FEATURE HAZARDOUS AREAS


LED encapsulation explained for users of hazardous area luminaires


In this article, Tony Oram of Barrier Ex explains why there is a misconception of the term ‘encapsulation’ particularly for LED luminaire applications, with the aim of raising awareness and to ensure end users know what they should be looking for when they select luminaires for hazardous area environments energy solutions to minimise consumption, costs and improve energy resilience


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eferring to relevant Standards’ Terms and Definitions, the definition of encapsulation is:


‘type of protection whereby parts that are capable of igniting an explosive atmosphere by either sparking or heating are fully enclosed in a compound or other non-metallic enclosure with adhesion in such a way as to avoid ignition of a dust layer or explosive atmosphere under operating or installation conditions’. So: It is very clear that parts must either be ‘fully


enclosed in a compound’, or ‘fully enclosed in a… non-metallic enclosure with adhesion’. The definition for adhesion is: ‘moisture, gas and dust tight permanent bonding of a compound to a surface’. Adhesion is not encapsulation. Within the relevant sections in the Standards


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we find the ‘Constructional Requirements’ state, ‘…the requirements differ according to whether the compound adheres to the enclosure. Where adhesion is specified…to maintain the type of protection, it shall be maintained after completion of all the prescribed tests… NOTE Tests for adhesion are under consideration’. The use of ‘non-metallic enclosure’


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encapsulation creates intentional air pockets, voids or ‘free space’ within the 26 Standards assembly. The definition for free space is: ‘intentionally created space surrounding components or space inside components’ Under ‘Free space in the encapsulation’


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for Group II equipment, ‘The sum of the free spaces shall not exceed 100 cm3


level of protection “mb”’. For volumes between 10-100cm3


5 , the


Standard allows a 3mm minimum compound thickness adjacent to such free spaces, but only if proven by a high-pressure test. If the enclosure encapsulating the free space is also adhered to by encapsulant the 3mm may comprise both the enclosure plus compound. Therefore if the enclosure walls are 3mm


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thick there is no additional encapsulation compound required, again subject to passing the pressure test. Samples, which have already undergone thermal conditioning and are therefore in a weakened state, are prepared with a pressure connection into the free space. The pressure applied varies with the equipment’s specified minimum ambient temperature, between 1MPa and 1.62MPa which is considerable. When encapsulating luminaire LEDs, an


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optically clear encapsulant may be applied directly, avoiding free spaces and making a


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pressure test redundant. However, some lighting products are designed with a clear cover placed over the naked LED array, creating a ‘free space’. The cover flange may be glued down but is then surrounded by a moat filled with encapsulant. It should technically be re-categorised as an adhesive compound, as to ‘encapsulate’ is to completely surround or submerge. Adhesion is not encapsulation, nor does the cover fulfil the definition of a non-metallic enclosure. The cover is usually closed by sealing against the LED PCB surface. Unless the cover-flange seal used plus the surrounding adhesive compound are both extremely tough, and unless both have excellent adhesive quality joined to a high surface-energy adhesion surface, the high-pressure test will break through any possible leak paths. In any case, any exposed cover may still rupture. If an adhesive foam gasket were used to seal an LED cover, surrounded by flexible adhesive such as RTV silicone, it would easily rupture subject to the high pressure test. So a concession is introduced: ‘…if the component… prior to encapsulation, passes the Leakage test on sealed devices specified in IEC 60079-15 (without the conditioning, voltage, or dielectric withstand testing) it can be encapsulated without requiring the pressure test.’ Important Note: Sealing is a protection


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concept for ATEX Category 3 equipment, zone 2, EPL Gc, Ex‘nC’. This is a concession which moves outside of the Zone 1 protection concept thereby blurring the lines between acceptable Zone 1 and Zone 2 equipment. The Ex’m’ Standard concedes that any one of the 3no. tests in the Ex’nC’ Standard may be performed upon samples. The LED PCB subassembly is to be tested prior to ‘encapsulation’, therefore every leak path, machined hole, screw hole, wire entry etcetera must be sealed. The test sample must ensure that it withstands the leak test in its final designed production form and material specification. The Ex‘nC’ Standard is explicit that these tests are to be performed after the prescribed rigorous conditioning process. But the Ex‘m’ concession waives this


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mandatory conditioning stage. There is some ambiguity even with the test instruction, ‘The test samples… are suddenly immersed…to a depth of 25mm…’ Is this measured from the bottom of the test enclosure, or from the highest part of the sample? By comparison, the IEC IP67 test specifies the height of the sample


and the minimum height of water above the sample, so it should be reasonable to apply the same to this test. There is also the question of whether the water bath temperature should be maintained throughout. So to conclude the key points for anyone


looking at acquiring lighting bound for hazardous areas are: ‘Free space’ allowance is to provide for


mechanical, physical or thermal functionality within small components such as relays, fuses etcetera. Since LEDs do not require this to function they should typically be encapsulated in clear compound. Adhesion is NOT encapsulation! The Ex’m’ Standard allows a 3mm-thick


walled ‘free space’ enclosure without further compound encapsulation if a high-pressure test is passed. (for ‘mb’, 10 – 100cm3


). But a


concession is made on the high pressure test in favour of a sealed enclosure leakage test, conducted before encapsulation. The Ex‘nC’ Standard stipulates the leakage


test for adhesion is to be conducted only after conditioning. But the Ex‘m’ concession waives this requirement also, thereby weakening the protection concept further. There is no provision within the relevant standards for partial encapsulation.


Barrier Ex www.barrier-ex.co.uk / INDUSTRIALCOMPLIANCE


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