Chassis arrangement for reducing flame emissions

Abstract

A chassis arrangement that minimizes lateral flame spread from card to card within the chassis, and increases heat dissipation from the chassis, comprising a heat sink attached to the inside of the chassis cover to dissipate excess heat from within the chassis, and PCB shields disposed between the circuit cards of the chassis and acting in concert with the heat sink to act as physical and thermal barriers to prevent lateral flame spread from card to card due to flame and hot gas wrap-around.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to chassis for electronic enclosures, and more particularly to an apparatus for reducing flame emissions from such chassis.

BACKGROUND OF THE INVENTION

[0002] For telecommunications equipment installed in central offices (COs) and other telephone buildings, it is recommended that the equipment meet certain minimum equipment-building interface, physical dimension and environmental performance criteria. One such set of criteria is Telcordia standards document GR-63-CORE, “Network Equipment-Building System (NEBS) Requirements: Physical Protection,” Issue 1, October, 1995.

[0003] With certain equipment failures, it is possible for some components to overheat to the extent that they ignite a fire. The NEBS standard includes criteria, described in Section 4.2, that provide minimum fire resistance recommendations for CO telecommunication equipment. These criteria are designed to minimize the fire propagation hazard, and to minimize the likelihood that an equipment assembly fire will spread beyond the structural elements of the equipment on fire. These criteria address measurable conditions that include ignition of material in adjacent equipment frames and assemblies, and visible flames extending beyond the horizontal or vertical confines of the equipment on fire. Compliance to the standard is tested by introducing a flame source into the equipment under test, and observing how the equipment reacts once ignition has occurred.

SUMMARY OF THE INVENTION

[0004] The present invention is a chassis arrangement that minimizes the lateral flame spread from card to card within the chassis, and increases heat dissipation from the chassis. The invention comprises a heat sink attached to the underside of the chassis cover to dissipate excess heat from within the chassis, and PCB shields disposed between the circuit cards of the chassis and acting in concert with the heat sink to act as physical and thermal barriers to prevent lateral flame spread from card to card due to flame and hot gas wrap-around.

DESCRIPTION OF THE DRAWINGS

[0005] The FIGURE shows an exploded view of a preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0006] The FIGURE shows an exploded view of a preferred embodiment of the present invention. Chassis 1 and chassis cover 2 comprise an equipment enclosure that defines enclosed internal compartment 7 for holding printed circuit board (PCB) assemblies such as PCB 3. A PCB shield 4 is disposed between all circuit boards 3. Heat sink 5 is thermally coupled to the bottom face of chassis cover 2.

[0007] In the preferred embodiment, PCB shields 4 are of approximately the same height and depth as PCB cards 3, and are oriented approximately coplanar with PCB cards 3. More particularly, chassis 1 and PCB shields 4 are dimensioned such that the front and rear edges of PCB shields 4, when PCB shields 4 are installed in chassis 1, are approximately flush with the front and rear faces of internal compartment 7 in which PCB cards 3 are installed. In the context of the present invention, approximately flush can also mean actually flush. In addition, PCB shields 4 and heat sink 5 are dimensioned such that, when chassis cover 2 is installed on chassis 1, the top edges of PCB shields 4 are approximately flush with the bottom face of heat sink 5. Having an approximately flush relationship between the front, rear and top edges of PCB shields 4 and the front and rear faces of internal compartment 7 and the bottom face of heat sink 5, respectively, operates to reduce or eliminate lateral flame spread from card to card due to flame and hot gas wrap-around at the edges of PCB cards 3. In addition, PCB shields 4 act as a physical and thermal barrier between adjacent PCB cards 3 such that a PCB card that is on fire or emitting hot gasses does not expose the opposing face of an adjacent PCB card to flame, excessive heat or hot gasses, and directs excessive heat towards heat sink 5. Since heat, hot gasses and flames tend to naturally rise, a flush relationship between the bottom edge of PCB shields 4 and the bottom face of internal compartment 7 is not critical for the reduction of lateral flame spread from card to card due to flame and hot gas wrap-around at the edges of PCB cards 3.

[0008] In general, the dimensioning of PCB shields 4 and the proximal relationship of the edges of PCB shields 4 to the faces of internal compartment 7 and the bottom face of heat sink 5 is such that lateral flame spread from card to card due to flame and hot gas wrap-around at the edges of PCB cards 3 is greatly reduced or eliminated. For example, if internal compartment 7 is deeper than PCB cards 3, PCB shields 4 may only need to extend a small amount beyond the front and/or rear edges of PCB cards 3 to reduce lateral flame spread. Also, while design of PCB shields 4 to cover as much of PCB cards 3 as possible will tend to minimize lateral flame spread, the positioning of combustible material and concentration of fuel load on PCB cards 3 may allow PCB shields 4 to be dimensioned smaller than PCB cards 3 and positioned between PCB cards 3 so as to be effective in reducing or eliminating flame spread from card to card. Other considerations in the positioning, orientation and thickness of PCB shields 4 include that the shields not adversely affect the internal normal operating temperature of the equipment assembly because of excessive air blockage or air flow impedance.

[0009] In the preferred embodiment, PCB shields 4 comprise 0.019″ galvanized sheet steel with a backing of flame retardant non-conductive fiberglass cloth tape. The tape also acts to prevent shorts on the card during installation and removal. This combination of materials proves to perform satisfactorily, is easy to fabricate, and is relatively inexpensive. Other materials may be used that satisfy specific design, performance and economic criteria.

[0010] In the preferred embodiment, PCB shields 4 are attached to PCB cards 3 by screws and standoffs at the rear of the card at card handle 6, and at the front of the card. The flame retardant tape is applied to the face of PCB shield 4 that faces the PCB card to which the shield is attached. The specific means to dispose PCB shields 4 between PCB cards 3 is not important as long as the objectives of the present invention are met.

[0011] Heat sink 5 operates to dissipate heat from within internal compartment 7 so as to reduce the intensity of flames that may be present on PCB cards 3, and to reduce the incidence of flame ignition on PCB cards 3 due to excessive heat build-up in internal compartment 7.

[0012] Heat sink 5 is dimensioned laterally such that it at least spans PCB cards 3 such that excess heat generated by ignition of combustible material on a card can rise and be transferred to heat sink 5. In the preferred embodiment, heat sink 5 extends approximately the entire width of chassis cover 2. Generally, where internal compartment 7 may be wider than the portion reserved for card installation, heat sink 5 need only span sufficiently to dissipate excess heat generated from ignition and flames from PCB cards 3, and also satisfy the desire to reduce card to card flame spread from flame wrap-around over the top edge of PCB cards 3. Thus, heat sink 5 may not need to span the entire width of internal compartment 7. Similarly, generally, the longitudinal dimension of heat sink 5 need only be sufficient to dissipate excess heat generated from ignition and flames from PCB cards 3, and also satisfy the desire to reduce card to card flame spread from flame wrap-around over the top edge of PCB cards 3. In the preferred embodiment, heat sink 5 extends from the front of internal compartment 7 to the vent holes at the rear portion of chassis cover 2.

[0013] The selection of material for heat sink 5 is based on criteria that include mass, flammability, thermal conductivity, linear thermal expansion, cost, availability and ease of fabrication. In the preferred embodiment, 0.3125″ plate aluminum is used. However, any suitable material that satisfies the design and selection criteria may be used.

[0014] Heat sink 5 is sufficiently thermally coupled to the bottom face of chassis cover 2 so as to allow the transmission of heat from heat sink 5 through chassis cover 2 and to the environment. The coupling means for attaching heat sink 5 to chassis cover 2 is not, per se, important, and may comprise, for example, screws, rivets, retaining tabs, or thermal coupling adhesive.

[0015] An alternative embodiment for heat sink 5 and chassis cover 2 comprises a heat sink 5 having a longitudinal dimension that is shorter than that of PCB cards 3, and includes non-flammable material, such as foam or tape, attached to chassis cover 2 adjacent to heat sink 5, and above and approximately or actually flush with PCB cards 3. In this arrangement, heat sink satisfies the thermal related criteria, and heat sink 5 and the non-flammable material act in concert to reduce or eliminate lateral flame spread from card to card due to flame and hot gas wrap-around at the top edges of PCB cards 3.

[0016] A second alternative embodiment includes a thermal fuse attached to any cooling fans in the equipment assembly that may vent hot gasses and flames outside the equipment enclosure. In the event of a fire that produces excessive heat within the enclosure, the cooling fans shut down.

[0017] While the present invention has been shown and described with respect to exemplary embodiments, it will be understood by those skilled in the art that modifications may be made thereto without departing from the scope and spirit of the invention. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims

1. A chassis arrangement for reducing flame emissions, a chassis and chassis cover defining an internal compartment having printed circuit board (PCB) cards installed within, said chassis arrangement comprising: a heat sink thermally coupled to the bottom face of the chassis cover so as to allow the transmission of excess heat from the internal compartment through said heat sink and the chassis cover and to the environment; and PCB shields disposed between the PCB cards, said PCB shields acting as physical and thermal barriers between adjacent PCB cards, said PCB shields acting in concert with said heat sink to prevent or reduce lateral flame spread from PCB card to PCB card due to flame wrap-around and to direct excess heat toward said heat sink.

2. A chassis arrangement according to claim 1, wherein the top edges of said PCB shields are approximately flush with the bottom face of said heat shield.

3. A chassis arrangement according to claim 1, wherein the top edges of said PCB shields are approximately flush with the bottom face of said heat shield, and the front and rear edges of said PCB shields are approximately flush with the front and rear faces, respectively, of the internal compartment.

4. A chassis arrangement according to claim 2, further comprising non-flammable material attached to the chassis cover, adjacent to said heat sink, and approximately flush with the top edges of the PCB cards, said non-flammable material acting in concert with said heat sink to prevent or reduce lateral flame spread from PCB card to PCB card due to flame wrap-around.

5. A chassis arrangement according to claim 1, the chassis further comprising a forced air cooling system, said chassis arrangement further comprising a thermal fuse acting to shut down the forced air cooling system, thereby reducing the venting of hot gasses or flames outside the chassis.