During the production, processing, storing and transportation of flammable substances such as fuel, alcohol, liquid gas, explosive dust, etc., potentially explosive atmospheres may be present. To ensure the safety and availability of a chemical or petrochemical plant, a procedure is required to protect electrical and electronic installations from lightning currents and surges which could be the cause of an explosion.
Intrinsically safe measuring circuits are frequently used in potentially explosive atmospheres. Fig. 1 shows the general design and lightning protection zones of such a system. Since maximum system availability is required and numerous safety requirements must be observed in hazardous areas, the following areas were divided into lightning protection zone 1 (LPZ 1) and lightning protection zone 2 (LPZ 2):
Fig. 1: Basic division of an installation into lightning protection zones (LPZs).
According to the lightning protection zone concept as per IEC 62305-4 (EN 62305-4), adequate surge protective devices, which will be described below, must be provided for all lines at the boundaries of the lightning protection zones.
External lightning protection
The external lightning protection system includes all systems installed outside or inside the structure to be protected for intercepting and discharging the lightning current to the earth termination system.
Table 1: Arrangement of air-termination systems according to the class of LPS.
A lightning protection system for potentially explosive atmospheres is typically designed according to class of LPS II. Another class of LPS can be chosen in justified individual cases, in case of special conditions (legal requirements) or as a result of a risk analysis. The requirements described below are based on class of LPS II.
Air-termination systems
In potentially explosive atmospheres, air-termination systems must be installed at least according to class of LPS II (Table 1). To determine the relevant points of strike, it is recommended to use the rolling sphere method with a minimum radius according to class of LPS II. 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 explosive (Ex) zones (Fig. 2).
Fig. 2: Air-termination system for a tank with air-termination rods and air-termination calbes.
Natural components such as metallic roof structures, metal pipes and containers can also be used as air-termination systems if they have a minimum material thickness of 5 mm according to Annex D 5.5.2 of the IEC 62305-3 (EN 62305-3) standard and the temperature rise and reduction of material at the point of strike does not present additional risks (e.g. reduction of the wall thickness of pressure containers, high surface temperature at the point of strike – see Fig. 1).
Down conductors
Down conductors are electrically conductive connections between the air-termination system and the earth-termination system. To prevent damage when conducting the lightning current to the earth-termination system, the down conductors must be arranged in such a way that
The reinforcements of reinforced concrete buildings may also be used as down conductors if they are permanently interconnected in such a way that they can carry lightning currents.
Separation distance
If there is an insufficient separation distance d between the air-termination system or down conductor and metal and electrical installations inside the structure to be protected, dangerous proximities may occur between the parts of the external lightning protection system and metal as well as electrical installations inside the building. The separation distance d must not be smaller than the safety distance s (d > s).
Since in practice the lightning current splits between the individual down conductors depending on the impedances, the safety distance must be calculated separately for the relevant building/installation as per IEC 62305-3 (EN 62305-3).
Shielding of buildings
Another measure of the lightning protection zone concept is to shield buildings. To this end, metal facades and reinforcements of walls, floors and ceilings on or in the building are combined to form shielding cages as far as practicable (Fig. 3). By electrically interconnecting these natural metal components of the object to be protected to form closed shielding cages, the magnetic field is considerably reduced. Thus, the magnetic field can be easily decreased by a factor of between ten and 300, while an infrastructure for EMC protection can be established at low cost. When retrofitting existing installations, the room shielding must be adapted to the EMC requirements, for example, by means of reinforcement mats.
Fig. 3: Shielding of structures by using natural components of the building.
Protection in hazardous areas
The lightning protection and Ex zones are already harmonised at the design stage. This means that the requirements for the use of surge protective devices both in hazardous areas and at the boundaries of lightning protection zones must be fulfilled. Consequently, the place of installation of the surge arrester is exactly defined, that is it must be installed at the transition from LPZ 0B to LPZ 1.
This prevents dangerous surges from entering Ex zone 0 or 20 since the interference has already been discharged. The availability of the temperature transmitter, which is important for the process, is considerably increased. In addition, the requirements of IEC 60079-11 (EN 60079-11), IEC 60079-14 (EN 60079-14) and IEC 60079-25 (EN 60079-25) must be observed (Fig. 4):
Fig. 4: Surge protective devices in an intrinsically safe measuring circuit.
According to the definition in the protection concept, the LPC in the control room is defined as LPZ 2. A surge protective device is also provided at the transition from LPZ 0B to LPZ 1 for the intrinsically safe measuring line from the temperature transmitter. This surge protective device at the other end of the field line which extends beyond the building must have the same discharge capacity as the surge protective device installed on the tank.
Downstream of the surge protective device, the intrinsically safe line is led via an isolating amplifier (Fig. 5). From there, the shielded line to the LPC is routed in LPZ 2. The cable shield is connected on both ends, therefore no surge protective device is required at the transition from LPZ 1 to LPZ 2 since the electromagnetic residual interference to be expected is significantly attenuated by the cable shield earthed on both ends.
Other selection criteria Insulation strength of equipment
To ensure that leakage currents do influence the measured values, the sensor signals from the tank are frequently galvanically isolated. The insulation strength of the transmitter between the intrinsically safe 4 to 20 mA current loop and the earthed temperature sensor is ≥500 V AC. Thus, the equipment is unearthed. When using surge protective devices, this unearthed state must not be interfered with.
Fig. 5: Surge protective devices for intrinsically safe measuring circuits.
If the transmitter has an insulation strength of <500 V AC, the intrinsically safe measuring circuit is earthed. In this case, surge protective devices which in case of a nominal discharge current of 10 kA (8/20 µs wave form) have a voltage protection level below the insulation strength of the earthed transmitter must be used (e.g. Up (core/PG) ≤35 V).
Type of protection: Category ia, ib or ic?
The transmitter and the surge protective device are installed in Ex zone 1 so that type of protection ib is sufficient for the 4 to 20 mA current loop. The surge protective devices used (ia) fulfil the most stringent requirements and are thus also suited for ib and ic applications.
Permissible maximum values for L0 and C0
Before an intrinsically safe measuring circuit can be put into operation, it must be demonstrated that it is intrinsically safe. To this end, the power supply unit, the transmitter, the cables and the surge protective devices must fulfil the conditions of intrinsic safety. If required, energy buffers such as the inductances and capacitances of the surge protective devices must be taken into account. According to the EC type examination certificate (PTB 99 ATEX 2092), the internal capacitances and inductances of BXT ML4 BD EX 24 surge protective devices (Fig. 6) are negligible and do not have to be taken into account for the conditions of intrinsic safety (Table 2).
Fig. 6: Example of an intermeshed earth-termination system.
Table 2: Example of a temperature transmitter.
Values for voltage Ui and current Ii
According to its technical data, the intrinsically safe transmitter to be protected has a maximum supply voltage Ui and a maximum short-circuit current Ii when used in intrinsically safe applications (Fig. 7). The rated voltage Uc of the arrester must be at least as high as the maximum open-circuit voltage of the power supply unit. The nominal current of the arrester must also be at least as high as the short-circuit current Ii of the transmitter to be expected in the event of a fault. If these marginal conditions are not observed when dimensioning the surge arresters, the surge protective device can be overloaded and thus fail or the intrinsic safety of the measuring circuit is no longer ensured due to an impermissible temperature rise on the surge protective device.
Fig. 7: Example of the shield treatment of intrinsically safe cables.
Coordination of surge protective devices
NE 21, the NAMUR (an international user association of automation technology in process industries) recommendation, defines general interference immunity requirements for process and laboratory equipment (e.g. transmitter). The signal inputs of such equipment must withstand voltages of 500 V between the cable cores (transverse voltage) and 1000 V between the cable core and earth (longitudinal voltage). The measurement set-up and the wave form are described in the IEC 61000-4-5 (EN 61000-4-5) basic standard. Depending on the amplitude of the test impulse, a specific immunity level is assigned to terminal equipment.
These immunity levels of terminal equipment are documented by test levels (1 to 4) while test level 1 is the lowest and test level 4 the highest immunity level. The test level can be usually found in the documentation of the device to be protected or requested from the manufacturer of the device. In case of a risk of lightning and surge effects, the conducted interference (voltage, current and energy) must be limited to a value within the immunity level of the terminal equipment. The test levels are documented on the surge protective devices (e.g. P1).
Intermeshed earth-termination
In the past, separate earth-termination systems were often used in practice (lightning protection and protective earthing separated from the functional earthing). This turned out to be extremely unfavourable and can even be dangerous.
In case of a lightning strike, voltage differences up to some 100 kV can occur which may lead to the destruction of electronic components, risks for persons and explosions in potentially explosive atmospheres due to sparking.
Therefore, it is advisable to install a separate earth-termination system for every single building or part of an installation and to intermesh them. This intermeshing (Fig. 6) reduces potential differences between the buildings or parts of the installation and thus conducted partial lightning currents. The closer the mesh of the earth-termination system, the lower the potential differences between the buildings or parts of the installation in case of a lightning strike. Mesh sizes of 20 x 20 m (mesh sizes of 10 x 10 m are recommended in potentially explosive atmospheres and when using electronic systems) have proven to be economically feasible. When selecting the earthing material, it must be ensured that the buried pipes do not corrode.
Equipotential bonding
Consistent equipotential bonding must be established in all potentially explosive atmospheres to prevent potential differences between different and extraneous conductive parts. Building columns and structural parts, pipes, containers, etc. must be integrated in the equipotential bonding system so that a voltage difference does not have to expected even under fault conditions.
The connections of the equipotential bonding conductors must be secured against automatic loosening. According to IEC 60079-14 (EN 60079-14), supplementary equipotential bonding is required which must be properly established, installed and tested in line with the IEC 60364-4-41 (HD 60364-4-41) and IEC 60364-5-54 (HD 60364-5-54) standard. When using surge protective devices, the cross-section of the copper earthing conductor for equipotential bonding must be at least 4 mm2.
Lightning equipotential bonding outside the hazardous area
The use of surge protective devices in low-voltage consumer’s installations and measuring and control systems outside the hazardous area (e.g. control room) does not differ from other applications. In this context, it must be pointed out that surge protective devices for lines from
LPZ 0A to LPZ 1 must have a lightning current discharge capacity which is described by the 10/350 µs test wave form. Surge protective devices of different requirement classes must be coordinated with one another.
Shield treatment in intrinsically safe measuring circuits
The treatment of the cable shield is an important measure to prevent electromagnetic interference. In this context, the effects of electromagnetic fields must be reduced to an acceptable level to prevent ignition. This is only possible if the shield is earthed on both cable ends. Earthing the shield on both ends is only permitted in hazardous areas if absolutely no potential differences are to be expected between the earthing points (intermeshed earth-termination system, mesh size of 10 x 10 m) and an insulated earthing conductor with a cross-section of at least 4 mm2 (16 mm2 is preferred) is installed in parallel to the intrinsically safe cable, and is connected to the cable shield at any point and is insulated again. This parallel cable must be connected at the same equipotential bonding bar as the shield of the intrinsically safe cable (Fig. 6).
Moreover, permanently and continuously connected reinforcing bars can be used as equipotential bonding conductor. These are connected to the equipotential bonding bar on both ends.
Conclusion
The risk of chemical and petrochemical plants due to a lightning discharge and the resulting electromagnetic interference is described in the relevant standards. When using the lightning protection zone concept for designing and installing such plants, the risks of sparking in case of a direct lightning strike or discharge of conducted interference energies must be safely minimised with economically acceptable efforts. The surge arresters used must fulfil explosion protection requirements, ensure coordination with terminal equipment and meet the requirements resulting from the operating parameters of the measuring and control circuits.
Contact Alexis Barwise, DEHN Protection South Africa, Tel 011 704-1487, alexis.barwise@dehn-africa.com
