TELECOMMUNICATIONS CABINET ARMORED AGAINST ELECTROMAGNETIC INTERFERENCE
DESCRIPTION OF THE INVENTION The present invention relates to a cabinet for telecommunications equipment, and in particular relates to the shield against electromagnetic interference (EMI) provided by a cabinet. Telecommunications services, such as telephony, Internet and cable television signals, can be distributed among a group of commercial or residential users through telecommunications equipment that is housed in a cabinet dedicated to that group of users. The cabinet may comprise a metal cabinet resting on a concrete bearing. The cabinet has an access opening for the installation and service of telecommunications equipment inside the cabinet, and has an access panel to close the access opening. Since the telecommunications equipment emits and is susceptible to E I, the enclosure is constructed to provide shielding against EMI when the access panel is closed. However, the shielding against EMI can be discontinued through a joint between the access panel and the adjacent edge portion of the cabinet structure around the periphery of the access opening. Therefore, the access panel and the edge portion of the cabinet structure are constructed to be joined to each other in the electrically conductive contact. This contact provides a low resistive impedance path (LRI) across the joint so that the shielding against EMI is not unduly inhibited by the joint. The electrically conductive contact is established by and between electrically conductive blankets, electrically conductive tape, and / or electrically conductive paint on access panels and the cabinet structure. In accordance with the present invention, a telecommunications equipment enclosure shielded against EMI comprises a metal cabinet structure and an access panel. The cabinet structure defines a compartment configured to contain in electrical equipment that emits and is susceptible to EMI. An edge portion of the cabinet structure defines a peripheral boundary of an access opening of the compartment. The access panel includes a metal panel structure. When the access panel is closed, the metal panel structure that extends through the access opening and lies on the edge portion of the metal cabinet structure. The apparatus further includes a layer of electrically non-conductive material that is interposed as a dielectric between the metal panel structure and the edge portion of the metal cabinet structure when the access panel is closed. In this configuration, a low capacitive impedance path (LCI) is provided for the shielding against EMI between the access panel and the metal cabinet structure when the access panel is closed. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be apparent to those skilled in the art upon reading the following description in view of the accompanying drawings, wherein: Figure 1 is an isometric view of an electrical equipment enclosure shielded against EMI comprising a first embodiment of the invention; Figure 2 is a sectional view of the cabinet of Figure 1; Figure 2A is an enlarged partial view of the parts shown in Figure 2; Figure 3 is a sectional view of the parts of a cabinet comprising a second embodiment of the invention; Figure 4 is a sectional view of a part of a cabinet comprising a third embodiment of the invention; Figure 5 is a sectional view of the parts of a cabinet comprising a fourth embodiment of the invention; Figure 6 is a view similar to Figure 5 showing the parts of a cabinet comprising a fifth embodiment of the invention; Figure 7 is a view similar to Figure 2 showing the parts of a cabinet comprising a sixth embodiment of the invention; and Figure 7A is an enlarged partial view of the parts shown in Figure 7. An apparatus 10 comprising a first embodiment of the present invention is shown in Figure 1. The apparatus 10 is an external enclosure for telecommunications equipment, and includes a cabinet structure 12 with an access opening 14. The cabinet 10 in the first embodiment further includes an access panel 16 in the form of a door. A compartment 18 in the cabinet structure 12 is configured to contain telecommunications or other electrical equipment that emits and is susceptible to EMI. The shielding against EMI provided by the cabinet structure 12 is discontinuous through the access opening 14. When the door 16 is closed, it extends completely through the access opening 14, although a seal is then defined around the periphery of the opening 14 between the door 16 and the cabinet structure 12. The present invention completes the shielding against EMI through the joint by providing a low capacitive impedance path (LCI) between the door 16 and the cabinet structure 12 when the door is closed. As shown in Figure 1, the cabinet structure 12 in the first embodiment includes a rectangular metal frame 20 on which the rectangular metal wall panels 22 are mounted to close the upper, lower and three sides of the compartment 18. The frame 20 in the first embodiment is defined by enlarged members 24 of elongated aluminum structure which are interconnected by the wall panels 22. The lower and upper metal cover structures 26 and 28 are mounted on the mount 20 to provide protection against the weather when the cabinet 10 is installed in the open. As shown in Figure 2, each structure member 24 has three main sections 32, 34 and 36 in a cross-sectional configuration somewhat coiled. The first section 32 defines a respective angle 38 of the mount 20. The second section 34 projects from the angle section 32 in a generally C-shaped configuration. A terminal edge portion 40 of the second section 34 has a plurality openings 42 longitudinally separated. The adjacent wall panel 22 has a corresponding plurality of recessed portions 44 with aligned openings 46 through which the fasteners 48 are received to hold the wall panel 22 to the mount member 24. The generally C-shaped configuration provides a space 50 between the terminal edge portion 40 and a base portion 52 extending inwardly. An edge-edge portion 54 of the adjacent wall panel 22 is received in the space 50. The third section 36 of the frame member 24 has almost the same size and configuration as the second section 34, as seen in the cross section in FIG. Figure 2. The third section 36 of this form has a space 55 between the terminal edge portion 56 and a base portion 58 extending inwardly. As further shown in Figure 2, the third section 36 projects from the angle section 32 orthogonally relative to the second section 34, and is oriented in opposite manner relative to the second section 34. Each third section 36 is it is a shape configured to engage and support an adjacent wall panel 22 or, as shown in Figure 2, to define a peripheral boundary of the access opening 14. The door 16 in the first embodiment of the invention is a multi-part structure that includes a main door panel 60 and a thermal insulation panel 62. The main door panel 60 defines the size and peripheral rectangular configuration of the door 16. The flanges 66 project outwardly along the four side edges of the main door panel 60. One of the tabs 66 that extends vertically is connected to the adjacent frame member 24 by a piano hinge 70 in the corresponding space. When the door 16 is in the closed position of Figure 2, the gaskets 72 in the spaces 55 are deflected by means of compression between the frame members 24 and the flanges 66. This provides an external environmental seal between the door 16 and the. structure. 12th, metal cabinet. The joint covers 72 are strips of elastomeric material, as is known in the art, and are preferably mounted on the mount members 24 by adhesively bonded joints 74. The insulation panel 62 is a laminated structure that includes a polyethylene bubble layer 80 sandwiched between outer and inner layers 82 and 84. A bond 86 with adhesive supports the insulation panel 62 on the main door panel 60. When the door 16 is in the closed position of Figure 2, the inner aluminum layer 82 connects the four frame members 24 on the periphery of the access opening 14. Specifically, the inner aluminum layer 82 lies on the third section 36 of each adjacent frame member 24 in the terminal portion 56 of the third section 36. In the first embodiment of the invention, the outer surfaces 90 of the portions 56 of terminal edge are not painted, while all or substantially all of the other surfaces of the frame members 24 in the access opening 14 are preferably covered by at least one paint coating 92. A thin layer 94 of aluminum oxide is shape on each exterior surface 90 not painted. Each layer 94 of aluminum oxide has a constant dielectric of five or more. When the door 16 is closed, the aluminum oxide layers 94 interpose as dielectrics between the aluminum edge portions 56 of the cabinet structure 12 and the inner aluminum layer 82 of the insulation panel 62. The cabinet structure 12 and the multi-part door structure 16 are thus configured in accordance with the present invention to define elongated capacitor structures that extend completely and continuously around the four sides of the access opening 14. This results in an LCI path for the shielding against EMI between the door 16 and the cabinet structure 12 when the door 16 is closed. A second embodiment of the present invention is partially shown in Figure 3. The second embodiment is also a cabinet 100 of outdoor telecommunications equipment. The cabinet 100 has many parts that are substantially the same as corresponding parts of the cabinet 10 described above. This is indicated by the use of the same reference numbers for the corresponding parts in Figures 2 and 3. For example, the cabinet 100 includes a metal cabinet structure 12 with frame members 24 defining an access opening 14, and further includes a multi-part door structure 16 for closing the access opening 14. As the first embodiment, the exterior environmental seals are defined by joint covers 72 on the frame members 24 when the door structure 16 in the second embodiment is closed. An aluminum layer 82 on a thermal insulation panel 62 then lies on the terminal edge portions 56 of the adjacent frame members 24. However, each outer surface 90 in the end edge portions 56 in the second embodiment is covered by an electrically non-conductive paint coating 102. The paint 102 (like the paint 92 described above) is preferably a product commonly referred to as powder paint. When the door structure 16 in the second embodiment is closed, the paint coatings 102 on the outer surfaces 90 interpose as dielectrics between the aluminum edge portions 56 of the cabinet structure 12 and the interior aluminum layer 82 of the panel 62 isolation. The cabinet structure 12 and the multi-part gate structure 16 in the second embodiment are thus configured to provide an LCI path for the EMI shielding according to the present invention. In a third embodiment of the invention, a cabinet can include a main alternative door panel similar to the main door panel 150 shown in Figure 4. Unlike the main door panels described above, which are formed of metal sheet, the main door panel 150 Figure 4 is formed of an electrically non-conductive material, for example, a plastic material. The plastic material may include additives such as stabilizers, fillers, reinforcements and the like. The use of the thermal insulation panel 62 according to the present invention allows the use of the main plastic door panel 150 because LCI is provided by the inner metallic layer 82 of the thermal insulation panel 62. The fourth and fifth embodiments of the invention are partially shown in Figures 5 and 6 respectively. As shown in Figure 5, the aluminum layers 82 and 84 of the thermal insulation panel 62 are interconnected by strips of electrically conductive tape 160 at their peripheral edges. This is found to improve the shielding against EMI provided by the invention. In a variation of this feature, the aluminum layers 82 and 84 can be folded, fused, or otherwise joined together directly, as shown in Figure 6. An exterior cabinet 170 comprising a sixth embodiment of the invention is partially shown in Figure 7. Cabinet 170 also has many parts that are substantially the same as corresponding parts of cabinet 10 described above, as indicated by the use of the same reference numerals for corresponding parts in Figures 2 and 7 The cabinet 170 in this manner includes a metal cabinet structure 12 with frame members 24 defining an access opening 14. The end edge portions 56 of the frame members 24 have layers 94 of aluminum oxide on uncoated exterior surfaces 90 that extend completely around the four sides of the access opening 14. An LCI path for shielding against EMI is provided according to the invention when the inner aluminum layer 82 in the insulation panel 62 joins the aluminum oxide layers 94, as shown in Figure 7. ~ The cabinet 170 in the sixth modality, 172 alternative alternatives are included. The cabinet 170 further includes a main alternative door panel 174 with peripheral tabs 176. The flanges 176 have interior side surfaces 178 parallel to the outer "side" surfaces 180 on which the flashing 172 is attached to the frame members 24. These surfaces 178 of the flanges 176 are sized to engage the corresponding coverings 172 over the areas that they are wider than the areas in which the tabs 66 of Figure 2 engage the corresponding flashes 72. This provides the cabinet 170 with a more extensive environmental seal, each of the other door panels 60 and 150 shown in Figures 2. -4 can be modified according to this feature of the invention - The invention has been described with reference to the preferred embodiments Those skilled in the art will notice improvements, changes and modifications For example, the preferred embodiments described above are exterior cabinets that They have environmental seals that surround the access openings. can be used with an exterior cabinet that does not have an environmental seal, or with an interior cabinet, either with or without an environmental seal. Such improvements, changes and modifications are intended to be covered by the appended claims.