In short. A good power HMI shows state at a glance and helps decisions during stress. It uses a clear one-line mimic, consistent symbols, restrained colour, safe command confirmation and links to alarms. The test is whether a tired operator can understand the situation in seconds.
Screen hierarchy
| Level | Content |
|---|---|
| Overview | Whole plant: sources, buses, key loads, mode and readiness |
| Area | Generator plant, UPS, distribution, BESS |
| Equipment | Single generator or breaker with detailed values |
| Diagnostic | Trends, events, sequences, interlock status |
Mimic rules
- Draw the single-line diagram with true topology, so power flow can be traced.
- Show live or dead status of buses by colour or line style.
- Use standard symbols and consistent positions for breakers.
- Display breaker positions clearly: closed, open, racked out, tripped, unknown.
- Use colour sparingly: reserve red and amber for abnormal states.
- Show mode and ownership, such as local, remote, auto.
Commands
- Select before operate, with confirmation.
- Show the permissives and interlocks that block a command, with reasons.
- Require authority levels for critical commands.
- Log every command with user and time.
- Avoid commands near touch areas that can be hit by accident.
Interlock and permissive display
When an operator tries to close a breaker and the close is blocked, the HMI should list which conditions failed. This reduces guesswork and unsafe bypassing. See interlocking.
Outage-ready screens
- A blackout screen showing source status, battery autonomy, fuel remaining and restoration progress.
- A restoration screen showing steps and which loads are on.
- A summary of alarms with first-out. See alarm rationalisation.
Illustrative example
A plant HMI shows a green running generator with a grey breaker symbol. Operators misread the grey as closed. After a human factors review, open breakers show as hollow, closed as solid, tripped as flashing, and the legend is placed on every screen. Confusion falls in later drills.
A mimic shows state at a glance
An electrical mimic is the single-line diagram brought to the screen. The operator should understand the plant state in seconds: which sources are live, which breakers are closed, where power is flowing.
| Principle | Practice |
|---|---|
| Consistent colours | One colour scheme for energised, de-energised, alarm; do not rely on colour alone, add shape or text |
| Muted background | Grey or dark neutral; colour reserved for meaning |
| Clear breaker states | Open, closed, tripped, racked out, local mode |
| Hierarchy | Overview, then area, then device details |
| Navigation | Fixed alarm banner and menu |
Operator action safety
- Commands need select-before-operate or a confirmation.
- Show the interlock state, and why a command is blocked.
- Make the active control mode obvious.
- Use consistent naming that matches field labels.
Avoid
- Cluttered screens with many animations.
- Red used for both normal and alarm states.
- Unlabelled icons.
Test with operators
Run a scenario drill: ask a person unfamiliar with the design to find the failed source and the next action. Where they hesitate, the screen needs work.
See alarm rationalisation and SCADA architecture.
The master engineering question
During an outage at 3 a.m., can an operator tell in ten seconds which sources are live, which loads are on and what to do next?
What to send us
Send one-line diagrams and operating modes. Send your application details.
Questions engineers ask
Should the HMI copy the physical panel?
It should follow the one-line diagram and operating logic, not panel layout.
How many colours?
As few as possible, with consistent meaning.
Can operators trust the mimic?
Only if it is driven by confirmed feedback and shows unknown states.
Do I need a local HMI at the panel?
Yes, for local operation when remote systems fail.
This guide gives general principles. Confirm with human-factors review.