Electrical and control requirements are where non-standard automation projects are most often re-opened after the design is signed. Mechanical changes are visible; control changes are argued over in emails. Writing the requirements down in the following form, before the build starts, keeps the machine and the buyer's plant on the same page.
Fix the utility and supply assumptions in writing
Start with what the site actually delivers: supply voltage and frequency, available breaker rating, phase configuration, compressed-air pressure and flow at the drop point, and the location of the water inlet and drain if the machine uses either. Record the measured values, not the nameplate rating of the building, and state which side is responsible for bringing services to the machine boundary.
These numbers constrain the electrical design immediately. Heating loads, pump sizes and drive selection all follow from the available supply, and a machine designed against the wrong figure needs a transformer or a second service the buyer never budgeted for.
Specify the control platform your plant can support
Choose the PLC, drive and HMI family that your existing maintenance team can service, and name the exact generation and firmware level you expect. A supplier may quote a cheaper controller that your plant has no spare of and no one trained on. Ask for the project programme to be delivered with the machine, along with the HMI source project, the alarm list, and a written map of every input, output and axis address.
Confirm the password arrangement for the HMI and controller, and who is authorised to change recipes. This is an ownership question, and it belongs in the requirement document, not in a conversation at commissioning.
Define the interfaces to upstream and downstream equipment
Non-standard machines rarely run alone. Document every hand-off: to a conveyor, a robot, a press, a test bench or an existing line controller. For each interface, state the signal list, the direction of each signal, the handshaking sequence, the timeout behaviour when either side stops, and whether the link is hardwired I/O or a fieldbus such as PROFINET or EtherNet/IP, including the protocol version.
Write the stop and restart behaviour explicitly. Which side holds the machine in a fault, what happens to work-in-progress on a line stop, and what an operator must do to resume. These cases are almost never tested until the line is live.
If your plant reports machine data, say so at the requirement stage. Name the tag list you expect, the update rate you need, and whether the supplier or your integrator builds the gateway. Retrofitting a data interface is far more work than declaring it in the requirement.
State the safety functions and how they are proved
List the guards, interlocks, light curtains, two-hand controls, e-stop devices and their expected behaviour when a guard opens mid-cycle. State the required performance level for each function and ask how the supplier intends to achieve it — a safety relay, a safety controller, or safe torque off on the drives — and how that is verified at acceptance.
Ask for the safety-related part of the control system to be documented separately from the standard I/O list, with a wiring diagram and the forced-stop test procedure your team will repeat after maintenance.
Set the documentation and labelling deliverables
Require the as-built electrical drawings delivered on the same revision control you use: power distribution, control schematic, terminal schedule, panel layout, bill of materials with orderable part numbers, and a wire number list matching the physical labels in the panel. Labelling on the machine must correspond to the drawings, and that correspondence should be an acceptance item.
Ask also for the I/O test sheet showing every input and output exercised, the alarm manual with cause and corrective action, and the routine training record for your staff.
Agree the acceptance tests for the control system
Before the design is frozen, write the factory and site acceptance items for the electrical side: interlock function test, e-stop circuit test under power, a power-loss and recovery test that proves the machine resumes safely, an interface simulation with your line signals, a cycle-time run, and an inspection of panel build quality against the drawings. State the pass criteria, and hold the final payment release against them.
For a supplier whose capabilities cover system integration, customised configuration and technical support alongside equipment manufacturing, the electrical requirement should also say where the machine leaves off and where your line control begins — that boundary is the most common source of disputed work on integrated builds.
A checklist to send with the enquiry
- Measured site supply, air, water and drain values, plus the service boundary
- Named PLC, drive and HMI family, version, and programme delivery terms
- Signal lists, protocol versions and handshaking sequences for every interface
- Machine-data tag list and reporting expectation, or a written statement that none is required
- Safety function list with performance level and verification method
- As-built drawing set, I/O schedule, alarm manual and training record
- Acceptance test items with pass criteria tied to payment release
Send that checklist with the enquiry rather than after the order. Suppliers who answer it concretely are usually running a controlled engineering process; suppliers who treat it as a detail will hand you change orders later.
Planning the build next
If you are specifying a customised machine or a line integration and want the electrical scope reviewed against your plant's standards before the design is frozen, you can start that conversation through Solutions, or contact our engineering team on the contact page. More about the company's engineering and integration capability is on the Engineering Services page.
