In chemical, petrochemical and many industrial plants, potentially explosive areas develop frequently during the manufacturing, processing, storage, and transportation of flammable materials (e.g. gasoline, alcohol, liquid gas, explosive dust).
Special explosion protection measures must be taken in industrial sectors where gas, vapour, fog or dust occurs during the processing or transport of flammable substances which, in combination with a mixture of air, may present a dangerous explosive atmosphere.
Lightning currents and overvoltages in potentially explosive atmospheres
When assessing the risk for potentially explosive atmospheres, the following lightning related ignition sources should be considered:
• • Melting at the point of strike
• • Heating of discharge paths
• • Uncontrolled flashover in case the separation distance is not maintained
• • Induced voltages in cables and lines
• • Lightning strikes into metallic lines entering potentially explosive atmospheres
In case of lightning-related risks, BS EN 62305:2011 is the current UK standard for lightning and surge protection. BS EN 62305:2011 covers in depth the general principles of lightning protection, risk assessments, external lightning protection and surge protection.
The overall lightning protection concept includes:
• • A risk analysis to determine the necessity of lightning protection and to select the required lightning protection measures.
• • Measures to protect structures from material damage and damage to persons caused by direct lightning strikes by installing a lightning protection system (LPS) consisting of air-termination systems, down conductors, earth-termination system, lightning equipotential bonding and separation distance.
• • Protection measures against the effects of lightning electromagnetic impulses (LEMP) by installing a LEMP protection system (LPMS) for structures with electrical and electronic systems. An individual combination of protection measures are earthing and equipotential bonding, spatial shielding, cable routing and shielding, coordinated SPD protection.
Not only do the effects of a direct lightning strike pose a risk, but the electromagnetic effect of the lightning current on the installation of the electrical system also presents an enhanced risk in potentially explosive atmospheres. The lightning current energy and subsequent magnetic field induce high voltages (ignitable energy) in various lines (extraneous conductive parts) that form a loop. If a potentially explosive atmosphere is present at the same time (for example on the terminals in the enclosure of a piece of intrinsically safe equipment) this ignition energy may at any time cause fire or explosion.
Surge protection in intrinsically safe circuits
Surge protection devices for use on intrinsically safe circuits must meet the stringent requirements placed on these circuits. DEHN have a range of devices specifically designed for use in intrinsic circuits such as the Blitzductor XT EX range of surge protection devices.
With ATEX and FISCO approval, the BLITZDUCTOR XT EX is a pluggable, four-pole, universal DIN rail mounted surge arrester designed for the most stringent requirements on the availability of intrinsically safe measuring and control circuits and bus systems.
With regard to intrinsic safety, the arrester is considered earth-free and its self-inductance and self-capacitance are negligibly small. The low impedance arrester design ensures a high impulse current discharge capacity (min. 10x) and a low voltage protection level.
The built in LifeCheck function allows quick and easy testing of arresters. However, the protection modules may only be read out by means of the hand-held DRC LC reader in non-explosive atmospheres.
Integrated into the protection modules, LifeCheck permanently monitors the current condition of the arrester. Like an early warning system, LifeCheck reliably detects imminent electrical or thermal overload of the protection components. The LifeCheck status can be read out within a matter of seconds by means of the hand-held DEHNrecord LC reader via contactless RFID technology and also shows the date of the last test of the protection module.
To ensure safe operation and reliability in harsh environments, the arresters have successfully passed vibration and shock tests to EN 60068-2 26/27/28. The design of the devices also allows quick and easy replacement of the protection modules which house all the relevant protection elements.
Installed BLITZDUCTOR XT EX
Two-part design with universal base part and application-specific protection
module. Space-saving design for DIN rail mounting.
BLITZDUCTOR XT EX
Universal base part accommodates all types of protection modules. The protection
modules can be easily removed from the base part without signal interruption
thereby minimising storage requirements as well as prewiring and maintenance
operations
.
BXT ML4 BD EX 24
Universal LifeCheck-equipped surge arrester module for protecting two intrinsically safe circuits such as 4-20 mA, HART, PROFIBUS.
BXT ML2 BD S EX 24
LifeCheck-equipped universal surge arrester module for one intrinsically safe circuit such as 4-20 mA, PROFIBUS.
Optionally available with direct/indirect shield earthing.
BXT ML4 BC EX 24
For protecting, for example three or four wire temperature measuring systems.
BXT ML2 BD HF EX 6
For protecting one pair for fast data transmission and supply currents up to 4.8 A.
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