Electrical engineering: A quick guide to earthing and bonding
Explosive atmospheres can occur wherever flammable gases, vapours, mists or dusts mix with the surrounding air. That situation can arise in a wide range of industrial environments, from chemical processing, storage or transportation activities to food manufacturing.
One of the major dangers in explosive atmospheres is ignition by electric spark. Those sparks can come from many sources. They can be generated by faults in equipment, or by static electricity produced in industrial processes such as powder mixing or the pumping of liquids. They can also be induced by lightning, or radio transmission.
Protection of personnel and equipment where explosive atmospheres may be present requires a comprehensive approach, taking into account the design, operation and maintenance of the facility. One critical, but sometimes poorly understood, element of that approach is appropriate earthing and bonding.
- Earthing, as defined by BS 7671 Requirements for Electrical Installations is the “connection of the exposed conductive parts of an installation to the main earthing terminal of that installation.”
- Bonding, correctly called ‘equipotential bonding’ is an electrical connection maintaining various conductive parts at substantially the same potential.
BS 7671 further defines the types of conductive parts that might be encountered:
- An ‘exposed conductive part’ is a conductive part of an installation which can be touched and which may become live under fault conditions, for example a metal cabinet or housing.
- An ‘extraneous conductive part’ is a conductive part liable to introduce a potential that is not part of the electrical installation, for example a pipe or conduit made of conductive material.
Guidance on the appropriate sizing of earthing and bonding conductors is given in BS 7671, and the same rules apply to all types of equipment, although only equipment specifically designed for the environment should be used in hazardous areas.
Another important regulation, BS EN 60079-14:2014, Explosive atmospheres. Electrical installations design, selection and erection, requires potential equalization for equipment installed in hazardous areas. In practice, this means that all exposed and extraneous conductive parts must be connected the equipotential bonding system. Those connections can’t be left to chance, they must be made through designed connection points, and all connections should be secure against self-loosening.
The bonding system may include protective conductors, metal conduits, and metallic parts of structures and pipework. Exposed conductive parts need not be separately connected to the equipotential bonding system only if they are firmly secured to and are in metallic contact with structural parts or piping which are connected to the equipotential bonding system. Beware of bolted flanged connections between structures which can corrode and provide very poor continuity. Such plates are often painted and on the surface look good, but may be badly corroded underneath.
Sometimes pipe flanges are deliberately insulated for cathodic protection and other purposes, so it is often folly to depend on the structural integrity of an installation to provide a guaranteed path for bonding purposes.
GSE Systems has helped clients in wide range of industries to ensure their electrical installations are fully compliant with the relevant electrical safety regulations and best practices. We can survey existing installations, identify issues and deliver appropriate solutions.