Created by: Glen Zhu | Updated Date: January 8th, 2024
Explosive atmospheres form when a sufficient concentration of flammable substances, such as gases, vapors, or combustible dust, mixes with oxygen in the air. The presence of an ignition source, such as a spark, flame, or heat, can then trigger a rapid combustion reaction, leading to an explosion.
Chemical industries and petrol stations are common examples of hazardous areas with explosive atmospheres. These areas are defined as hazardous due to the high risk of fire and explosions resulting from lightning strikes and voltage spikes.
It is important to note that safety protection measures must be taken to minimize the risk of explosions, prevent property damage from lightning and ensure plants availability. This includes using intrinsically safe equipment, effective earthing system and necessary surge protective devices.
Two ATEX directives requires manufacturers and employers to comply with the requirements outlined to enhance health and safety of workers, equipment, and protective systems at risk from potentially explosive atmospheres.
Areas with potentially explosive atmospheres are classified into zone based on the frequency and duration of the presence of explosive atmospheres. The classification helps in determining the level of protection required (Table 1).
Table 1 – Classification of hazardous areas
The ATEX directives applies to a wide range of industries, including oil and gas, chemical, pharmaceutical and mining. Zone classification plays a crucial role in risk assessment, equipment selection, safety measures and overall hazard management in areas where explosive atmospheres may occur (Figure 1).
A comprehensive lightning protection for hazardous areas should be designed including air-termination system, earthing and intermeshed equipotential bonding. The system should meet the requirements set out in IEC 62305.
The class of LPS II is determined by using the rolling sphere method for potentially explosive areas. However, in case of a lightning strike to the air-termination system, sparking may occur at the point of strike. To prevent ignition sparks, the air-termination systems should be installed outside the Ex zones.
Metal structures can be natural air-termination systems if reaching the minimum material thickness of 5mm. An isolated LPS is required with adequate separation distance to remove the risk of sparking or explosion of combustible materials.
Figure 1 – Basic division of an installation into lightning protection zones
All conductive parts are connected via intermeshed equipotential bonding system to the ground with the intervals of 20 m at ground level.
Intermeshed equipotential bonding system involves interconnecting all conductive elements to create a network where they share the same electrical potential. The cross-section of the copper earthing conductor for equipotential bonding must be at least 4 mm2.
Earthing helps prevent the buildup of static charges on equipment or surfaces, reducing the likelihood of sparks and that could ignite flammable gases, vapors, mists, or dust in explosive atmospheres. It is advisable to install a separate earth-termination system for every single building or part of an installation.
Using corrosion-resistant conductors, bonding methods, and protective coatings is crucial for maintaining long-term integrity in explosive areas.
Besides, proper separation distance between the air-termination system or down conductors must be regulated in accordance with requirement of IEC 62305. High-voltage resistant, insulated down conductors could be applied when the separation distance is not easy to be maintained.
Figure 2 – An intermeshed earth-termination system
In areas with explosive atmospheres, intrinsically safe (IS) electrical systems and devices are often required. These systems are designed to limit electrical energy to levels below the ignition threshold, preventing sparks or thermal effects that could cause ignition.
IS systems contribute to the overall safety by minimizing the risk of electrical equipment becoming an ignition source. IS barriers acts as a protective barrier between the control system and the hazardous location, preventing excessive energy from reaching potentially explosive atmospheres.
However, IS barriers are not surge protectors. IS barriers are specifically designed to limit energy in hazardous areas to prevent ignition, whereas surge protectors are intended to safeguard electronic equipment in non-hazardous areas from transient voltage spikes.
Therefore, surge protection still should be applied to protect IS circuits from the transient over-voltages.
The work principle of an intrinsically safe electrical system is ensuring electrical and thermal energy level below the ignition thresholds in potentially explosive atmosphere.
Intrinsically safe systems incorporate components such as sensors, transmitters, and switches that are specifically designed to minimize energy levels. Galvanic isolation is often used to separate electrical circuits and prevent the flow of excessive current.
These systems are certified by regulatory bodies according to standards like ATEX or the National Electrical Code (NEC), and they are commonly installed in hazardous zones where the risk of explosive atmospheres exists.
One typical way of installing an intrinsically safe measuring circuit, consisting of a combination of an associated electrical apparatus, intrinsically safe cable installation, a temperature transmitter (galvanically separated from the sensor element), and the corresponding required surge protection devices (SPDs).
The isolator is located in the control room’s instrumentation and control cabinet, while the temperature transmitter, situated on a tank with combustible liquid, has its sensor element directly in Ex Zone 0.
The transmitter, in Ex Zone 1, is securely and permanently connected to the metallic tank through its metal enclosure. The connection between the two devices is established by a shielded, intrinsically safe cable. Both the measuring station and tanks are part of a meshed earthing system.
Figure 3 – Surge protective devices in an intrinsically safe measuring circuit
To safeguard against lightning-related risks in both the control room and hazardous areas, the installation requires two surge protection devices (SPDs) in the intrinsically safe circuit. One SPD shields the isolator in the control room, and the other protects the tank transmitter.
The tank SPD not only prevents hazardous spark discharge to the sensor conductor but also provides explosion protection. By connecting signal wires, unused wires, and open cable shields to the circuit, the SPDs prevent compensating current during normal operation. In the event of hazardous surges, the open cable ends are linked to equipotential bonding via the SPD, preventing spark creation.
FRD surge protective devices help protect costly measuring technology against the influence of surge voltages through atmospheric discharge. They offer optimum protection for two-pole or four-pole measurement and control application.
Surge Protector for MCR systems SRD2-24
SPD for intrinsically safe measuring circuits FDB2-24
Surge arrester with a low-capacitance protective circuit for protecting intrinsically safe measuring circuits and bus systems. Application: Flow sensors, temperature sensors.
Shielding in cables is often employed to prevent the ingress or egress of electromagnetic interference (EMI) or radio frequency interference (RFI). Shield could minimize the impact of external electromagnetic interference on the signals transmitted within the cable. This is especially important in sensitive circuits used in hazardous areas.
Figure 4 – The shield treatment of intrinsically safe cables
Proper grounding ensures the shields’ effectiveness, the shield acts as a grounded path for unwanted currents, preventing them from circulating within the cable conductors and potentially spark or hot spots.
Earthing the shield on both cable ends in hazardous areas only when no potential differences between the earthing points and an insulated earthing conductor is installed in parallel to the intrinsically safe cables, is connected to the cable shield at any point and is insulated again.
Moreover, permanently and continuously connected reinforcing bars can be used as equipotential bonding conductor to be connected to the equipotential bonding bar on both ends.
LSP’s reliable surge protection devices (SPDs) are designed to meet the protection needs of installations against lightning and surges. Contact our Experts!
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