Control Panels and Electrical Enclosures
BIRKET Engineering News, Jan/Feb 1994
Designing and assembling control panels is one of the engineering services provided by BIRKET Engineering. It is important that panels be built to recognized standards of quality and safety. ETL tests assembled equipment, such as control panels, for compliance to these accepted standards.
For the purposes of ETL, a control panel is a general term for what may be a computer cabinet, operator console, junction box, distribution box, sensor enclosure, or other miscellaneous enclosure that contains components necessary for a control system. Electrical enclosures housing controls components are specially designed to protect their contents from elements that might affect their performance unfavorably while providing ready access to these components. These unwanted elements include heat, moisture, dust and dirt, chemical or organic matter, or other things that might accelerate the maintenance requirements of the system.
A control system is, in the broadest sense, any interconnection of components to provide a desired function. The portion of the system to be controlled is called the process. It is affected by applied signals, called inputs, and produces signals of interest, called outputs. In the example of the control system for a roller coaster, the process is the vehicle speeding around the track and guests having fun. The inputs are sensors telling the computer where the vehicle is on the track, or the ride operator dispatching the vehicle from the station. The outputs are lift motor controllers and brakes along the track slowing the vehicles down.
There are several types of control panels or other electrical enclosures. The computer cabinet or rack is the enclosure that houses the computer or other processing unit. It is central to the control system. It receives inputs (sensors, switches, or other monitors), processes them, and initiates outputs (indicator lights, messages, solenoids, valves, motor drives, or other equipment action). Usually one of the larger enclosures, it often also houses the system power supply, whatever signal conditioning and communications equipment is required, and emergency stop hardware. The computer cabinet may be located a great distance away from the inputs it is receiving and the outputs it is controlling.
A person may be required to provide inputs to the system (though many systems do not). This person or operator may determine system parameters such as speed, quantity produced, or dispatch rate. The operator inputs this information into the system through switches and buttons at another type of control panel: the operator console. The operator console may also provide outputs from the system to the operator in the form of lights, buzzers, or alphanumeric message indicators to prompt action on the part of the operator.
Signals from inputs to the processor and from the processor to outputs are typically transmitted over electrical wires. Often two or more wires are dedicated for use by a single input or output. In a control system of any size, this means there are often thousands of wires of every description headed in every direction. Wires coming from the computer cabinet are connected to wires coming from remote input and output locations at another type of control panel: the junction box. A junction box is an enclosure that provides a ready means of protecting, terminating, reassigning, and labeling electrical wires and cables. If a cable enters an enclosure and the wires leave the enclosure and go to many different places, then the enclosure may be called a distribution box or breakout box.
A sensor enclosure houses a sensor. Limit switches, motion detectors, and proximity switches are common types of sensors. When a sensor must be placed in a hazardous or environmentally unfriendly area, the sensor is often protected by having its own enclosure.
Other examples of control panels or boxes include monitor cabinets, remote emergency stop button enclosures, and motor control centers, to name a few.
Some control system integrators providing design services similar to ours do not maintain in-plant assembly capability. We do because it allows a close working relationship between engineers and the production staff. In this way we can best accommodate short schedules and our customers’ inevitable last minute changes.