A programmable logic controller (PLC) is an industrial computer that reads inputs from the field, runs a control program and sets outputs, over and over, many times a second. The control panel around it supplies power, protects the circuits, organises the wiring and keeps the electronics in a suitable environment. A well-built panel with a well-documented program is easy to maintain; a poorly built one becomes a liability the day its builder leaves.
What a PLC does
A PLC works in a repeating cycle called a scan. It reads the state of every input into memory, runs through the program using those values, writes the results to the outputs, and then handles communications and self-checks before starting again. Because the inputs are read once per scan, the program works from a consistent snapshot of the process, which makes its behaviour predictable.
Inputs can be discrete (a switch is open or closed) or analog (a 4–20 mA signal from a transmitter, covered in our guide to 4–20 mA current loops). Outputs likewise switch a relay or starter, or drive an analog signal to a valve or a variable frequency drive. Communication modules connect the PLC to operator screens, other controllers, drives and supervisory systems over industrial networks.
Programs are commonly written in the languages defined by IEC 61131-3, such as ladder diagram, function block diagram, structured text and sequential function chart. Ladder remains popular in North America because electricians can read it like a wiring diagram. Whatever the language, a program that is laid out in sections that match the plant, with tag names that match the instrument index and comments that explain why, is far easier to support.
PLC, building automation controller or something else?
| Option | Suits | Limits |
|---|---|---|
| Relay logic | Very simple interlocks and small machines | Hard to change, no data, no diagnostics |
| Building automation controller | HVAC, lighting and building sequences, BACnet integration | Less suited to fast or interlock-heavy process work |
| PLC | Pump stations, plants, process skids, machines, harsh environments | Needs programming skills and software licences to maintain |
| Distributed control system | Large continuous processes with many loops | Scale and cost beyond most building and small plant work |
Many sites use more than one. A common arrangement has a PLC running a boiler plant or pump station and sharing data with the building automation system, which handles scheduling, trending and the rest of the building.
Inside a control panel
A control panel brings together power, protection, control and field connections in one enclosure. A typical layout keeps power at the top, control electronics in the middle and field terminals near the bottom, close to where cables enter. Wireways run between rows so wiring stays tidy and can be traced.
Typical contents include:
- A disconnecting means and overcurrent protection for the incoming supply and each branch.
- DC power supplies for the PLC and field devices, sometimes in a redundant pair, and often an uninterruptible supply for the controller.
- The PLC rack or modules, network switch and any remote I/O.
- Relays, motor starters or drives where the panel controls motors. See motors, starters and VFDs.
- Terminal blocks with fused or disconnect terminals for field circuits.
- Cooling or heating where the enclosure's heat load or the ambient temperature requires it.
Approval and ratings
In Canada, electrical equipment, including a control panel built for one project, must be approved before it is energised, either by a certification body's mark or by a special inspection under the provincial program. CSA C22.2 No. 286 covers industrial control panels and assemblies. The panel also needs a short-circuit current rating at least equal to the fault current available where it is installed; our guide to available fault current explains why that matters. The enclosure type must suit the location: dust, water, washdown, corrosion and, where applicable, a hazardous location classification. Check the current editions and your province's requirements.
Design practices that pay off later
- Spare I/O, spare terminals and spare backplate space, so the next change does not mean a new enclosure.
- Outputs that fail safe: decide for each output what should happen on loss of power or loss of the controller, and design for it.
- Emergency stops and safety interlocks hardwired, not dependent on the standard PLC program alone. Where a safety function is required, it follows its own standards and assessment.
- Heat calculated, not guessed: drives and power supplies add heat that shortens electronics' life.
- Every wire numbered at both ends, every device tagged, and drawings that match.
- A written sequence of operation agreed before programming starts.
Program and documentation management
The PLC program is part of the facility's records. Keep a current backup off the controller, record the software version needed to open it, and note every change with a date, a reason and who made it. Make sure the owner holds the rights and passwords, not only the integrator. Remote access for support is convenient but is also the most common way control systems are exposed to attack; limit it, log it and switch it off when not needed. Our guide to cybersecurity for building automation applies equally to PLCs.
Taking over an existing panel
When a facility inherits a panel with little documentation, a short review usually covers: backing up the running program and comparing it with any copies on file; walking the panel against the drawings and marking up differences; listing every input and output with its tag, range and current state; checking the condition of power supplies, batteries, fans and filters; and identifying loops left in manual and alarms that are permanently bypassed. Those last two often explain why operators no longer trust the system.
Questions to ask your integrator
- Will the panel be certified, or will it need a special inspection before it is energised, and who arranges that?
- What short-circuit current rating will the panel carry, and has it been checked against the fault current at its location?
- Which PLC platform and software version will be used, and is the product line current?
- Will we receive the program without password protection, the HMI project and all passwords at handover?
- How much spare I/O and backplate space is included?
- Will the factory acceptance test follow a written procedure we can witness, and will the site acceptance test check every loop end to end?
- What happens to each output on loss of power, loss of communication and controller failure?
How ehZee can help
We design control panels and PLC systems, write sequences of operation, review existing panels and programs, and coordinate fabrication, approval and commissioning. Design drawings are sealed by a licensed professional engineer where required, and installation is done by licensed electrical contractors. See our instrumentation and controls service.
Common questions
Can a PLC replace our old relay panel?
Usually, yes. A migration starts by documenting what the relay logic actually does, including any quirks operators rely on, then writing a sequence of operation, programming it and testing it before cutover. The panel will still need approval before it is energised.
How long does a PLC last?
The hardware often runs for many years, but support for a product line, its spare parts and the software to program it can end earlier. Check the manufacturer's lifecycle status and plan a migration before parts become hard to find.
Who should own the PLC program?
The owner should hold a current copy, the passwords and the rights to modify it, written into the contract with the integrator. Without them, every future change depends on one supplier.