Type C FIBC:
This section describes which hazardous zones the four bag types (A,B,C,D) are suitable for use in. The “zones” are based on the IEC method of hazardous area zoning classification which have been adopted in ATEX legislated countries, the U.S. NEC 505 & 506 and the Canadian CEC. It also specifies the types of liners (L1, L2, L3) and FIBC bag type combinations that may or may not be used in hazardous areas. This section heavily references IEC 61340-4-4: “Electrostatics – Part 4-4: Standard test methods for specific applications – Electrostatic classification of flexible intermediate bulk containers (FIBC)” which specifies the construction, testing and labelling criteria for Type A,B,C,D FIBC bags.
Under the requirements of this standard Type C bags must be grounded and for those manufacturing or using Type C bags the resistance from any part of the bag to a groundable point on the bag shall be less than 10 Meg-ohm. This means that the conductive fabric or filaments that make up the grounding path for electrostatic charges should not have a resistance of more than 10 Meg-ohm to the bag’s grounding point. This is a change from CLC/TR: 50404 which recommended a maximum resistance of 100 Meg-ohm.
The safest method of grounding Type C bags is to ensure the resistance through the bag is less than the benchmark value set by the standard(s) the site wishes to comply with. If a grounding system is to be specified, it is important to ensure the system tests the full range of permitted resistance. The specified resistance range for an IEC 61340-4-4 compliant bag will be any value under 10 Meg-ohm.
This section states that if Type D bags are used they do not need to be grounded, but conductive objects within the vicinity of the bag must be grounded.
The precautionary guidance regarding the grounding of road tankers reflects the stated goal of the technical specification providing the latest state of the art guidance. Section 184.108.40.206.3, part C, states that the “earth cable” earthing the tanker should be part of a static grounding system that continuously monitors the resistance between the road tanker and the designated grounding point located on the loading gantry. It states that the grounding system should be interlocked with the transfer system to shut down the product transfer operation if this resistance exceeds 10 ohms.
It also states that the grounding system should be capable of recognising when it is not connected to the chassis/tank of the road tanker. This ensures that situations where the tank of the road tanker is not connected to the grounding system, for example, where an operator could connect the clamp to an isolated metal mud-guard or wheel-nut, will not result in a permissive condition for the transfer operation, thereby eliminating the risk of electrostatic charging of the road tanker.
The precautionary measures for the grounding of vacuum trucks state that the truck should be connected to a “designated” site earth before proceeding with the transfer operation. A “designated” site earth is a grounding point that has been verified as having a low resistance connection to “true earth” by suitably qualified engineers. These points are usually identified by tags or labels.
The guidance states that wherever a designated grounding point is not provided and portable ground rounds are permitted for use, or there is doubt regarding the quality of the site designated grounding point, the integrity of that grounding point should be verified prior to the transfer operation. This means that there should be a way of ensuring that the grounding point is actually connected to true earth in order to ensure the charges generated by the transfer operation do not accumulate on the truck or the hoses connected to the truck. This can be achieved via a truck mounted ground verification system or by engineers with ground testing meters. The resistance between the verified grounding point and the vacuum truck should not exceed 10 ohms and it is recommended that this resistance is verified by a truck mounted grounding system which will automatically indicate this to the driver or be verified with an ohmmeter (which should be used by a suitably competent electrical person).
This section also states that hoses used in the vacuuming operation should be compliant with the section covering hoses and hose assemblies. Table 18 in section “220.127.116.11 Properties and usage of ISO 8031 hose assembly grades”, describes the types of hoses that can be used for materials with varying electrical properties.
If non-metallic connections are used to ground the truck up to 1 Meg ohm resistance between the truck and the grounding point is permitted. It should be advised that if a 1 Meg ohm limit is being adopted that the grounding point to which the truck is connected has a verified low connection resistance to true earth, of not more than a few ohms. If a 1 Meg ohm limit is adopted for powder transfers it would be advisable to estimate the charging current generated by the vacuuming operation as very high charging rates are common place with truck vacuumed non-conductive powders.
Personnel grounding, flooring and footwear:
Clause 11 provides guidance on minimising the risk of static electricity accumulating on people and describes various scenarios that can generate electrostatic charges on people. It also describes what methods can be adopted to dissipate charge off people, ranging from guidance on flooring, footwear and additional accessories that can be used to ground personnel working in zoned atmospheres.
Technical Specification IEC 60079-32-1 is probably the most comprehensive guidance produced on avoiding the hazards of static electricity to date and with the content being developed and approved by a wide cohort of IEC national technical committees, represents a cohesive global approach to controlling electrostatic hazards in the workplace. This article provides a limited overview of the guidance contained in the document. More information regarding the material, processes and equipment that can be susceptible electrostatic discharges, how charging can be minimised and what additional grounding and bonding practices can be put into action are described throughout the different clauses.
Mike O’Brien, Managing Director for Newson Gale
If you have any questions relating to the topics discussed in this article,
please contact Newson Gale.