1. Introduction
This document outlines acceptable practices for earthing and bonding electrical apparatus in hazardous areas. While the topic is straightforward, it spans multiple areas of electrical expertise, making a systematic approach beneficial. Numerous codes of practice specify methods for earthing and bonding; however, the fundamental requirements remain consistent regardless of geographic location. Consequently, significant variations in requirements should not occur.
This document primarily focuses on practices accepted in the United Kingdom and Europe. If a national code of practice exists and deviates significantly from this document, it should be carefully reviewed. While adherence to such a code might be practical, it is essential to ensure that compliance results in a safe installation.
Some parts of this document state what are well-known basic principles to practising electrical and instrument engineers. They are restated primarily for the sake of completeness, and ease of reference.
2. Reasons for Earthing and Bonding
The primary purposes of earthing and bonding are as follows:
a) To establish a reliable, dedicated low-impedance return path for fault currents, enabling prompt detection of faults and rapid disconnection of the power source.
b) To eliminate potential differences that could pose an electrocution hazard to personnel or cause sparking capable of igniting flammable substances.
c) To mitigate the impact of lightning strikes, whether directly on the installation or in its vicinity.
d) To control or prevent the accumulation of electrostatic charges.
e) To reduce electrical interference and provide a stable signal reference for instrumentation systems.
f) To meet segregation requirements and define fault paths essential for ensuring the safety of explosion-proof apparatus.
3. Fault Isolation
Fault currents should be removed as quickly as possible, ideally in less than one second, to prevent the energy dissipated at the fault site from causing a fire or explosion. Most gases require temperatures exceeding 200°C to ignite spontaneously, and such temperatures can also cause fires and damage conventional insulation.
Interestingly, the potential difference deemed unacceptable from an electrocution perspective is comparable to that required to ignite gases. The human body's sensitivity to electricity is complex, as it depends on both frequency and exposure duration. Numerous references address this topic in detail, one notable example being Touch Voltages in Electrical Installations by B.D. Jenkins.
A simplified analysis, illustrated in figure 1, suggests that limiting the current flowing between the body’s extremities to 5 mA can be achieved by restricting the voltage to 25 V RMS over a separation of approximately 2.5 meters.
This paper provides additional details for Earthing and Bonding in Hazardous Areas.
