What I means on a P&ID.
Watch what slot the I sits in. First-letter I means current, IT, II. Second-letter I means indicator, FI, PI, LI. When a junior engineer reads IT-505 as 'indicator-transmitter' the meaning flips. They hunt for an analog input where there's actually a current measurement coming from the electrical scope. Parse the first letter first.
Also stands for.
- Modifier. Indicator, when used as a second letter, e.g. FI, PI
Common second letters.
| Tag | Meaning |
|---|---|
| IT | Transmitter, IT. Current transmitter, e.g. Motor amp meter |
| II | Indicator, II. Current indicator |
| IE | Sensor, primary element, IE. Current sensing element, e.g. A current transformer |
| IY | Computing, relay, IY. Current relay or signal converter |
Example tags.
| Tag | Meaning |
|---|---|
| IT-505 | Current transmitter, often a motor-current monitor brought into the BPCS |
| II-505 | Current indicator, ammeter |
| IE-505 | Current sensing element, the current transformer feeding the transmitter |
Typical placement.
On electrical signals brought into the process control system. Motor current to detect bearing wear or overload, transformer current for utility-side metering. Like E-prefix tags, I-prefix tags are uncommon on a process P&ID because most electrical measurement lives on a separate electrical drawing set.
Common questions
What does the ISA 5.1 first letter I stand for?
In ISA 5.1, the first letter I stands for Current, electrical. It can also stand for Modifier. Indicator, when used as a second letter, e.g. FI, PI.
How is letter I typically used on a P&ID?
On electrical signals brought into the process control system. Motor current to detect bearing wear or overload, transformer current for utility-side metering. Like E-prefix tags, I-prefix tags are uncommon on a process P&ID because most electrical measurement lives on a separate electrical drawing set.
What second letters commonly follow I in ISA 5.1 tags?
Common combinations include IT (Transmitter, IT. Current transmitter, e.g. Motor amp meter), II (Indicator, II. Current indicator), IE (Sensor, primary element, IE. Current sensing element, e.g. A current transformer), IY (Computing, relay, IY. Current relay or signal converter).
What are example ISA 5.1 tags starting with I?
IT-505 (Current transmitter, often a motor-current monitor brought into the BPCS); II-505 (Current indicator, ammeter); IE-505 (Current sensing element, the current transformer feeding the transmitter).
Run it on your drawings.
Upload a P&ID set and see every tag read in the convention the drawing already uses.