As is well known, electrical enclosures exposed to corrosive or other contaminant laden air are susceptible to and may corrode or otherwise be attacked thereby. Contaminated atmospheres exist in many chemical plants and frequently where corrosive and other chemicals are used in the manufacture or processing of various products. The use of plastic enclosures in such atmospheres is a decided advantage because, depending upon airborne contaminant concentration and time, enclosures made of economically feasible metals may chemically react with those contaminants so as to freeze cover to box or become so eaten away as to become useless for their intended purpose.
Where electrical enclosures containing arcing devices are used in and around explosive atmospheres, various codes and standards prescribe that they be of explosion proof design to avoid potential injury to life and property. The underlying philosophy of explosion proof design where arcing could occur inside the enclosure is not to prevent explosive gases from gaining entry into the enclosure, but rather to contain any explosion that does occur therein so that the explosive flame front cannot propagate outside of the enclosure and ignite the surrounding atmosphere. Thus, explosion proof enclosing permit, but contain, a limited explosion therein rather than attempt to exclude an explosive gas.
Two principally plastic enclosures displaying somewhat different explosion proof design philosophy are disclosed in U.S. Pat. Nos. 3,927,249 to Pearse and 4,260,863 to Appleton. Both patents show a box and cover having a common interface when assembled that is not airtight and that can thus breathe so as to admit and expel whatever atmosphere surrounds the device, that atmosphere potentially containing contaminants. Various code requirements dictate the design of this interface, and when the interface is flat, some minimum amount of the material forming the interface must be metal. The Pearse patent incorporates a flat interface, and therefore this largely plastic enclosure incorporates metallic inserts that form a large part of the interface to satisfy this requirement of having metal at the interface. However, to satisfy explosion proof design requirements, the maximum allowable gap at the interface is so small that the Pearse device would more than likely require a secondary machining or grinding operation to achieve the necessary gap restrictions between box and cover where they join. Yet, some gap is desirable because any explosion occurring within the enclosure is vented through the gap to hold down internal pressure and prevent the enclosure from rupturing and thereby releasing the flame front to the surrounding atmosphere. Thus, the internal pressure is rather quickly relieved by venting expanding internal gases through the gap, and the flame front is quenched by the metallic surfaces defining the gap as it escapes therethrough.
If the contaminated atmosphere acts to eat away at the metallic interface, the interface gap between box and cover may so increase that the box loses its ability to quench any flame front attempting to escape. Alternatively, if the contaminated atmosphere reacts with the metal at the interface to fill up the gap, such as by products of corrosion, the vent may close up tightly that a later explosion within the box might create a pressure so intense as to rupture or blow the largely plastic box apart, thus also causing it to fail in its explosion proof function.
But even in applications where the electrical enclosure need not be explosion proof, the necessity to occasionally get inside the enclosure renders it most desirable to construct the enclosure of materials that are not attacked by corrosive or caustic gases. And although the use of plastic electrical enclosures and plastic sheathed cable is widespread under these conditions, many local building codes (such as in the City of Chicago) do not allow the use of plastic enclosures in conjunction with plastic sheathed cable.
In those locations where caustic, corrosive or otherwise contaminated atmospheres are present, and where local codes do not restrict the use of plastic enclosures, it would be desirable to use plastic electrical enclosures that can be conventionally connected into a metallic conduit system, yet provide all of the ground continuity advantages of a completely metallic electrical enclosure. Such an enclosure would have essentially all of the advantages, yet few or none of the disadvantages, of both plastic and metallic electrical enclosures. And, when such an electrical enclosure is designed for explosion proof applications, the absence of metal at the explosion proof interface ensures that the enclosure will not be altered in time by the effect of atmosphere laden contaminants so as to lose its explosion proof characteristics.