1. Field of the Invention
The field of the present invention relates to a door latch for an electrical equipment enclosure generally, and more particularly to a door latch which can prevent the door of an electrical equipment enclosure from being forced open during a short circuit over current condition without requiring bolts within the latch, of which the following is a specification, reference being had to the drawings accompanying and forming a part of the same.
2. Description of the Related Art
In conventional electrical distribution and control systems, electrical switching devices are often enclosed in a housing having an openable cover or door. Conventional electrical equipment enclosures such as those containing, for example, a motor starter, electric switch, or circuit breaker require durable latches to prevent the enclosure door from blowing open under the arc gas pressure generated upon occurrence of a short-circuit overcurrent condition within any of the enclosed electric equipment.
In FIG. 1, a conventional switch device enclosure 100 is having a switching device (not shown), such as a circuit breaker or switch installed therein. A hinged cover or door 104 is openable via at least one hinge 133 to provide access to the interior of enclosure 100. When closed, the door 104 prevents direct operative access to the enclosed switch (not shown). An operating handle 102 mounted external to the enclosure 100 and movable in the directions indicated by arrow 119 is configured to drive a mechanism (not shown), which in turn acts to toggle the switch (not shown) from a power ON position to a power OFF position. Labels having text such as “ON” and “OFF”, are positioned on enclosure 100 to correspond to operating handle 102 positions that likewise correspond to, and thus indicate, the state of the enclosed switch (not shown). The door 104 is retained in a closed position by at least one releasable door-latching mechanism 128 (FIG. 2A) having a releasable pawl or latch member 108 (FIG. 2A) comprising a tab 118 extending therefrom.
Referring to FIG. 2A, a cut-away side view of the interior of the enclosure of FIG. 1 is shown in the vicinity of the latch mechanism 128. A conventional latch member 108 is rotatably mounted to enclosure 100 by a rivet or pin 138 which provides an axis of rotation A1 for latch 108. A center line CL1 through the center axis of rotation pin 138 and generally orthogonal to the surface of door 104 is shown in FIG. 2A for reference. Latch member 108 comprises a tab 118 having a latching surface 119 configured to latchably cooperate with a latching portion 134 the outer surface of door 104. When enclosure door 104 is closed, an aperture or slot 114 disposed in the door 104 is configured to allow tab 118 to protrude through to the exterior of enclosure 100. To secure the door 104 in a closed position, a bias spring 120 is anchored between latch member 108 and enclosure 100 and disposed to apply a bias force F1 in a first latching direction D1 to maintain at least a portion of latching surface 119 proximal to a latching portion 134 of the outer surface of door 104. Generally, a small air gap 137 is provided between latching surface 119 and latching portion 134. The latching portion 134 of the outer surface of door 104 is conventionally disposed, with respect to the centerline CL1 of the axis of rotation A1, in first latching direction D1. In this way, the latching surface 119 of tab 118 interferes with the surface of door 104 to prevent inadvertant opening of door 104.
To allow the door 104 to open, the latch member 108 is unlatched by manually applying a force F2 to latch member 108 in a second de-latching direction D2 generally opposite to the first latching direction D1, sufficient to cause latch member 108 to rotate in a second de-latching direction D2 around the axis of rotation A1 and allow tab 118 to pass through slot 114.
Latch member 108 is provided with an aperture 112 configured to receive a locking member (not shown) such as the hasp of a lock (not shown) for locking the cover 104 closed.
As shown in FIG. 2B, in the event of a high-pressure condition in enclosure 100, for example, if the switching device (not shown) in the enclosure 100 experiences a short circuit fault, a relatively high instantaneous pressure is generated inside the enclosure 100. Under such a high internal pressure, a resultant expansive force vector Fe is applied generally orthogonal to the enclosure door 104 which causes the door 104 to deflect or move in an outward direction. The door 104, at latching portion 134, in turn contacts the latching surface 119 of tab 118, thus applying the expansive force vector Fe to tab 118. The latching surface 119 of tab 118 is conventionally configured to create a moment arm of length R1 in the first latching direction D1, between the centerline CL1 of the axis of rotation A1 and the latching surface 119 of tab 118. It will be appreciated that, in the event of a high expansive force Fe applied to the latching surface 119 in a direction generally orthogonal to the interior of enclosure door 104, a rotational force, or torque, TR1, is developed in a second de-latching direction D2, is applied to latch member 108 having a magnitude that is the product of the expansive force Fe and moment arm R1, such that TR1=Fe×R1. The rotational force TR1 biases the latch 108 in the second de-latching direction D2, and, if of sufficient magnitude, for example greater than the force applied by bias spring 120, results in the rotation of latch 108.
As shown in FIG. 2C, and as discussed above, in the event of a high-pressure condition in enclosure 100, the conventional latch 108 may unlatch or move out of position, and allow the door 104 to open, thus releasing hot gasses and debris.