The International Electrotechnical Commission has published a new Technical Specification called IEC 60079-32-1: “Explosive atmospheres – Part 32-1: Electrostatic hazards – Guidance”. This Technical Specification is a guidance document which is the latest addition to the IEC series of 60079 “Explosive Atmospheres” standards that are designed to limit fires and explosions caused by electrical malfunctions within hazardous locations.
The 168 page document is the first of two documents to be published by the IEC under the “60079-32” designation and is intended to aid the designers and users of process equipment minimise the risk of incendive electrostatic discharges within potentially explosive atmospheres. It covers a broad range of process scenarios that can lead to the generation of electrostatic charges, provides examples of what measures can be taken to reduce charge generation and accumulation and outlines how process equipment should be grounded and bonded.
The second part, IEC 60079-32-2, is entitled “Electrostatics hazards – Tests” and outlines test methods to determine factors like surface resistance, earth leakage resistance, powder resistivity, liquid conductivity, capacitance and the incendivity of electrostatic discharges.
The stated objective of IEC 60079-32-1 is to provide:
“the best available accepted state of the art guidance for the avoidance of hazards due to static electricity”.
To date guidance documents that address the ignition hazards of static electricity have either been published by national institutions like the NFPA or pan-European organisations like CENELEC. IEC 60079-32-1 has been collectively developed by a large number of technical committees from IEC member countries, making this document a truly global collaboration. It also builds on the work of national and regional guidance documents addressing electrostatic hazards, including CENELEC/TR: 50404, NFPA 77, BS 5958, TRBS 2153:2009 and JNIOSH TR42.
Although the Technical Specification can be purchased from the IEC’s webstore, it will be the responsibility of national standards institutes like the ANSI in the U.S., BSI in the U.K. and DIN in Germany to administer the circulation of the document in their respective national territories. The ANSI has the document available for purchase from its website. Cenelec has withdrawn CLC/TR: 50404 and replaced it with CLC/TR 60079-32-1.
Overview of the Technical Specification:
The Technical Specification is sub-divided into what are termed “clauses” that highlight the electrostatic hazards associated with various categories of materials, the hazards associated with people, including physiological shocks, and what grounding and bonding measures should be put in to practice. The clauses are presented as:
1) The handling of solids.
2) The storage and handling of liquids.
3) The handling of gases and vapours.
4) The storage and handling of powders.
5) The storage and handling of explosives.
6) Electrostatic problems caused by people.
7) Avoidance of electrostatic shock.
8) Earthing and bonding of plant and machinery.
There are also several Annexes that provide informative material, examples of which include a description of the various types of electrostatic discharges, the types of electrostatic discharges that can be expected from processes carried out within potentially flammable and combustible atmospheres and the provision of an illustrated flowchart for assessing electrostatic hazards.
Owing to the fact that the document is 168 pages long, it would be impossible to provide a comprehensive overview of the guidance contained in the document in just a few pages. However, it would be worth touching on guidance related to the grounding and bonding of specific processes that utilise portable equipment at risk of static charge accumulation.
The design and monitoring of grounding systems:
This section addresses the design and monitoring of systems dedicated to grounding permanent and portable plant equipment. Permanently installed plant equipment like reactors and pumps will most likely be grounded via the electrical grounding system for the plant. Electrical fault paths (and lightning protection paths) are more than adequate to dissipate electrostatic charge to ground.
For portable conductive equipment this section recommends that temporary connections using bolts or “pressure-type” clamps are capable of penetrating protective coatings, rust or product deposits that are typically present on the surface of such equipment, e.g. metal drums. It states that pressure-type clamps should be capable of establishing a connection resistance of less than 10 Ohms to the base metal of the conductive equipment.
Systems designed to monitor the resistance between equipment at risk of charge accumulation and earth (designated grounding points) should not only be capable of monitoring the resistance in the grounding circuit, but should also be capable of drawing attention to any changes in resistance. This is to ensure that malfunctions in the grounding circuit are detected as early as possible so that inspections and necessary repairs are made in good time.
Given that metal grounding circuits should not display a resistance above 10 ohms it would be prudent to specify grounding systems that are capable of identifying changes in resistance and alerting personnel as soon as 10 ohms in the ground path is exceeded.