ISA-101 vs ISA-18.2 vs ISA-5.1: which standard answers which question
The three standards answer three different questions. ISA-101 covers the operator interface (HMI): how screens are structured, what colour means, how the HMI is built and maintained. ISA-18.2 covers the alarm system: what counts as an alarm, how alarms are prioritised and how the system is managed over its life. ISA-5.1 covers instrumentation: how an instrument is tagged and drawn on a P&ID. They meet at the edges, and a project that follows one usually touches the other two, but each has a job the others do not do.
In short
- ISA-101: how the operator interface looks, behaves and is maintained.
- ISA-18.2: what an alarm is, how urgent it is, and how the alarm system is managed.
- ISA-5.1: the tag (
FCV-104) and the bubble that identify an instrument on the P&ID. - In a project: ISA-5.1 gives the tags, ISA-18.2 the alarms and priorities, ISA-101 decides how both appear on screen.
- None of them is a symbol library: equipment shapes and screen layouts come from the project’s own style guide.
The short answer
Start from the question you are trying to answer:
| If your question is… | Look at | What you get |
|---|---|---|
| How should the screens be organised, what does each colour mean, how big is the text? | ISA-101 | A design approach and an HMI lifecycle, normally recorded in a project style guide |
| What counts as an alarm, what priority does it get, how do we stop alarm floods? | ISA-18.2 | An alarm management lifecycle, from the plant's philosophy to audit |
| What do we call this transmitter and how is it drawn on the P&ID? | ISA-5.1 | Tag format, letter codes and the bubble that shows where the instrument lives |
| What does a pump, tank or conveyor look like on the drawing? | None of the three | Other drawing standards (see P&ID symbols) and the project's legend sheet |
The sections below take each standard in turn: what it covers and, just as useful, what it leaves to someone else. This is a practitioner's summary, not a substitute for the standards themselves; the project specification and the documents it cites are what count when a requirement is in question.
ISA-101: the operator interface
ISA-101 is the standard for human-machine interfaces in process automation. It treats the HMI as something with a lifecycle (a style guide written first, then design, implementation, operation and maintenance under change control) and it deals with the questions an HMI designer actually meets: how displays are organised into a hierarchy, what colour is allowed to mean, how text is sized, and how the interface supports the operator's awareness of what the plant is doing. It is deliberately independent of the platform.
What it covers
- The HMI lifecycle, with a project style guide at the start.
- Display hierarchy and navigation (screen hierarchy).
- The colour philosophy: muted by default, colour for abnormal conditions and state changes (calm palette).
- Typography and consistency of objects.
- How alarm priority is shown on the screen. The priorities themselves come from alarm management.
What it does not
- It does not define every symbol, pixel or faceplate. It gives the constraints to design within; the shapes come from the project's own symbol set.
- It does not run the alarm system. It leaves that to ISA-18.2.
- It does not choose your HMI software or hardware.
The full walk-through is in the ISA-101 guide.
ISA-18.2: the alarm system
ISA-18.2 is about managing alarm systems in the process industries; it is also published internationally as IEC 62682. Its starting point is a definition: an alarm indicates an equipment malfunction, a process deviation or an abnormal condition that requires a response. If nothing needs doing, it is not an alarm. Around that idea it frames alarm management as a lifecycle: the plant's alarm philosophy, identification of candidate alarms, rationalisation (cause, consequence, operator action, time to respond, priority), detailed design, implementation, operation and maintenance, monitoring and assessment, and change management and audit. The per-operator alarm-rate benchmarks people quote usually come from EEMUA 191, a separate and older guide.
What it covers
- The definition of an alarm and the plant's written alarm philosophy.
- Rationalisation and priority, so that urgency is decided by consequence and time to respond.
- Handling of shelved, suppressed and out-of-service alarms.
- Monitoring and assessment of how the alarm system performs, and the controlled change of it.
What it does not
- It does not design the screen. How a priority is drawn is an HMI decision, which has to match the philosophy but is made in the style guide.
- It does not give you one universal set of alarm colours. The priorities, and what each one means, are recorded in the plant's own philosophy.
- It does not tell you which alarms your plant needs. That is what rationalisation is for, done by people who know the process.
The practical side, from rationalisation to showing priority on screen, is in alarm management.
ISA-5.1: instrument identification and symbols
ISA-5.1, “Instrumentation Symbols and Identification”, is the drawing-side standard. It defines the tag (function letters plus a loop number, such as FCV-104 or LSHH-401), the letter codes for the measured variable and the function, the bubble that shows where an instrument lives (field, DCS, PLC or shared display), and the line types between instruments. It is P&ID-centric: it exists so that everyone who reads a drawing arrives at the same picture of the loop.
What it covers
- Tag format and letter codes.
- The mounting bubbles: plain circle for field, line through the circle for DCS, circle in a square for PLC, double line for shared.
- Signal line types.
What it does not
- It does not cover mechanical equipment such as pumps, tanks or conveyors. Those glyphs come from other drawing standards and the project's legend sheet.
- It does not say anything about alarm priority or how an alarm is handled.
- It does not tell you what a screen looks like. On the HMI the tag is simply carried over exactly as the P&ID drew it, so operators can cross-reference.
Details and the letter table are in the ISA-5.1 reference and the instrument symbols guide.
What none of them covers
Some gaps catch teams out because they assume “the standard” settles them:
- Equipment symbols. Pumps, valves, tanks and conveyors are drawn from other standards and the legend sheet of each drawing set. See P&ID symbols.
- Exact screen layouts and faceplates. ISA-101 asks for consistency. The layouts themselves are part of the project's style guide; see faceplate design.
- Graphic symbols for process displays. This is a separate standard, ISA-5.5, which the ISA-5.1 reference explains.
- Functional safety. Safety instrumented systems have their own standard (ISA-84). The three standards here do not replace it.
- Your platform. How to build a multi-state object in WinCC, Ignition, FactoryTalk or EasyBuilder is vendor documentation; the integration guides cover the import steps.
How they fit together in one project
The dependency runs in one direction: the screen shows tags and priorities that the other two standards produce.
- ISA-5.1 first. The P&ID gives every instrument its tag and its mounting. These tags are the names everything else uses.
- ISA-18.2 next. Alarms are rationalised against those tags: cause, consequence, operator action, time to respond and priority.
- ISA-101 throughout. The HMI style guide fixes palette, sizes, symbols and hierarchy, and decides how tags and priorities appear: the tag exactly as on the P&ID, the priority as a colour plus a second cue.
In real projects the three overlap in time. The style guide is being written while the P&ID is still changing, and the alarm list is rationalised while screens are drawn. What matters is that none of the three is left to chance: when a tag changes, the screen follows; when a priority changes, the badge follows.
Apply them on the screen
The Symbol Builder starts from the calm palette and exports one image per state, with alarm priority badges and instrument bubbles in the ISA-5.1 outline. The builder is free to use, no account needed to draw; downloads need a free account, 5 a month on Free.
Open the builder → Read the ISA-101 guide →Frequently asked questions
What is the difference between ISA-101 and ISA-18.2?
ISA-101 is about the operator interface: how screens are organised, what colour means, how the HMI is built and maintained. ISA-18.2 is about the alarm system behind it: what counts as an alarm, how it is prioritised and how the system is managed over time. ISA-101 leaves alarm management to ISA-18.2, and the HMI then shows the priorities that the ISA-18.2 process produces.
What is the difference between ISA-101 and ISA-5.1?
ISA-5.1 is a drawing standard: tags, letter codes and bubbles that identify instruments on a P&ID. ISA-101 is a design standard for the operator screen. A good HMI uses ISA-101 for layout and colour and carries the ISA-5.1 tags over exactly as the P&ID shows them, so operators can cross-reference screen and drawing.
Do I have to follow all three standards on a project?
ISA standards are voluntary on their own. They apply to a project when a contract, a company specification or a regulator requires them, so the project specification is where to look. Where nothing requires them they are still a useful common language: ISA-5.1 for the drawings, ISA-18.2 for the alarm list, ISA-101 for the screens.
Where do alarm colours come from, ISA-101 or ISA-18.2?
Both play a part. The alarm priorities and what each one means come from the alarm philosophy and the rationalisation that ISA-18.2 describes. How a priority looks on the screen, with its colour and a second cue that does not rely on colour, is an HMI decision made in the style guide, which is ISA-101 territory. The two have to agree.
Which standard should I read first if I design HMIs?
Start with ISA-101, because it covers the screens you build. Then learn enough of ISA-18.2 to ask the right questions about alarm priorities, and enough of ISA-5.1 to read the tags on the P&ID. The guides linked below follow that order.
Keep reading
- ISA-101 HMI design standard: complete guide — lifecycle, colour, hierarchy and typography.
- Alarm management — the ISA-18.2 lifecycle in practice.
- ISA-5.1 instrumentation symbols reference — tags, letter codes and mounting bubbles.
- HMI design mistakes — what goes wrong when these ideas are skipped.
- The calm HMI colour palette — what colour means on the screen.
- HMI / SCADA / P&ID glossary — the terms used above.
- Ask the community — a question about which standard applies to your project.