The short answer
Panel PLC Faults
A controller that faults in a panel is often faulting because of the panel: a 24 volt supply that sags, a ground that floats, a module that overheated, a terminal that loosened, water that reached a module, or noise from a drive coupled into the backplane. The indicators on the controller and the modules say a great deal before the software is opened, and the fault record in the software says whether the fault is a program fault, an I/O fault, a power fault, or a hardware fault. The panel-side checks are the ones a technician can do with a meter and a thermometer: the supply voltage under load, the ground bond, the module temperatures, the torque of the power terminals, and the routing of drive cables. A fault that recurs after the program has been proven is a panel fault until the panel is proven.
Key points
- Read the indicators on the controller and every module first; the pattern names the class of fault.
- The fault record in the software separates program, I/O, power, and hardware faults.
- Supply sag, floating ground, heat, loose terminals, and drive noise are the panel causes of controller faults.
- An I/O fault names a module and a slot; check the module, its power, its terminal block, and its field wiring in that order.
- A fault that recurs with a proven program is a panel problem; measure the panel.
Read the lights
| Indicator pattern | Class | First check |
|---|---|---|
| Controller run indicator off, fault indicator on | Major fault: program, hardware, or power | The fault record in the software |
| Controller off, all modules dark | No power to the rack | The 24 volt or 120 volt supply to the rack and its fuse |
| Controller on, one module fault indicator on | Module fault: hardware, configuration, or field | The module status in the software; the module power and field wiring |
| Controller on, I/O fault indicator on | A module or a remote rack not responding | The I/O tree; the remote rack power and network |
| Controller cycling between run and fault | Recurring fault: supply sag, watchdog, or an intermittent module | Supply voltage under load; the fault log times |
| Everything normal, outputs not working | Output power or a field problem | The output module supply and fuses |
| Communication indicators dark or erratic | Network or serial problem | The port status; the switch; the cable |
Panel causes
- Supply sag
- A 24 volt supply that dips below the controller minimum when a load switches or a motor starts; the controller resets or faults on power. Measure with a recording meter during the events.
- Floating or noisy ground
- A rack ground that is bonded through paint or a long lead, or a ground that carries drive current; analog inputs wander and communications fault. Measure the bond resistance and the voltage between the rack and the ground bar.
- Heat
- Modules derate and fault above their rated temperature; a panel without the heat calculation done, or with a failed fan, cooks the rack. Measure the temperature at the top of the rack.
- Loose terminals
- Vibration from a nearby pump or the panel door loosens power terminals and module terminal blocks; an intermittent fault that follows the vibration. Torque check.
- Water
- Condensation or a leaking conduit reaching a module; corrosion on the terminals, a module that faults in wet weather. Look for the water path.
- Drive noise
- A drive output cable routed beside the rack or the I/O wiring couples noise into the backplane and the inputs. Reroute, shield, and bond.
- Wiring errors after work
- A 120 volt wire landed on a 24 volt input, a field wire on the wrong terminal; the module faults or is destroyed. Compare with the drawing.
Order of checks
- 1
Indicators
Controller and every module, before touching anything; photograph them.
- 2
Fault record
The fault type, code, and time from the software, and the fault log for recurrence.
- 3
Power
Supply voltage at the rack under load, the rack supply indicators, the fuses; a recording meter if the fault recurs.
- 4
Ground
Bond resistance from the rack to the ground bar, and voltage between them with the plant running.
- 5
Temperature
At the top of the rack and at the supply; compare with the module ratings.
- 6
The module
For an I/O fault: reseat, check the terminal block, check the field wiring against the drawing, substitute the spare.
- 7
Connections
Torque on power terminals, module terminal blocks, and the ground bond.
- 8
Routing
Drive cables and power wiring near the rack and the I/O wiring.
- 9
The program
Only when the panel has been proven, the fault record points at the program, or the fault is clearly a program fault by its code.
I/O module faults
An I/O module fault names the module and the slot. The module may have failed, may be a different type from what the project expects, may have lost its field power, may have a channel with an open wire or a short, or may have been damaged by a wiring error or a surge. The software module status names the reason; the terminal block, the field power fuse, and the field wiring are checked in that order; and the spare module of the same part number and firmware proves the module. A module that fails repeatedly in one slot is a slot, a backplane, or a wiring problem, not a bad batch of modules.
Frequently asked questions
- The controller faulted once during a storm and has been fine since.
- A surge or a sag reached it. Check the surge devices, the supply hold-up, and the grounding; the next storm is the test. Log it and alarm on supply monitoring so the next one is seen.
- Why does the fault clear when I reseat the module?
- A connector with corrosion or a loose module retention, which reseating cleans temporarily. Inspect the connector, replace the module if the contacts are damaged, and check the panel for humidity and corrosive air.
- How do I tell a program fault from a panel fault?
- The fault code. Program faults name a routine and a rung; power and hardware faults name the supply, the module, or the backplane. A watchdog fault can be either and is worked from the task times.
- Should the controller be on the UPS?
- Yes, with the switch and the radio, so that the controller rides through sags and short outages and can report a power failure. The outputs may or may not be on the UPS depending on what the process should do without power.
Related topics
- PLC Will Not RunA controller that stays in program mode, faults on every attempt to run, or will not power up: the mode switch, the major fault log, the power supply, the memory and battery, the firmware and program mismatch, and the I/O configuration errors that stop a processor, in the order that finds them.
- Processor FaultedA PLC with its fault light on and no logic running. How to read what it is telling you, the causes in order of likelihood, and how to restart it without repeating the fault.
- Program FaultsFaults the program causes: an index out of range, division by zero, an invalid indirect address, a missing routine, a type mismatch, or task overlap. Reading the fault record, what a fault handler should do, clearing safely, and logic that does not fault.
- PLC PowerPowering the controller section of a panel: the control transformer and 24 volt supplies, separating controller power from field I/O power, sizing with margin, the UPS and what rides through, protection and monitoring, the DC ground, and restart sequencing.
- Ground FaultsGround faults in and around control panels: what happens on grounded 120 VAC circuits, on grounded and ungrounded 24 VDC systems, and on drive outputs; where they hide, from chafed wire to flooded boxes; and the divide-and-conquer method that finds them.
- Enclosure Heat CalculationsEstimating the temperature inside a control panel from the power its contents dissipate and the surface that carries it out, what to do when the answer is too hot, and why drives change everything.
Direct contact
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