The control cabinet maintenance procedure has 7 steps: planning, visual inspection, LOTO isolation, cleaning and checking connections, servicing the internal devices, re-energising and functional testing, and writing the report and scheduling the next visit. The interval runs from 3 to 6 months depending on the installation environment, following the recommendations of IEC 61439 and TCVN 7447.
The control cabinet is the coordinating centre for a plant's entire electrical system. When the cabinet becomes unstable, the production line stops immediately. Tan Long's engineering team — with more than 20 years designing and installing PLC/SCADA control cabinets for industrial refrigeration systems — carries out all 7 of these steps on every maintenance visit. The article below sets out each step in detail, the maintenance interval by item, common faults and the criteria for choosing a maintenance contractor.
What is a control cabinet, and why does it need scheduled maintenance?
Control cabinet (control panel) is a unit bringing electrical components together to coordinate, protect and monitor industrial equipment. In an industrial refrigeration system, the control cabinet manages the operation of the compressors, air coolers, pumps, fans and temperature sensors under a PLC or SCADA program.
Control cabinets need scheduled maintenance because dust, humidity, vibration and heat accumulate over time and cause contact faults, loose connections and declining insulation. A study by EPRI (the Electric Power Research Institute) found that more than 70% of cabinet failures originate in loose connections and contact oxidation, not in components failing spontaneously. In other words, most failures can be prevented by maintaining on schedule.
At Tan Long, our engineering team once dealt with a refrigeration control cabinet at a seafood processing plant in Can Tho where an interposing relay burned out after 18 months without maintenance. The cause was thick dust on the board creating a local hot spot reaching 85°C (185°F), above the 70°C (158°F) rating of a standard relay. The cost of the emergency repair and 36 hours of lost production was 8 times the cost of maintaining every 6 months.
The control cabinet maintenance procedure: 7 steps to standard
The control cabinet maintenance procedure comprises 7 sequential steps, with no step skipped and no reordering. The sequence directly affects both the safety of the maintenance technician and the accuracy of the results.
Step 1 – Planning and preparation before maintenance
Before going to site, the technician needs to establish the type of cabinet (distribution board, control cabinet, PLC/SCADA cabinet), the current revision of the electrical drawings, the previous maintenance history and the list of spare parts held. The mandatory instruments are: a multimeter (measuring to 1,000V AC), an insulation tester (megohmmeter, MΩ range), an infrared thermal gun, an industrial vacuum cleaner and a torque screwdriver set.
The technician must confirm that the maintenance window fits the production plan, avoiding isolation during peak operating hours. For industrial refrigeration systems that must hold temperature continuously, such as freezer stores, arrange in advance to switch to manual or standby operation for the duration of the work.
Step 2 – Visual inspection and operating readings while energised
This step is carried out while the cabinet is still live, to record its actual state before isolation. The technician checks the readings shown on the meters (voltage, current, frequency), notes any alarms on the HMI or fault lamps, then uses the thermal gun to scan for hot spots.
The normal temperature at a terminal inside the cabinet should not exceed 55°C (131°F) relative to ambient. A hot spot above 70°C (158°F) indicates a loose contact or a local overload and needs dealing with immediately. Check at the same time whether the enclosure shows rust, distortion or water ingress.
Step 3 – Safe isolation under the LOTO procedure
LOTO (Lockout/Tagout) is the mandatory safety procedure before any electrical maintenance work. At Tan Long, the engineering team applies 5 LOTO steps on every cabinet maintenance job:
(1) Notify and coordinate with the operations department at least 15 minutes before isolating. (2) Open the main breaker, then each branch breaker in turn, working from the load back to the supply. (3) Physically lock the main breaker with a dedicated lock and hang the warning sign "MAINTENANCE IN PROGRESS – DO NOT ENERGISE". (4) Confirm the supply is dead with a voltage tester or multimeter at every outgoing terminal. (5) Earth the enclosure and busbars before working inside.
An absolute rule: never trust the state of a breaker by eye. Measure the actual voltage before touching any terminal.
Step 4 – Cleaning the cabinet and checking mechanical connections
Once the supply is confirmed dead, the technician cleans inside the cabinet. Use an industrial vacuum cleaner (not high-pressure compressed air, which drives dust into the electronic boards) to remove dust from the components, the copper busbars and the cable trunking. Then clean the remaining surfaces with a soft dry cloth or a non-conductive brush.
Next check every terminal screw: re-tighten to the manufacturer's specified torque (typically 0.5–2.5 N·m for small terminals, 4–10 N·m for busbars). A loose terminal is the most common cause of cabinet fires. Check the condition of the cable insulation inside the cabinet, particularly at bends and at cable clamps.
Step 5 – Inspecting and servicing the internal electrical devices
This is the step with the highest technical complexity. The devices to check, in order:
- Aptomat (MCB/MCCB/ACB): Check the operating mechanism and spring pressure; test both manual and automatic operation; measure the insulation resistance between phases and from phase to enclosure (the minimum under IEC 60364 is 1 MΩ)
- Contactors and interposing relays: Check the main contacts for pitting or wear. Contacts worn more than one third of their original thickness need replacing. Check the coil with a resistance meter — a 220V AC contactor coil typically measures 100 to 500 Ω, a 24V DC coil 50 to 200 Ω depending on the manufacturer.
- PLC and I/O cards (where fitted): Check the status lamps (POWER, RUN, ERR). At Tan Long, technicians always back up the PLC program to a laptop before doing anything inside a cabinet containing a PLC — a rule that is never waived. Check the backup battery holding the PLC memory (usually a CR2032 or a 3.6V lithium cell), replacing it every 3–5 years or when its voltage falls below 2.8V.
- Sensors and transmitters: Check the analogue signals (4–20 mA or 0–10 V) from the temperature, pressure and flow sensors. Compare against the values shown on the HMI to identify a sensor that has drifted or failed.
Step 6 – Re-energising, functional testing and comparing readings
Before re-energising, the technician checks again: no tools left inside the cabinet; every cable connected in its correct position; the cabinet door closed. Then remove the LOTO locks and energise in the reverse order of step 3: from the supply out to the loads.
Watch for at least 5 minutes after energising: monitor the status lamps on the cabinet, check the starting current of the loads, and record the temperature at the terminals with the thermal gun. If you notice a burning smell, sparking, or a current above 110% of rating, isolate immediately and resolve the cause before running again.
Step 7 – Writing the report and scheduling the next visit
The maintenance report is an important technical document, not an administrative formality. It must record: the date and time; the technician's name; the items inspected; the actual measured values (insulation resistance, temperatures at the test points, load currents); the problems found and what was done about them; and the recommendations for the next visit together with the spare parts to hold.
The maintenance history makes it possible to predict how equipment is degrading. For example, if a breaker's insulation resistance falls from 5 MΩ to 3 MΩ to 1.2 MΩ across three consecutive visits, the technician can forecast when it needs replacing before it fails.
Control cabinet maintenance intervals: a schedule by item
The control cabinet maintenance interval depends on the installation environment, how hard the system runs and the component manufacturers' requirements. The table below sets out:
| Maintenance item | Monthly | Quarterly (3 months) | 6 months | Annually |
|---|---|---|---|---|
| Check operating readings (voltage, current) | ✓ | ✓ | ✓ | ✓ |
| Infrared thermal scan of the terminals | ✓ | ✓ | ✓ | ✓ |
| Clean and vacuum inside the cabinet | ✓ | ✓ | ✓ | |
| Re-tighten terminals and screws | ✓ | ✓ | ✓ | |
| Test breaker operation | ✓ | ✓ | ||
| Measure insulation resistance | ✓ | ✓ | ||
| Check contactors and relays (contacts, coils) | ✓ | ✓ | ||
| Check the PLC battery, back up the program | ✓ | |||
| Test the protection relays | ✓ | |||
| Check earthing and the lightning protection system | ✓ | |||
| Update PLC/HMI firmware (if a new release exists) | ✓ |
Dusty environments (cement plants, timber processing) or humid ones (cold stores, seafood plants) need the cleaning and insulation check interval shortened to 1–2 months rather than 3.
Common control cabinet faults and how to deal with them
Control cabinet faults fall into 3 groups: switching components (breakers, contactors), the connection system (terminals, busbars), and the controller (PLC, relays). Recognising them early reduces downtime.
| Fault | Symptom | Common cause | What to do |
|---|---|---|---|
| Breaker tripping repeatedly | Trips again 1–5 minutes after being reset | Overload, a short circuit in the load, or a breaker that is weakening | Measure the actual load current and identify the cause of the overload before resetting |
| Contactor buzzing and vibrating | A continuous chattering noise while the contactor is pulled in | Coil voltage low (below 85% of rating), or dirty pole faces | Measure the coil voltage, clean or replace the contactor |
| Local hot spot in the cabinet | A spot above 70°C (158°F) on the thermal gun | Loose terminal, overloaded conductor, or oxidised contacts | Isolate, re-tighten the terminals, check the conductor cross-section |
| PLC loses its program | After a power loss, the PLC will not start when power returns | The PLC battery is flat and there is no standby UPS | Replace the battery and reload the program from the backup |
| Protection relay operating incorrectly | The line stops for no apparent reason | Incorrect relay settings, electromagnetic interference, or a faulty sensor | Re-check the settings and measure the sensor signal |
| Oxidation of the copper busbars | The busbar surface turns green or black and the contact resistance rises | High humidity, with no desiccant in the cabinet | Clean with fine abrasive paper (P400) and apply proper contact grease |
Who should carry out control cabinet maintenance?
Control cabinet maintenance must be carried out by a technician holding electrical safety certification under Circular 05/2020/TT-BLDTBXH of the Ministry of Labour, Invalids and Social Affairs. This is a legal requirement, not a recommendation.
For cabinets with a PLC or SCADA, the technician needs additional training in programming and operating automatic control systems. A wrong move in a PLC program not only stops the plant but can destroy operating data accumulated over years. At Tan Long, every technician maintaining PLC/SCADA cabinets completes at least 120 hours of practical training before working on site independently.
If the plant has an in-house engineering team, divide the work clearly: monthly checks of readings and cleaning can go to in-house technicians, while insulation resistance measurement, relay testing and PLC maintenance should go to a professional firm with instruments calibrated to ISO/IEC 17025.
Tan Long provides scheduled maintenance for PLC control cabinet and SCADA control cabinet with engineers trained in depth, serving food and seafood processing plants and cold stores nationwide. Support hotline: 0933.357.058.
Signs a control cabinet needs maintenance immediately
Do not wait for the scheduled visit if the control cabinet shows any of the signs below. Reacting early stops a small fault becoming a long production shutdown.
A burning smell or the smell of melting plastic from the cabinet is the clearest sign of a local thermal fault. Isolate immediately and use the thermal gun to locate the hot spot. Fault lamps on the HMI, or protection relays operating repeatedly for no clear reason, indicate a sensor or control circuit problem that needs diagnosing.
A load current abnormally higher than the normal operating figure (more than 10% higher, sustained across several hours of operation) is an early signal of a mechanical fault in the motor or of oxidised contacts raising the resistance. Unusual vibration or noise from a contactor or inverter also needs checking immediately, not left until the next day.
The 7-step control cabinet maintenance procedure is not a formality. It is the engineering discipline that keeps a plant from stopping unexpectedly, extends equipment life and keeps operators safe. Three points to remember: isolate properly under LOTO before any work inside the cabinet; measure insulation resistance every 6 months to catch declining insulation early; and keep full maintenance records so replacement trends can be forecast.
If your plant needs advice on the PLC/SCADA control cabinet maintenance procedure, or technical support from the Tan Long team, call the hotline 0933.357.058 or visit tanlongvn.com/lien-he/.
Frequently asked questions about the control cabinet maintenance procedure
How often should a control cabinet be maintained?
The minimum control cabinet maintenance interval is every 6 months as a general recommendation. However, in high-humidity environments (above 80% RH) or dusty ones such as seafood plants and cold stores, the interval should come down to 3 months. In an air-conditioned office environment, 12 months is acceptable.
Does control cabinet maintenance require isolation?
Control cabinet maintenance requires isolation before any work inside the cabinet such as tightening terminals, cleaning, measuring insulation resistance and inspecting components. Only checking the operating readings and the infrared thermal scan can be done while energised, and then only at a safe distance and in compliance with electrical safety regulations.
What insulation resistance meets the standard?
The insulation resistance of the electrical devices in a control cabinet must be at least 1 MΩ under IEC 60364. In practice, new equipment usually measures 10–100 MΩ. Once the value falls below 1 MΩ, the device needs replacing or its insulation treating immediately.
Can you maintain a PLC control cabinet yourself?
Self-maintenance of a cabinet containing a PLC should be limited to items that do not require opening it: checking status lamps, reading the HMI and cleaning the outside. Work inside — checking I/O cards, replacing the PLC battery, measuring insulation resistance and testing relays — needs a professionally trained technician, to avoid losing the control program or causing an electrical accident. Reliable cold storage installation for orange storage at a good price. Restaurant cold rooms: construction, function and detailed pricing. Cold storage for sausage products: consulting, design and fast installation
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