10 steps to control electrical cabinet problems

In brief:

• Interactions between power and control wiring inside a single electrical cabinet can cause performance anomalies.
• There is a simple 10-step procedure for minimizing those problems.
• One must be aware of the wiring color codes currently in effect before troubleshooting cabinet wiring.

Mixing 480-V three-phase cables and lower voltage 24- or 120-V control wiring and communication cabling in the same cabinet can result in erratic operation or even complete failure of electronic equipment inside the cabinet. Knowing what’s inside the cabinet before you open it, the specific wiring variables to look for once inside, what values to measure, and simple ways to correct problems can help alleviate many erratic and sometimes mysterious control and communication problems.

“Be aware that an arc-flash label usually provides the maximum voltage in the cabinet and doesn’t mention other supply voltages that might be present.”

Electrical cabinets often are designed as a central control point for automation and process control equipment. Inside the cabinet are the programmable logic controllers (PLCs), variable frequency drives (VFDs), and their associated communication and control wiring.

Because the equipment being controlled typically is powered by 480 V, the cabling often must be routed through the same cabinet as the electronic controls — an advantage for both troubleshooting and maintenance. You can observe the indicator lights on a programmable controller, measure voltage on a motor starter, and adjust the drive in the same cabinet.

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.

Industrial control panels are required to have durable and legible labels that indicate the rated voltage, number of phases, and the frequency of any supply in the cabinet. Older panels might not be marked. Many panels now have an arc-flash warning label on the panel door. Be aware that an arc-flash label usually provides the maximum voltage in the cabinet and doesn’t mention other supply voltages that might be present. In addition to any label, refer to electrical diagrams and vendor manuals, and even walk down the systems if necessary to help determine voltage supplies present in the cabinet.

Figure 1. This automation cabinet contains power, control, and communication wiring. A 480-V disconnect handle is at the upper right of the cabinet. PLCs near the top of the cabinet use 24-V inputs and outputs to control a conveyor system while 480-V VFDs near the cabinet bottom drive the conveyor motors.

It’s generally best to stand to the side of the cabinet, if possible, to operate disconnects, release latches, and open doors, just in case something goes wrong. Once the cabinet door is open, make a visual inspection for obvious abnormalities and the smell of burnt insulation (Figure 1). Refer to the appropriate wiring and control circuit diagrams to identify components and terminal strips.

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.

To reduce EMI, power conductors should be as far away from control and communication wiring as possible. There’s no standard distance definition for what constitutes “close proximity.” Use reasonable judgment. Keep power and control conductors in separate wiring trays inside the cabinet. If for some reason it’s necessary for power conductors and control wires to cross, ensure that they cross at a right angle to reduce the effects of EMI.

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. Refer to wiring diagrams as needed.