Electric Control Cabinet

Non-Electrical Components
We will look into non-electrical components first because there are only usually two of them and they are straightforward to understand.

Enclosure
The enclosure is the metal box that houses the other components of an electrical control panel. It comes in varying sizes, materials, and specifications to match the exact needs of the processes the panel is used for and the environment the enclosure is in. The number of doors indicate the sections of the enclosure and plays a part in determining its size. The enclosure is typically made from steel, but different materials can be used based on the application.

Back Panel
Back panels are metal sheets mounted inside the enclosure. These panels hold DIN rail mounting and wiring ducts. DIN metal rails provide a standardized mounting structure for the electrical devices that are to be attached. The wiring ducts serve as pathways for wires connecting the electrical devices. The ducts help in keeping components organized and noise between devices managed.

Electrical Components
These are the components that use electricity. They can further be classified into two categories: components that route power and components that control and monitor the mechanical processes. While most of these components are inside the enclosure, there are devices located outside such as the HMI.

Main Disconnect
Electricity for all of the devices comes into the control panel through the main disconnect. Disconnects can be fused, non-fused, or a circuit breaker. It usually has a handle outside the panel that lets users shut off power. The typical power that enters the panel can be anywhere between 120 to 480 volts.

Surge Arrester
After the power goes through the main disconnect, it will then be routed to a surge arrester. This component protects all the other devices inside a control panel from electrical surges. These surges can come from utility power surges or lightning strikes.

Transformer and/or Power Supply
Transformers change the voltage of AC power. Normally mounted externally to save space, these are essential for small AC loads, like conveyor motors, heaters, and pumps.

Terminal Blocks
Terminal blocks are also used to route power to the devices inside the control panel. Each block has two terminals that can join wires together. These blocks can be arranged in a strip along the DIN rail to route the power to the other components.

Programmable Logic Controller or PLC
The PLC is akin to a computer’s CPU in that it serves as the brain of the entire panel. In fact, this component does have a central processing unit where the logic program is stored. The PLC has inputs and outputs where the terminal blocks are connected. These connections enable control of the mechanical processes.

Sensors
Sensors such as proximity switches and photo eyes relay information to the PLC. They monitor the processes and give necessary feedback to the PLC in order to control the operations.

Relays and Contactors
Relays and contactors are essentially switches that turn devices on and off depending on the need of each process. Smaller relays are used to control simpler devices such as lights or fans. Contactors are used to control higher load devices like motors.

Network Switch
A network switch connects multiple devices via ethernet. It serves as the communication hub connecting the PLC and network compatible devices outside the panel, usually along the assembly line.

Human Machine Interface or HMI
The HMI is one of the network compatible devices connected to the network switch. As the name describes, the HMI serves as the tool that lets users interact with the control panel. It usually has a screen, buttons, switches, joysticks and other input devices that allow the user to monitor and control the processes along the assembly line.

Safety First
Safety always should be the first concern before opening a cabinet. As a technician or engineer begins work on electronic controls, it’s natural to maintain a narrow focus on the suspect low-voltage equipment and controls and easily forget that working inside a mixed-voltage cabinet exposes workers to dangerous voltages and short-circuit currents. Know the voltages you’ll see before opening that door.

Electromagnetic interference
Notice how wiring enters the cabinet. The 480-volt power conductors and low-voltage control wiring generally will be brought in through separate conduits.
Running such conductors in separate conduits in the field helps to minimize the possibility of electromagnetic interference (EMI). If power conductors are too close to the control wiring and electronic components, you can expect erratic equipment operation.

Separation of power and control circuitry
Ensure adequate separation of power and control circuitry. To help differentiate between them, notice the conductor sizes and color-coding schemes in use. Control circuit wiring typically is 16 AWG or 18 AWG. Power conductors generally will be no smaller than 12 AWG and often are considerably larger. Grounded conductors are white, are gray, or have three continuous white stripes on any color insulation except green, blue, or orange. Control circuit wires that are white with a blue stripe are the grounded conductor for a dc control circuit. Any control wire that is orange or white with an orange stripe is an ungrounded conductor that remains energized after the main supply disconnect is switched to off. Additionally, red insulation indicates an ungrounded conductor in an ac control circuit and blue insulation indicates the ungrounded conductor in dc control circuits. Conductors that enter the cabinet as part of a multi-conductor cable can have different color schemes.

Low voltage instrumentation wiring
Usually, either a twisted pair of conductors or a shielded cable helps minimize the effects of EMI in low-voltage instrumentation wiring. In a twisted pair, one conductor twists around another with the number of twists per inch specified. A shielded cable is a twisted-pair cable with a braid or foil covering running the entire length of the conductors. It also has a thermoplastic jacket for physical protection.

Voltage measurements
Once you correct any abnormalities found during the visual inspection, take voltage measurements to ensure there’s no EMI from power conductors. Use a properly rated digital multimeter to measure and record voltage levels. Input voltage to electronic equipment such as PLCs and VFDs is generally specified as plus or minus 10% of the rated voltage.

Control wiring terminations
Check control wiring terminations for tightness. Any effects of electromagnetic induction the controls might normally have been able to handle will be exacerbated at a loose termination point, and electronic inputs are affected. Occasionally, wires come loose from pressure connectors, usually because of improper installation. Check each individual wire at its termination point to ensure it’s secure in its connector or under its terminal screw.

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