Hazop assessments, and the reports that follow on from them, are a great way of capturing and identifying processes and practices that could lead to the ignition of flammable atmospheres through discharges of static electricity. What Hazop reports are not so great at doing is identifying what the earthing solution to eliminate the risk should look like.
The task of identifying the right earthing solution falls to people like you and members of your team and it’s not likely to be something you deal with on a day to day basis. For most people, identifying and specifying the right static earthing solution is probably the kind of project they’ll handle once or twice in their career. But get it right first time and it quickly becomes an area where you can bring value to the table throughout your career. This guide is about helping you get started on the right path and can be best described as a door opener to the subject of hazardous area static control.
The guide is broken down into three distinct sections. The first section deals with industry guidelines that provide guidance on controlling static electricity in hazardous areas The second section helps you work out the “best-fit” for controlling electrostatic hazards at your site and the third section touches on hazardous area equipment approvals, specifically what you should be looking for when selecting an EX Certified approved static earthing solution.
1. Static Earthing Benchmarks.
Before embarking on this guide to specifying and sourcing static earthing solutions it should be asserted from the jump-off point that hazardous area certified equipment that carries the mark of a Notified Body like SIRA or BASEEFA is not a validation of a earthing system’s performance characteristics when it relates to providing static earthing protection. Although a lot of time and effort can be put into sourcing earthing solutions that match or exceed your EX zoning requirements, the first recommendation this buyer’s guide will make is to take account of hazardous process industry associations that provide guidance on preventing ignitions caused by static electricity. There are several documents published by highly authoritative and respected associations around the world that identify the industrial processes that can be the source of electrostatic ignitions.
The committees that are assigned the task of developing and updating these guidance documents in line with the latest state of the art techniques are employees of companies and consultancies active in the hazardous process industries.
Demonstrating compliance with the recommendations outlined in these guidance documents will virtually ensure all of the electrostatic hazards presented by your company’s operations are under your control. If you can specify earthing solutions that display compliance with the publications listed in Table 1, you will be ensuring your static earthing protection methods display the latest state of the art in preventing fires and explosions caused by static electricity.
Table 1: Hazardous process industry guidelines for preventing fires and explosion caused by static electricity.
The guidelines in Table 1 describe how and why certain operations, whether it involves liquids, gases or powders, can generate static electricity and result in the static electricity accumulating on the equipment being used in the process. The primary means of preventing ignitions caused by static electricity is to ensure all conductive and semi-conductive equipment, including people, are bonded and earthed to a verified “true earth” earthing point. This ensures electrostatic charges cannot accumulate on equipment and discharge a spark into an ignitable atmosphere.
Because the Earth has an infinite capacity to balance positive and negative charge, if equipment is connected to it, that equipment is at “earth potential” meaning it can’t charge up in response to static generated by the movement of material.
Fig. 1: To ensure equipment cannot accumulate electrostatic charge, the equipment should be connected to the general mass of the earth by means of a true earth grounding point. The resistance between the grounding point and true earth must be low enough to allow the electrostatic charge generated by the process flow to earth.
Just as many other safety related functions have benchmarks designed with factors of safety in mind, earthing and bonding circuits can, and should, work to benchmarks that exceed the minimum safety requirements. The minimum theoretical requirement for earthing electrostatic charges is usually described in academic circles as having an electrical resistance not exceeding 1 meg-ohm (1 million ohms) between the object at risk of charge accumulation and the general mass of earth.
However, it is well recognised that metal objects at risk of charge accumulation, e.g. road tankers, and the earthing and bonding circuits providing earthing protection, should never display an electrical resistance of more than 10 ohms if they are in good condition. This value of 10 ohms is the one value of resistance that is consistently recommended across all of the publications listed in Table 1. So wherever a earthing solution is being sourced for operations that involve metal objects like road tankers, railcars, totes, barrels and containers, earthing systems that display earth monitoring values of 10 ohms or less should be specified.
Another reason why the theoretical value of 1 meg-ohm does not have a role in real world applications is the requirements related to earthing Type C FIBCs. Although CLC/TR: 50404 (2003) states that the resistance through a Type C FIBC bag should not exceed 100 meg-ohm, the latest state of the art guidance published in IEC 60079-32-1 (2013) and NFPA 77 (2014) states that resistance through the bag should not exceed 10 meg-ohm. So clearly, a “theoretically acceptable” value of 1 meg-ohm is impractical when discussed in the context of metal objects that should display a benchmark resistance of 0 to 10 ohms or less, and Type C FIBCs that should display benchmarks of either 0 to 10 meg-ohm or 0 to 100 meg-ohms (depending on what standard the bag is manufactured to).
NOTE: If you are engaged in sourcing a earthing solution for Type C FIBC bags you must ensure you know what standard the bags are manufactured to. If you don’t know what standard your bags are manufactured to the bag supplier should be consulted. Once you know what standard your bag is manufactured to you should source a Type C FIBC earthing system that monitors the earthing circuit from 0 ohms up to 10 meg-ohms (NFPA 77 / IEC 60079-32 compliant) or from 0 ohms up to 100 meg-ohms (CLC/TR: 50404 compliant). Avoid earthing systems that do not monitor the full range of resistance as they are likely to fail bags that are designed to work up to 100 meg-ohms and pass bags that should only work up to 10 meg-ohms.