Successful Cases
21 Rules for Maintenance and Upkeep of PLC Control Cabinets
1. Why Maintenance is Needed
1. To extend service life;
2. To ensure stable operation of the equipment;
3. This is also a reflection of 5S.
2. Problems That May Occur Without Maintenance
If the electrical cabinet is too dirty, it can cause short circuits, arcing, and damage to the equipment;
If the electrical cabinet fan filter is clogged, poor heat dissipation will lead to poor equipment stability;
Excessive humidity can cause equipment short circuits;
Certain high-current areas that overheat locally for a long time can damage the equipment.
3. Preparations
Various tools: screwdrivers, wrenches, insulation resistance tester, multimeter, clamp ammeter; vacuum cleaner; heat gun, marker pen, file, etc.;
Spare parts: fan filter, auxiliary contacts for contactors, relay coils, heat shrink tubing, conductive paste.
Personnel: Not only electricians are needed, but also programmers. If programmers participate and maintain the software, many soft faults can be prevented in advance.
4. Precautions
Make a good plan, develop detailed steps, and proceed step by step. Never rely on a sudden idea; instead, consider all aspects carefully and follow the plan step by step:
1. Safety first: cut off power before disassembling equipment, pay attention to safety;
2. When cleaning the electrical cabinet, be careful not to spread dust everywhere;
3. Mark all places that have been worked on and observe carefully. For example, some wires have been reconnected or terminals re-crimped. After restarting the equipment, pay attention to these areas for any issues.
5. Data Backup
First back up the program, which is very necessary. If the equipment fails one day and there is no program, it will be a mess. With the program, at worst, you just replace the controller. So this is a must-know for maintenance personnel.
1. For data that needs long-term archiving (such as historical trend data in the military industry), back up the historical trend data to a portable hard drive, then shut down the Windows-based operating system, such as an HMI system with WinCC Intouch; then cut off the main power;
2. For controllers that cannot communicate, such as soft starters, manually record internal parameters as a precaution.
6. System Power Off
After data backup is complete, perform switching operations. When cutting power, first disconnect the various protection switches below;
Then disconnect the main switch (when powering on, first turn on the main switch on the control cabinet, then sequentially turn on the protection switches below);
Safety lock: after power off on site, it is best to lock it safely to prevent accidental power supply.
7. Appearance and Environmental Inspection
Temperature environment conditions: 0~40℃
Relative humidity below 85%
Vibration amplitude less than 0.5mm (10~55Hz)
No large amounts of dust, salt, or iron filings
24VDC output voltage measurement: check if within normal range
After power off, test system insulation using an insulation resistance tester
8. Control Cabinet Cleaning
Power off the equipment, cover electronic components with heat dissipation holes such as PLCs and inverters, preferably remove the PLC modules.
Only use a vacuum cleaner; do not use compressed air because it can blow dust inside the equipment and often contains moisture, which can cause short circuits.
After vacuuming, check if wiring is loose.
9. Cable Inspection
Check if large cables are loose: if loose, they are prone to damage.
Check if large cable joints are blackened: if yes, check if terminals are not tightly crimped or if current is too high.
10. Equipment Grounding Inspection
Profibus grounding measurement: check if the shielding layer is aging. Aging shielding causes poor grounding and may lead to Profibus node loss.
Analog signal grounding measurement: same principle as above.
Check if grounding wires are rusted; if rusted, treat accordingly.
11. Contactor Inspection
Check if mounting screws and incoming/outgoing wires of large contactors are loose.
Large contactor contact status: check if main contacts have burn marks, and check if arc extinguishing covers are blackened or damaged.
Contactor wiring terminal status: check for blackening.
Pull-in time and voltage test: check contactor pull-in time and continuity of incoming/outgoing wires.
Check if contactor pull-in sound is normal and noiseless. If a buzzing sound occurs after pull-in, disassemble and inspect the contacts.
12. Copper Busbar Inspection
Check if heat shrink tubing is damaged;
Check copper busbar connections: if copper surface is discolored, it may be overheated. If needed, disassemble the busbar, align it flat, apply conductive paste, and tighten screws.
13. Relay Inspection
If solenoid valves fail to pull in or release during production, check as follows.
a. Check if the LED on the relay corresponding to the solenoid valve is lit. Lit means the solenoid valve is powered; unlit means no power;
b. Use a multimeter to measure if there is voltage at relay pins A1 and A2.
If the solenoid valve malfunctions during the production process, the following actions can be taken:
Generally, only one set of contacts will be damaged. If the relay has two sets of contacts and one set is damaged, the other set can be considered. For example, if the on-site wiring is for contacts 11-14 and they are damaged, you can switch to 21-24. At this time, loosen the wires on pins 11 and 14 and move them to pins 21 and 24.
If replacing the contacts does not solve the problem, consider replacing the coil. You can remove the coil from a less important circuit and replace it, making sure not to insert it in the wrong direction.
14. Replacing the filter screen
Replace the filter screen every six months or once a year depending on the site conditions;
If the filter screen is clogged, it will seriously affect the equipment's heat dissipation.
15. Inspection of the inverter and power regulator fans
Check whether the inverter and power regulator fans are operating normally.
Check if the ventilation openings are blocked. If the fan blows out dust when turned on, it indicates that cleaning is needed.
16. Equipment heat inspection
Electrical cabinet temperature
Temperature of large cables and copper plates
Cable temperature
It is best to have an imaging device; if not available, use a temperature gun.
Regularly check the temperature inside the cabinet, the temperature of large cables, and the temperature of on-site cables to prevent damage to cables and components caused by dust accumulation or other reasons.
17. Clamp ammeter testing
1. Generally, one person operates and another supervises. During measurement, maintain a safe distance of more than 10 cm from live parts.
2. Before measuring, estimate the load current and voltage level, and select the range accordingly. If estimation is not possible, start measuring at the maximum range to prevent damage to the clamp meter, then switch to a lower range based on the reading.
3. Do not change the range setting during measurement. Because the secondary side is equivalent to a short circuit during measurement, switching ranges can cause a momentary open circuit on the secondary side, inducing a high voltage that may cause insulation breakdown inside the clamp meter.
4. To reduce errors, place the measured conductor as close to the center of the clamp meter opening as possible. The clamp opening should fit tightly. If there is noise, open and close the clamp again. If noise persists, check carefully and remove any dirt or debris before measuring again.
5. After measurement, set the range switch to the highest range to prevent damage to the instrument due to neglect in the next use.
6. Clamp meters are generally used to measure current on the low-voltage side of distribution transformers or motors. They must not be used on high-voltage lines to avoid insulation breakdown and electric shock.
18. Software maintenance - PLC diagnostic buffer
1. Record and organize hardware alarm logs.
2. Equipment problems do not necessarily cause shutdowns, but they are recorded in the PLC diagnostic buffer. Therefore, regularly checking these records is very important for equipment maintenance.
3. Timely understand the problematic stations, find causes, and prepare accordingly.
4. Some may say they don't understand this; it's okay, you can save it as a txt file and send it to someone else for help.
Key diagnostic buffer alarms:
A. A certain station is lost and then returns. This is quite dangerous and needs to be checked.
The danger is that communication may be interfered with, or the DP bus connector or shielding layer may be oxidized causing poor grounding. Sometimes the station is lost and then returns, but it may eventually be lost permanently.
B. A certain analog signal is missing, which also requires attention:
Possible cause: the analog signal may be interfered with, but overall the problem is not serious.
C. Another software issue:
Similar to example B above, but this alarm indicates that a certain data in the DB block cannot be found. The cause is a software pointer problem pointing to a non-existent address.
For example, if DB80 has only 1000 bytes and you write a pointer addressing program that points beyond DB1000, it will report an error. If it only reports an error, it's not too bad, but if it causes an infinite loop, the system will crash.
19. Software maintenance - HMI alarms
Check equipment alarms, especially those that occur frequently, and eliminate their causes:
For example, if a certain water flow switch frequently alarms, it can be disassembled and cleaned;
If a valve switch often times out, check whether the valve is excessively worn.
20. PLC power supply battery replacement
1. Before disassembly, power the PLC for more than 15 seconds (this charges the capacitor in the memory backup power supply, so when power is cut off, the capacitor can provide temporary power to protect the RAM data);
2. Disconnect the PLC AC power supply;
3. Open the battery cover of the basic unit;
4. Remove the old battery and install the new one;
5. Close the battery cover.
Note that the battery replacement time should be as short as possible, generally not exceeding 3 minutes. Otherwise, the program in RAM will be lost.
21. Holiday precautions
1. During holidays, the main power of the equipment can be turned off, but keep the lighting and fan circuit power on.
2. Be sure to turn on the control cabinet ventilation fan or air conditioner; otherwise, on rainy days, moisture may accumulate inside the equipment causing short circuits.
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