Hazardous Area Classification
Oil & gas installation has areas which are classified as being Zone 1 or Zone 2 according to IEC 60079. With regards to explosion ignition of flammable gases and vapours, it is necessary to take special precautions during installations. Whever possible, live conductor installation is avoided as the possiblity intentional or accidental contact with bare live conductors may cause a spark to occur. The energy of a spark that is needed to ignite an explosive mixture of air and gas is surprisingly small as stated in IEC 60079-11 and IEC 60079-15.
Importance of Earthing in Hazardous Areas
An important aspect in the design of earthing systems for hazardous areas is to maintain an "equi-potential" on all non-current carrying conducting system within the area. In this sense "equi-potential" means maintaining equal potential across the system hence reducing the risk of explosion as low as reasonably practicable. Only a few millivolts of difference should occur in the event of fault currents flowing in the conductor system. This is usually achieved by adequately sizing and positioning the earthing busbars, bonding cables, terminal systems and connections for a prospectively high I-squared-t duty.
IEC 60079-14, Clause 6.2 emphasizes the methods of earthing the neutrals, and to the use of neutral conductors and protective earthing conductors.
The three methods of earthing TN, TT and IT as well as the use of Safety Extra Low Voltage (SELV) and Protective Extra Low Voltage (PELV) systems and equipment as defined in IEC60364 require proper earthing when installed in hazardous areas.
SELV and PELV Systems and Equipment
A SELV system is an extra-low voltage system (50 Vac or 120 Vdc free of ripple when measured between any two conductors), which is electrically separated from the earth (or ground) and other systems (such as the primary winding of an isolating transformer) in such a way that a single fault cannot give rise to the risk of electric shock. A PELV system is also an extra-low voltage system, but is one that is not electrically separated from earth. In all other respects it must satisfy the requirements of a SELV system.
References
- IEC 60079-14 - Explosive atmospheres - Part 14: Electrical installations design, selection and erection
- Handbook of Electrical Engineering: For Practitioners in the Oil, Gas and Petrochemical Industry. Alan L. Sheldrake
© 2003 John Wiley & Sons, Ltd ISBN: 0-471-49631-6