The present invention relates to a housing for electrical components and, more specifically, to a housing for one or more electrical components that are connected to sources of a plurality of voltages.
Automation and remote monitoring and control are continuing trends in industrial environments. For example, referring to FIG. 1, in an automated or remotely controlled system 20, a computer or other controller 22, responding to particular input conditions, typically outputs a low voltage, digital control signal 24 to control the operation of a remotely located load 26. The digital control signal is commonly a few milliamps of current at 5-25 volts (v) ac/dc while the load is typically connected to a source 28 of 120 or more volts, 60 Hertz (Hz) ac electrical power and may draw considerable current while in operation. A pair of relays, typically, provides the means for controlling the connection of the higher voltage to the load with a lower voltage, low current control signal.
The control signal 24 output by the controller is input to a control relay 30, commonly a solid state switching device, that, in turn, controls conduction of an input or “coil” voltage 32 to a load relay 34. In the exemplary control circuit, the control relay 30 comprises an opto-isolator. The control signal is communicated to a light emitting diode (LED) 36 in the opto-isolator. When the control voltage is present, the LED emits light causing a photo-transistor 38 in the opto-isolator to conduct the load relay coil voltage, typically, 10-30 v. dc, from the controller or another source to the load relay. When the control signal is not present, the LED does not emit light and the photo-transistor does not conduct the coil voltage to the load relay.
The application of coil voltage to the input or coil 40 of the load relay causes a change in the conductive state of the load relay's main contacts 42, 44. The exemplary load relay 34 includes both normally open 44 and normally closed 42 main contacts and the logic of the circuit can be altered by changing the connection of a conductor connecting the load relay and the load. Closure of the normally open main contacts in response to the presence of a control signal connects the load to its source of power. Conversely, when the control signal is not present, the phototransistor 38 does not conduct causing the conduction state of the load relay's main terminals to be switched to the normal state, for purposes of the example, the open state, disconnecting the load from its source of power.
Referring also to FIG. 2, to reduce the complexity of constructing and servicing systems that utilize multiple voltages, such as a remote control or monitoring circuit, it is desirable to locate circuit components that are connected to the circuit's lower voltage(s) and circuit components connected to the circuit's higher voltage(s) in a single housing 50. For example, the number of devices that must be separately mounted during installation of the exemplary remote control circuit can be significantly reduced by purchasing a housing in which the control relay 30, the load relay 34 and a load switch 52, enabling manual or automated control of the load, are pre-installed. In addition, the components installed in the housing are typically pre-wired so that the installer need only to make the external connections to the controller, the source of load relay coil voltage, the load and the source of power for the load. Similarly, if service is necessary, the presence of the three components in a single location simplifies testing by eliminating long wire runs connecting separately mounted components.
The housing 50 comprises a box portion 54 and a cover 56 that is securable to the box with a plurality of screws 58. The housing typically includes a plurality of apertures through which the lower voltage conductors, for example, the conductors for the control signal and the load relay coil voltage, and the higher voltage conductors, for example, the conductors for the load current, can be introduced to the housing. One of the apertures 60, commonly the aperture for the load current conductors, is often aligned with the center of a projecting, externally threaded conduit nipple 62 that is affixed to the box portion of the housing. The housing can be mounted on another electrical enclosure 64 by inserting the nipple into an aperture in the other enclosure and securing the nipple and the housing with a conduit nut 66. Additional apertures are provided in the housing to accept threaded conduit connectors 68, 70 through which the control signal conductors and the load relay coil voltage conductors can be introduced to the housing.
While locating components that are connected to lower voltage(s) and components that are connected to higher voltage(s) in the same housing can simplify installation and service of circuits such as a remote monitoring and control circuit, electrical codes require that the housing be treated as an enclosure for conductors of the highest voltage unless the conductors of lower voltage(s) are physically separated from the higher voltage conductors. It is often desirable to have installation and service of the low voltage portions of a circuit, for example the control portion of the remote control circuit, performed by a computer or control technician. However, electrical codes require the work to be performed by a licensed electrician if a conductor in an enclosure is connected to source of voltage exceeding 42.4 v.
To enable computer and other technicians to install and service the lower voltage contents of a multi-voltage housing, the housing is typically separated into two compartments by a dielectric barrier 72. The insulating barrier commonly comprises fibrous material, typically paper, that is bent to form a first portion 74 that extends substantially parallel to the exposed edge of the box portion of the housing to cover the load relay, the load switch and the higher voltage conductors when the housing's cover 56 is removed. The insulating barrier also comprises a second portion 76 that extends substantially normal to the first portion and into the opening in the box for a substantial portion of the box's depth to separate the higher voltage contents of the box from the lower voltage contents.
The insulating barrier reduces the likelihood of inadvertent contact with the higher voltage and the possibility of injury while working on the lower voltage portion of the circuit. However, while fitting snuggly in the box portion the insulating barrier is retained in the box by friction and can be easily removed or even dislodged from the box by inadvertent contact during work on the lower voltage contents of the box. In addition, the wiring that connects the control relay and the load relay passes under the edge of the projecting portion 76 of the insulating barrier and movement of the wires may dislodge the barrier. The insulating barrier is a loose piece and once removed from the box, either intentionally or inadvertently, can be easily misplaced and not reinstalled. Further, when the cover has been installed it is not possible to determine if the insulating barrier has been installed without removing the cover of the housing, increasing the likelihood that a person attempting to service the low voltage portion of the contents of the box will, upon removing the cover, discover that the insulating barrier has been removed and attempt to perform the work in the housing without the barrier being in place.
What is desired, therefore, is a housing for electrical components providing a secure division of the housing enabling secure separation of contents connectable to lower voltage(s) and contents connectable to higher voltage(s).