Abstract:
A protective housing designed as a pressure proof capsule has a pressure relief valve which comprises a porous body  23  and a pore closure  31  about its perimeter for preventing the transition of a flame about an edge of the porous body. Alternative embodiments of pore closures  31  are disclosed, including a soft metal ring about an edge of the porous body that is depressed, such as by a pressure ring, into a gapless imbedding relation to the porous body. An edge region of the porous body can be modified by jacketing materials such as synthetic materials, metals and the like for creating a form fitting bond with the body of the protective housing.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a pressure relief device for protective housings and more particularly, to a pressure relief device for protective housings effective for the pressure-proof encapsulation of operating means that are capable of forming ignition sources. 
       BACKGROUND OF THE INVENTION 
       [0002]    Protective housings of the foregoing type are employed to encapsulate electrical operating means that can be used in an environment that poses an explosion hazard. Under certain circumstances, such electrical operating means may act as ignition sources. If explosive gases or gas mixtures have entered the pressure-proof housing, these can detonate or explode. The resultant pressure must not damage or destroy the housing in a manner that flames or hot particles move to the outside. The housing must withstand the occurring maximum explosion pressure. 
         [0003]    U.S. Pat. No. 4,180,177 suggests that a window be provided in a pressure-proof encapsulated housing, wherein a plate of foam is provided consisting of stainless steel. This metal foam plate has a considerable pore volume and thus allows uncombusted, as well as combusted, gases to flow out of the housing volume, in which case the metal foam acts, at the same time, as an effective flame arrester due to the cooling effect of the foam. 
         [0004]    DE 1170346 also suggests porous metal bodies such as metal wool. Furthermore, ceramic filters and porous sintered iron bodies are mentioned as flame arresters. 
         [0005]    In order to effectively prevent flame transmission, each channel leading from the housing interior to the outside must have a narrow gap width and a considerable length. The flame arrester ensures this by appropriate dimensions of the mesh size or the pore size and the pore volume. The conditions regarding the occurring gap width and thus the flame transmission safety must be maintained not only on the filter body itself but also at its transition to the enclosure. This must also be ensured in the event of any increased internal housing pressure that might act on the filter body, such pressure potentially occurring during an explosion or detonation in the housing interior. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0006]    It is an object of the present invention to provide a pressure-proof encapsulation housing having a pressure relief valve arrangement that enhances safety against flame transmission from the housing. 
         [0007]    The pressure relief device according to the invention comprises a receiving body with a passage in which a porous body is arranged. This porous body is enclosed on its edge or on a receiving body that may be configured so as to be a separate component or part of the housing wall. The edge comprises the outside circumferential surface as well as the two radially outside edge zones of preferably flat surfaces through which the gases can flow in and out. The edge is already provided with a pore closure when at least one of the aforementioned surfaces comprises closed pores. As a result, it is possible to prevent flame transmissions along the edge of the porous body. Consequently, the edge-side pore closure improves the safety of the pressure relief device against flame transmission. 
         [0008]    The porous body, for example, may be a fiber structure. This may be irregularly ordered fibers of metal, for example, said fibers being connected with each other by sintering, for example, so that a stiff, felt-like metal body displaying a large pore volume is formed. The fibers may consist of one uniform metal or of different metals. 
         [0009]    However, it is also possible to use wire for producing the porous body. For example, a braided or knit fabric of wire, a single-layer or multi-layer woven fabric of wire, wire screen structures or the like may be used. Preferably, multi-layer woven wire fabric composite plates, for example, consist of flat wire mesh arrangements that are connected to each other. In doing so, the same or different wires having the same or different diameters and material properties may be used in the individual layers as well as in the different layers. 
         [0010]    Furthermore, it is possible to provide a configuration of several expanded metal layers or ribbon coil arrangements as the porous body. Furthermore, the porous body may be a sintered metal body that consists of several metal particles bonded to each other by sintering. The metal particles may be metal spheres having uniform or different diameters or may be metal bodies of the same or different metals and having different shapes. 
         [0011]    The edge-side pore closure of the body, according to an embodiment of the invention, blocks a flame that has formed inside the housing, for example, in its path into the edge area of the fiber structure. In particular, this edge-side pore closure prevents the flame from bypassing the porous body. Thus, it is also not possible for any flame transmission to occur at the edge of the porous body. 
         [0012]    In order to close the pores, the edge of the porous body may be biased against a seating surface provided on the receiving body in order to close the pores of the porous body toward the outside due to a firm abutment of the edge against the seating surface. The firm abutment between the enclosure and the seating surface is preferably achieved by a tensioning arrangement. For example, the seating surface may be conical. In this case, the edge is preferably also such a conical, mechanically machined abutment surface. Mechanical machining may include, for example, machining by grinding/polishing, laser cutting or a similar method of machining that leaves a smooth surface. In this context “smooth” is understood to mean a surface with a roughness height that is at least as great as the maximum pore cross-section of the porous body. Preferably, the roughness of the abutment surface is lower. Due to the resultant gapless seat of the porous body, there no longer exist any gaps larger than those of the porous material between the porous material and the surrounding material, i.e., the seating surface. Consequently, a bypassing of the porous body by hot gases, particles or flames is prevented. 
         [0013]    The pore closure has the effect that flames that are being formed remain locked in the housing and cannot exit toward the outside. However, the pressure relief device allows cooled gases to exit and thus minimizes the peak pressure occurring in the housing. The mechanical strength of the protective housing required for pressure-proofing can thus be lowered. 
         [0014]    It has been found to be useful to provide means that ensure the gapless joining of the porous body to the receiving body even when the porous body is being slightly moved or deformed, for example, as a result of a load acting on the body. Thus, the body may be mounted on edge-side annular zones or, preferably be firmly clamped in place. Furthermore, it may be advantageous to shape the edge of the porous body in a conical manner so that said body tapers toward the outside—viewed from the housing interior. Explosive pressure occurring in the housing thus effects a tighter abutment of the edge-side abutment surface of the porous body against the seating surface of the receiving body and thus an improved seal at this site. 
         [0015]    It is also possible to mount the porous body using an edge enclosure that develops a tension pressure such that the pores of the porous body collapse in the active region of the edge mount, thus, again establishing the pore closure. This is particularly useful in porous bodies of wire. 
         [0016]    Furthermore, it has been found to be advantageous to produce the edge-side pore closure of the body with a material that closes the pores, for example, in that it penetrates into the pores at least somewhat. This material may a soft, plastic material such as, for example tin, a synthetic material, for example a thermoplastic or a duroplastic synthetic material, an elastomer or the like. Preferably, this material forms a hoop enclosing the edge of the porous body, said hoop closing the pores, on the one hand, and being seated in a gapless manner in the receiving body, on the other hand. In addition, this hoop may be glued into the receiving body or be secured therein by other sealing means. The material enclosing the edge of the porous body (metal or non-metal) may be considered a seal. Preferably, this seal molds itself in a plastic manner into the edge-side pores of the porous body. 
         [0017]    A collar bonded to the receiving body, e.g., by a sealing compound, may be provided on the hoop. In doing so, the seal will be axially at a distance from the porous body. Deformations of the porous body, e.g., due to explosions in the housing interior do not damage the seal, so that edge-side gaplessness remains ensured. In addition, the hoop and the collar separate the applied sealing compound from the porous body so that the penetration of potentially thinly liquid sealing compound into the porous body is avoided. 
         [0018]    In addition, a membrane may be provided that seals the passage of the receiving body so as to be gas-permeable but water-impermeable. Additionally or alternatively, a spray guard may be provided. 
         [0019]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a diagrammatic depiction, partially broken away, of an illustrative protective housing having a pressure relief device in accordance with the invention; 
           [0021]      FIG. 2  is an enlarged perspective vertical section of the pressure relief device shown in  FIG. 1 ; 
           [0022]      FIGS. 3-6  show alternative embodiments of porous structures that can be used in the pressure relief device shown in  FIG. 2 ; 
           [0023]      FIG. 7  is a perspective vertical section of a modified embodiment of a pressure relief device in accordance with the invention; 
           [0024]      FIG. 8  is an enlarged perspective vertical section of the porous body of the pressure relief device shown in  FIG. 7 ; 
           [0025]      FIG. 9  is an enlarged perspective vertical section of the edge of the porous body shown in  FIG. 8 ; 
           [0026]      FIG. 10  is a perspective vertical section of another embodiment of a pressure relief device in accordance with the invention; 
           [0027]      FIGS. 11 and 12  are enlarged perspective vertical sections of the porous body of the pressure relief device shown in  FIG. 10 ; 
           [0028]      FIG. 13  is a perspective vertical section of a further alternative embodiment of a pressure relief device in accordance with the invention; 
           [0029]      FIG. 14  is an enlarged perspective vertical section of a further alternative embodiment of the pressure relief device; and 
           [0030]      FIG. 15  is a perspective section of a further alternative embodiment of the pressure relief device in accordance with the invention. 
       
    
    
       [0031]    While the invention is susceptible of various modifications and alternative constructions, certain illustrative embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions and equivalents falling within the spirit and scope of the invention. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    Referring now more particularly to  FIG. 1  of the drawings, there is shown an illustrative protective housing  10  in accordance with the invention that can accommodate various types of components, such as for example, electrical installations such as circuit boards, driving devices, electronic or electrical components and the like, which can potentially act as ignition sources during operation or in error in faulty situations. The illustrated protective housing  10  has a pressure-proof wall  11  that defines a tightly enclosed interior  12  of the protective housing  10 . The interior  12  is not hermetically sealed with respect to the environment. Consequently, flammable gases may enter the interior  12 . However, the wall  11  is tight in so far that no openings or gaps are provided at any point through which a flame transmission could occur. 
         [0033]    The wall  11  in this case comprises lateral parts  13 ,  14 , an upper cover  15  and a bottom  16 . The lateral parts  13 ,  14 , as well as the upper cover  15  and/or the bottom  16 , may have an opening  17  in which a pressure relief device  18  is installed. The opening  17  in this case is in the bottom  16 , and the pressure relief device  18  establishes a gas-permeable connection between the interior  12  and the environment which exhibits a low flow resistance for reducing pressure peaks in the event of explosion-like reactions in the interior  12  such that developing gases can flow out rapidly and easily. 
         [0034]    A first embodiment of the pressure relief device  18  is depicted in  FIG. 2 . The pressure relief device  18  comprises a receiving body  19  having a central passage channel  20 . As illustrated, the receiving body  19  may be a separate component or, alternatively, also be a part of the housing wall, in all the embodiments. This receiving body is disposed to connect the interior  12  of the protective housing  10  with the environment in a pressure-equalizing manner. The cross-section of the passage channel  20  may have round or polygonal boundaries. In the present example, the cross-section is circular. 
         [0035]    The receiving body  19  may be provided with a flange  21  on its upper end. As depicted in  FIG. 2 , the flange  21  is at the end facing the interior  12  of the protective housing  10  when in use and abuts against the inside of the wall  11 , in this case the bottom  16 . The outside circumference of the receiving body  19  may be provided with fastening means such as, for example, an external thread  22  effective for fastening the pressure relief device  18  in the opening  17 . To accomplish this, the housing wall may be provided with a matching internal thread. Additionally or alternatively, the receiving body  19  can receive a locknut or be provided with other securing or fastening means. 
         [0036]    Arranged in the passage channel  20  is a porous body  23  that acts as a flame arrester. In the present exemplary embodiment, the porous body  23  is designed as a flat, disk-shaped body having a circular contour and uniform thickness. In order to support the porous body  23 , the receiving body  19  comprises a screen bottom  24  extending transversely through the passage channel  20 , said screen bottom being, for example, a stiff one-piece component of the receiving body  19 . The screen bottom  24  has a large number of holes that allow gas to pass freely, and the porous body  23  is in contact with the side of the screen bottom  24  that faces the interior  12 . 
         [0037]    Preferably, the porous body  23  has a tight mesh structure. For example, as shown by  FIG. 2 , it comprises several superimposed wire grid layers  25 . The plurality of grid layers  25  may be arranged loosely on top of each other or be connected with each other. If each grid layer  25  comprises groups of parallel-arranged wires, wherein the wires of one group crosses the wires of the other group and wherein, for example, weld spots are provided at the intersection points. The grid layers  25 , among each other, may also be connected by weld spots, solder spots or the like. However, the grid layers  25  may also be a woven wire fabric or a braided wire material, wherein the different grid layers  25  may be connected among each other or be loosely arranged on top of each other. 
         [0038]    The porous body  23  has a surface  26  facing the interior  12 , a surface  27  located on the screen bottom  24 , and an edge  28 . The edge comprises  28  an edge surface  28   a  following the surface of a cylinder jacket, as well as radially outer annular zones of the surfaces  26  and  27 . The edge  28  is at least partially enclosed—at least on the outer edge surface  28   a —by a cuff  29  of soft metal, for example tin, said cuff additionally transitioning—on least one side of the porous body  23 , for example the side  26 —into an annular collar  30  that is seated on an outer annular zone of the surface  26 . The cuff  29  and the annular collar  30  are soft enough that they abut—under pressure—in a sealing manner at least against the outer annular zone of the surface  26  and thus form a pore closure  31  in the form of a plastic seal. 
         [0039]    In order to firmly press the annular collar  30  against the porous body  23 , a clamping nut  33  is screwed into the passage  20  by means of an internal thread  32  provided there. The clamping nut  33  in this case has, on its side facing the surface  26 , a grove for the accommodation of the annular collar  30 . When the clamping nut  33  is tightened, it presses the annular collar  30  in axial direction against the outer edge of the surface  26  and allows the plastic material of the cuff  29  to flow in axial direction and then, optionally, also radially inward, so that the outer pores of the porous body  23  are closed on its edge  28 . Any gap that might potentially still exist at this point will be sealed to the extent that no flame transmission bypassing the porous body  23  is possible. 
         [0040]    Optionally, the passage  20  may additionally be closed by a membrane  34 . For example, this membrane may consist of a material such as used for many types of weather-proof clothing. It may be a breathable polymer film of a block polymer, wherein the polymers alternate, for example, between hydrophilic and hydrophobic sections. The membrane  34  may be secured by a rubber cuff  35  or similar means to the outer fitting of the pressure relief device  18 . In addition, the membrane  34  may be supported from the inside by a grid  36 . 
         [0041]    The pressure relief device  18  described by the foregoing operates as follows: 
         [0042]    The pressure relief device  18  screwed into the opening  17  holds the porous body  23  in a gapless bond with the receiving body  19 . The porous body  23  creates a pressure relief filter that is enclosed in its edge area by a soft metal ring in this case having the form of the cuff  29 . By tightening, i.e., axially tightening, the clamping nut  23  and due to the concomitant deformation of the soft cuff  29 , a gapless embedding of the porous body  23  in the receiving body  19  is achieved. 
         [0043]    The membrane  24  closes the passage  20  toward the outside to make it water-tight. Preferably, the membrane  34  is breathable and can thus allow gaseous water, i.e. water vapor, present in the interior  12  to escape from the interior  12 , thus avoiding an over-humidification of the interior  12 . 
         [0044]    If an explosion or detonation occurs in the interior  12 , this is accompanied by a sudden pressure increase in the interior  12 . An outward gas flow that is hardly impaired by the porous body  23  may develop. This may explode the membrane  34  off the pressure relief device  18 , tear it open or otherwise open it. The gas stream exiting through the porous body  23  is sufficiently cooled by the body  23  so that no hot particles or flames escape from the pressure relief device  18 . Therefore, an ignition of an ignitable gas mixture that might potentially be present outside the protective housing  10  is safely precluded. 
         [0045]    Due to the low flow resistance and the relatively large surface of the porous body  23 , an excessive pressure increase is prevented in the interior  12 . This can be taken into account when the pressure-resistance parameters of the protective housing  10  are determined. Consequently, by installing the pressure relief device  18  in accordance with the invention, the material use and the complexity of the design of the protective housing  10  can be minimized. In addition, it may be possible to design the protective housing  10  for exceptionally low temperatures, at which a larger amount of ignitable gas may enter into the housing and the material used for the construction of the wall  11  may display decreasing mechanical strength due to increasing brittleness. 
         [0046]    In the pressure relief device  18  as depicted in  FIG. 2 , it is possible to insert various suitable approximately disk-shaped porous bodies  23  that are effective to allow a largely unimpaired gas outflow, on the one hand, and thus have narrow gap or pore widths that reliably preclude a flame transmission, on the other hand. 
         [0047]    An alternative embodiment of such a body  23  is depicted in  FIG. 3 , which shows a section of the plan view of such a body. As can be seen, the body  23 —in this case—consists of a fabric of a cooling material such as, for example, wire, wire rope, ceramic rope or the like. Several such layers may be connected with each other to form a flexible, or also rigid, body. 
         [0048]    Alternatively, it is possible, as is shown in  FIG. 4 , that the body  23  may be formed by a subordinate arrangement of fibers or filaments that, for example, may consist of a metal or also of other materials such as ceramic or a mixture of different fibers, e.g., also ceramic fibers and metal fibers.  FIG. 4  shows a plan view of a sintered metal fiber body that is designed in such a manner that the pores in it have different sizes, however no pore is large enough to permit a flame transmission. 
         [0049]      FIG. 5  shows another embodiment of the porous body  23 . In this case, the porous body  23  consists of a number of small metal spheres that are bonded to each other and, preferably, are the same size. They also may be bonded to each other by a sintering process. 
         [0050]    Alternatively, as is shown in  FIG. 6  by a sectional plan view, the molded body  23  may be formed by undulated metal ribbons that are spot-connected to each other, with the metal ribbons being arranged in several layers on top of each other, for example, in order to create the air-permeable body  23 . Alternatively, the ribbons formed in the body  23  as in  FIG. 6  may also have a width that corresponds to the total height of the body  23 , in which case—in that embodiment—advantageously straight passage gaps are formed that offer a particularly low flow resistance. 
         [0051]    It will be understood that the above-described embodiment, as well as all the embodiments described hereinafter, may be supplemented with additional details. For example, it is possible to arrange, in the space below the screen bottom  24  or also between the screen bottom  24  and the molded body  23 , a molded body or powder. These may adsorb foreign substances from the air. As a result, such foreign substances may be prevented from entering the pores of the body  23  or from entering the housing. 
         [0052]    In order to ensure the function of the pressure relief device  18  over the long term, the interior sides of the passage opening  20  may be provided with a coating that continuously releases silver ions. As a result of this, the surfaces are protected against germs and bacteria over an extended time so that no microorganisms that would clog the pores of the filter or the porous body  23  are able to develop. It is also possible to provide the pores of the porous body  23  with a coating that releases silver ions in order to preclude microbial fouling. 
         [0053]    Provisions may be made that, following an explosion, the entire pressure relief device  18  or parts thereof such as, for example, the body  23  and/or the membrane  34 , are exchanged or replaced. In the simplest case, it is only the rubber cuff  35  or other possible fastening ring and the possibly torn membrane  34  that are removed and replaced by new parts. 
         [0054]      FIG. 7  shows a modified embodiment of the pressure relief device  18 , to which the above description applies correspondingly, with the exception of the special features explained hereinafter: 
         [0055]    The passage body  19  in this case is designed without a screen bottom, which also is optional in all the embodiments. In place of a screen bottom  24 , a plane annular seating surface  36  is provided in the passage channel  20 , disposed for accommodating the body  23 . The body  23  may be designed in accordance with one of the aforementioned types, and in this case is depicted separately in  FIG. 8 . As is depicted, its edge  28  is provided with a pore closure  31  that is made, for example, of a synthetic material, or a metal. The material encloses the edge surface  28   a,  as well as the outer edges of the sides  26 ,  27 , in a gapless manner and penetrates into the outer regions of the pores located there. As is shown in particular in  FIG. 9 , the thusly formed hoop body  37  has an outside  38  shaped like a cylindrical shell as well as planar annular upper side  40 . Extending from the latter may be a tube-shaped axial extension  41  that adjoins the inner edge of the upper side  40 . As apparent from  FIG. 7 , the annular space formed between the wall of the passage channel  20  and axial extension  41  can be filled with the sealing compound  42 , said compound ensuring a gapless seat of the body  23  in the passage  20 . In addition, the sealing material  42  is prevented from advancing into the porous body  23  and sealing it. 
         [0056]    If needed, the clamping nut  33  may additionally be screwed into the passage channel  20  in order to firmly secure the filter body comprising the body  23  and its jacket in the passage  20 . 
         [0057]      FIGS. 10 and 11  illustrate another embodiment of the pressure relief device  18 . The descriptions of the foregoing embodiments correspondingly apply, with the exception of the explanations hereinafter: 
         [0058]    The porous body  23  in this case a conical edge surface  28   a,  instead of a cylindrical edge surface  28   a.  The edge surface preferably is precision-machined, for example, by grinding, laser cutting or the like, so that a mechanically machined abutment surface  43  is produced. The abutment surface is preferably associated with a similar conical seating surface  44  that is concentrically arranged with respect thereto and adjoins the support surface  36 . The annular face surface of the clamping nut  33  pushes against the outer edge of the surface  26  and thus presses the abutment surface  43  against the seating surface  44  in order to achieve a gapless seat of the body  23 . Any explosive pressure acting on the surface  26  increases the contact pressure between the abutment surface  43  and the seating surface  44  and thus prevents even an only short-time formation of gaps. Consequently, consistent with the previously described embodiments, the abutment surface  43  and the seating surface  44  together again form the pore closure  31 . 
         [0059]    As illustrated by  FIG. 12 , the pore closure  31  may be formed by a hoop-like enclosure of the body  23 , in which case this enclosure preferably again consists of a synthetic material, for example a thermoplastic or duroplastic, hardening synthetic material, or also of a soft metal, e.g., tin. The enclosure surrounds the edge  28  of the body  23  and penetrates, at least partially, into the open pores present there in order to close said pores. The resulting enclosure body  45  also may be provided with an exterior conical abutment surface  46  that interfaces without gaps with the seating surface  44  described above. Again, the clamping nut  33  can be used to bias the seat and suppress the formation of gaps. 
         [0060]      FIG. 13  illustrates another embodiment of the pressure relief device  18  comprising a passage body  19  that may be an adaptation to the embodiment of  FIG. 10 , for example. In this embodiment, the body  23  as in  FIG. 12  may be used. However, it is also possible, as shown, to insert the porous body  23  in an annular enclosure body  45  that, preferably, consists of a soft metal. The cylindrical edge of the body  23  abuts against the cylindrical inside surface of the enclosure body  45 , while the outside conical surface of said enclosure body abuts against the conical seating surface of the receiving body  19 . However, it is also possible to provide the embodiment of the body  23  as in  FIG. 11  or, if there is no conical seating surface  44  but a cylindrical enclosure, any other of the above-described embodiments as the enclosure for the body  23 . 
         [0061]    In the present case, an axial pressure spring  46  is interposed between the enclosure body  45  and the clamping nut  33  and, optionally a pressure distribution ring  47  is provided. In addition, the clamping nut  33  may be secured in place with a counternut  48 . The pressure spring  46  in this instance is in the form of a disk spring. The enclosure body  45  preferably consists of a soft metal, or a synthetic material, or an elastomer. 
         [0062]    On the outside of the body  23 , a molded element having a plurality of labyrinth-like openings may be provided. The molded body may act as a weatherproof protection. When properly installed, this molded element preferably faces downward. 
         [0063]    The previously described membrane  34  with a suitable mounting means, for example in the form of a rubber cuff  35 , may provide the lower closure. As, it is possible—as in all of the previously described embodiments—to also provide an extension above the flange  21 , with the extension holding another membrane  49 , for example, with a cuff  50 . The membrane  49  may be designed so as to match the membrane  34 . Preferably, it is divided in the middle in order to be able to easily fold up toward the body  23  in case of an explosion. In doing so, the passage channel is cleared when pressure builds up in the housing and cleavage products—e.g., resulting from the combustion of the membrane—are carried by the flame into the pores of the filter, thus clogging the filter. Here, again, a silver ion coating on the inside of the clamping nut  33  and/or other parts, e.g., the body  23 , is intended to prevent biological matter from settling and thus prevent a clogging of the pores. 
         [0064]      FIG. 14  shows another embodiment of the invention. In this case, the receiving body  19  is reduced to a threaded ring  51 . In its central passage, there is seated a tube-shaped extension  52  that is glued or molded into the threaded ring  51 . For example, the extension  52  may consist of a polymer, an elastomer or another synthetic material. Above the threaded ring  51 , the extension terminates in a flange  53 , whose flat annular lower side abuts against the inside of the wall  11  of the protective housing  10  in order to form a gapless seal. The flange  53  consists of a material that is molded around the edge  28  of the body  23 , whereby it partially penetrates the pores of the edge  28 . Preferably, this material also penetrates into the pores of an outer annular region of the surface  26 ,  27  of the body  33  thus enclosing said body in a substance-bonded manner while forming the pore closure  31 . When pressure relief device  18  is installed in a protective housing  10 , it operates consistent with the above description. 
         [0065]    If the synthetic material used for producing the extension  52  and the flange  53  is sufficiently strong, there is no need for the threaded ring  51 . The thread may be applied outside, directly to the extension  52 . In doing so, the extension  52  and the flange  53  form the receiving body that seals the porous body  23 , in which case the mentioned pore closure  31  is again provided on the edge  28 . Instead of the external thread on the threaded ring  51  or on the flange  52  or on the receiving body  19 , it is also possible to use any suitable, sealing fastening means including glued or welded connections. 
         [0066]      FIG. 15  shows another alternative for achieving the pore closure  31  at the edge  28  of the body  23 . The clamping nut  33  is provided with an annular rib  54  on its face facing the body  23  that presses into the outer edge of the surface  26  when the clamping nut  33  is tightened. Likewise, the surface  36  may be provided with a rib  55  that presses into the surface  27  of the body  23  when the clamping nut  33  is tightened. As a result of this, the edge  28  of the body  23 , which preferably consists of a wire material, is compressed to such an extent that many of the existing pores collapse. The pore volume at the edge  28  of the body  23  is reduced to such an extent that a pore closure  31  is created. Consequently the ribs  54 ,  55  form the means to produce the pore closure  31 . 
         [0067]    From the foregoing, it can be seen that a protective housing designed as a pressure-proof capsule is provided with a pressure relieve device comprising a porous body  23 . On its edge, said porous body is provided with a pore closure  31  in order to prevent the transmission of a flame to said edge. The enclosing component represents a receiving body  19  for the pressure-relief device. In conjunction with the receiving body  19 , the pore closure forms a gapless bond. For example, the porous—unmachined—body  23  may be enclosed by a soft metal ring along its edge region and, for example, may be pressed through a prespecified conical contour of a receiving body and a pressure ring in such a manner that a gapless embedding is achieved due to the deformation. In addition, the deformation may be maintained lastingly by resilient elements in order to compensate for any heat expansion of the various materials in case of temperature fluctuations. The receiving body  19  may also be formed by the wall  11  itself 
         [0068]    Alternatively, the edge region of the body  23  can be modified by jacketing with materials such as synthetic materials, metals, etc., in such a manner that the jacketing, together with the receiving body  19 , creates a form-fitting bond with the use of glue or a sealing compound, which bond complying with the Ex-protection requirements. 
         [0069]    It will be understood that the use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e. meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
         [0070]    Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.