.
- Who will be responsible for ensuring the equipment is earthed prior to, and during, the operation and how do we alert them to a situation where there could be an electrostatic discharge risk?
- If, for whatever reason, the equipment loses its earthing protection during the operation, do I want that process to continue building up electrostatic charge on the equipment?
- What type of equipment requires static earthing protection and does the application have unique characteristics that require a particular type of earthing solution?
2.1 Assessing the required Layers of Protection in the context of Question A:
With the exception of locations like laboratories that handle small quantities of flammable products, the act of earthing a piece of equipment identified as being a static discharge risk will be the responsibility of equipment operators, or in the case of road tanker and vacuum truck operations, the driver of the vehicle. Because static electricity is a complex technical subject (some would even say akin to witchcraft!) it can be hard for people who don’t deal with it on a day-to-day basis to grasp the fundamentals of why it is a serious risk when assessed in the context of operations conducted in flammable atmospheres. An unhealthy paradigm of it “can’t happen to me” can follow on from this lack of awareness especially when the hazard is neither a tangible nor visual risk that would trigger a natural safety related response from an individual.
As static electricity is neither a visible nor tangible hazard the main challenge is to get your company’s operators to take responsibility for their own safety and the safety of their colleagues. The most effective way of getting operators into the habit of earthing equipment on a routine basis is to deploy a earthing solution that requires a visual confirmation of a verified ground before the operation can start. If the operator has a visual reference point for knowing when the operation can begin, they can be trained to take responsibility for the action of earthing the equipment they are operating. The most effective method of indication is to use green indicators to communicate a “GO” situation and red indicators to communicate a “NO GO” situation. To really get their attention, pulsing LEDs can prove very effective at telling the operator that the resistance in the earthing circuit is being monitored on a continuous basis and that he/she needs to see a pulsing green light before, and throughout, the operation.
Fig. 2: Pulsing green ground status indicators provide operators with a visual reference point to ensure the equipment they are operating is grounded prior to, and during, the operation.
Some earthing solutions have in-built buzzers that can alert operators to a lost earthing connection, however, you do need to be careful when evaluating such equipment, as the audibility of the buzzers frequently become redundant when they are competing against the ambient noise levels of the immediate working environment, if the operator is out of the buzzer’s effective audible range or if the operator must wear ear mufflers or protective ear plugs.
The benchmarks that should be in place to monitor earthing and bonding circuits should be based on the guidance outlined in the publications listed in Table 1. This will ensure your earthing solutions, hence your company, displays compliance with the leading authorities and latest state of the art in static earthing protection. Just to recap, any equipment of metallic construction like road tankers, railcars, IBCs, drums and powder processing systems should be monitored with a resistance not exceeding 10 ohms back to a verified earth earthing point. Type C bags manufactured in line with IEC and NFPA requirements should be monitored with 10 meg-ohm earthing systems and Type C bags manufactured to CENELEC requirements should be monitored with 100 meg-ohm earthing systems.
2.2 Assessing the required Layers of Protection in the context of Question B:
Visual indication and continuous ground circuit monitoring are two fundamental layers of protection that tend to go hand in hand. However, when there is no active earthing of the equipment and the operation is still running (thereby rapidly accumulating hazardous static charges) there must be additional controls in place that will prevent the equipment from rapidly accumulating hazardous electrostatic charges. Shutting down the movement of the material being processed will stop the generation of static electricity.
A common action is for the equipment operator to hit an emergency shutdown button to prevent further generation and accumulation of static electricity on the equipment he/she is operating. Depending on the nature of the operation, and with the best will in the world, people’s attention can, and will, be diverted to other activities while the operation is running so in the event that earthing or bonding is lost an additional layer of protection that can be deployed is to shut down the operation automatically. Automatic shutdown can be achieved with earthing systems that carry output contacts which can be interlocked with a range of devices (switches, valves, PLCs) that can execute a shutdown in response to the monitoring circuit identifying a lost ground connection.
Visual indication is an effective layer of protection to get earthing in place before the process is started by the operator and interlocks are an additional layer of protection that ensure that an automatic shutdown, as opposed to a manual shutdown, prevents the rapid build-up of static electricity.