Loop diagrams: what an instrument loop diagram shows, how to read one, loop types, scaling 4 to 20 mA, and keeping loop drawings current
Loop diagrams explained: what an instrument loop diagram shows from the field device through junction box and marshalling to the control system channel, how to read one, two wire, four wire, analog and discrete loops, scaling a 4 to 20 mA signal and the NAMUR NE 43 fault levels, and how loop diagrams are used from design and loop checks to fault finding and modifications. With an illustrative loop drawing.
Key takeaways
- A loop diagram shows one instrument loop from the field device to the control system: every terminal, cable, core, barrier and channel on the way.
- The P&ID says what the loop does. The loop diagram says how it is wired, which is what a technician needs at a junction box.
- A 4 to 20 mA signal is scaled linearly over the calibrated range. Below about 3.6 mA or above 21 mA, under NAMUR NE 43, the reading is treated as a fault.
- Loop diagrams are used most at the loop check during commissioning, and again every time a loop is fault found or modified.
- Generate loop diagrams from the instrument index and IO list, and update them through management of change, or they drift from the plant.
A level transmitter reads zero on the operator screen. The technician opens the junction box, finds forty terminals, and the loop drawing in the file shows a box number that was changed in a modification five years ago. An hour of tracing later the fault turns out to be a single loose core, on a terminal the drawing would have given in a minute if it had been kept up to date.
This guide covers loop diagrams: what they show, how to read one, the common loop types, scaling a 4 to 20 mA signal, and where loop diagrams are used from design through operation. It sits beside the guides to P&ID symbols, control system design documents and single line diagrams. The loop shown is illustrative.
What a loop diagram shows
A loop diagram is read from left to right, from the field to the control system. It shows one loop, so everything a technician needs to install, test or fault find that loop is on one sheet. ISA-5.4 sets out the minimum content and uses the ISA-5.1 symbols and tags that the P&ID uses, so the tag on the loop diagram matches the tag on the process drawing and the instrument tag on the device.
| What it shows | Detail | Why it matters |
|---|---|---|
| Loop and tag numbers | The loop number and each device's tag, as on the P&ID | Ties the wiring to the process and the IO list |
| Field device | The instrument, its terminals and its location | Where to start a fault find |
| Junction box | The box number and the terminals each core lands on | The usual place a loop is broken for testing |
| Cables | Cable numbers, core or pair numbers and colours, from field to cabinet | The route the signal takes, and what a spare pair is |
| Marshalling and barriers | Cabinet terminals, fuses and any isolator or intrinsically safe barrier | Where the field wiring meets the control system wiring |
| Control system channel | The controller, rack, slot, card and channel | Matches the IO list and the configuration |
| Power | Whether the loop is powered by the card or by an outside supply, and the voltage | Two wire and four wire devices are wired differently |
| Shield and earth | Where each cable shield is earthed, normally at one end only | A shield earthed at both ends can carry current and noise |
| Range and set points | Calibrated range, units, and alarm or trip settings | What the current means, and what the operator sees |
| Title block and references | Revision, drawing number, and the P&ID, datasheet and IO list it relates to | Shows whether the drawing is current |
Types of loop
Most loops on a plant are a handful of types. A loop diagram makes the difference between them visible, which matters most for power: a two wire transmitter draws its power from the loop, while a four wire device has its own supply and drives the current itself, and wiring one as the other is a common commissioning fault.
| Loop type | How it works | Typical use |
|---|---|---|
| Analog input, two wire | A transmitter powered by the loop current from the input card | The most common loop, a flow, level or pressure transmitter |
| Analog input, four wire | A transmitter with its own power supply, driving the current | Analysers and flowmeters with heavier electronics |
| Analog output | The control system drives 4 to 20 mA to a valve positioner or drive | Control valves and speed references |
| Discrete input | A contact or proximity switch read as on or off | Limit switches, level switches, run feedback |
| Discrete output | The control system switches a solenoid, relay or starter | Solenoid valves, motor start commands |
| Temperature | A thermocouple or RTD wired to a head transmitter or a dedicated card | Thermocouples need compensating cable to the cold junction |
| Fieldbus segment | Several devices on one cable, shown on a segment drawing | Device addresses take the place of channels |
Scaling a 4 to 20 mA signal
A 4 to 20 mA loop carries its measurement as a current, so it is unaffected by cable resistance within the loop's limits. The current is linear over the calibrated range, with 4 mA at the bottom and 20 mA at the top. Starting at 4 mA instead of zero leaves a two wire transmitter enough current to power itself, and makes a broken wire, which reads zero, distinguishable from a real zero. Readings well outside the range are treated as a fault, so the control system can alarm on a failed instrument instead of acting on a false value.
| Current | Share of range | Value on 0 to 250 m³/h |
|---|---|---|
| 4 mA | 0 per cent | 0 m³/h |
| 8 mA | 25 per cent | 62.5 m³/h |
| 12 mA | 50 per cent | 125 m³/h |
| 16 mA | 75 per cent | 187.5 m³/h |
| 20 mA | 100 per cent | 250 m³/h |
| Below 3.6 or above 21 mA | A fault | Treated as a failed instrument, under NAMUR NE 43 |
A loop diagram with the wrong range is as dangerous as one with the wrong terminals. The value the operator sees depends on it.
Where loop diagrams are used
Loop diagrams are produced in detailed design, alongside the IO list and instrument index, and are among the documents most used afterwards. Generated from the same data, as part of document automation, they stay consistent with it. They are the record for the loop check during commissioning, where every loop is tested and signed off, and any difference is marked up and issued as an as-built. After that they are only as good as the management of change process that keeps them current.
| Stage | How it is used | What keeps it right |
|---|---|---|
| Design | Produced from the instrument index and the IO list | Generated from the data, so the three agree |
| Construction | The installer terminates each core to the diagram | Marked up with any change as it is built |
| Loop check | Each loop tested from the field device to the operator screen | Signed off loop by loop, and redlines raised |
| Operation and fault finding | The technician follows the loop terminal by terminal | The drawing in the hand at the junction box |
| Modification | Updated through management of change and issued as built | Any loop not updated is wrong the next time it is needed |
Common mistakes
- Loop diagrams drawn by hand and not checked against the IO list.
- Modifications wired in the field without updating the loop diagram.
- Shields earthed at both ends, or not at all.
- Two wire and four wire devices wired the same way.
- Ranges on the drawing that differ from the transmitter and the control system.
- Junction box and terminal numbers on the drawing that do not match the labels in the field.
- Loop diagrams held only as scanned PDFs that cannot be searched or regenerated.
Common questions
What is a loop diagram?
A loop diagram, or loop drawing, is an instrumentation drawing that shows a single control or measurement loop from the field instrument to the control system. It shows each device with its tag, the junction box and cabinet terminals, the cable and core numbers, any barriers or isolators, the control system channel, the power source, the shield earthing, and the calibrated range. ISA-5.4 describes the content of a loop diagram.
What is the difference between a loop diagram and a P&ID?
A P&ID shows the process: the equipment, piping and instruments, and how the instruments relate to the process and to each other. A loop diagram takes one loop from the P&ID and shows how it is physically wired, terminal by terminal. The P&ID is used to understand what the plant does, and the loop diagram to install, test and fault find it.
How do you read a loop diagram?
Start at the field device on one side and follow the signal across the drawing. Most loop diagrams are divided into columns for the field, the junction box, the marshalling cabinet and the control system. At each boundary read the terminal numbers, the cable number and the core or pair, then the control system rack, slot and channel. The title block gives the loop number, the range and the related drawings.
How do you convert 4 to 20 mA to a process value?
The value is the bottom of the range plus the fraction of the 16 mA span above 4 mA, times the range span. For a 0 to 250 m³/h flowmeter, 12 mA is (12 minus 4) divided by 16, which is 50 per cent, so 125 m³/h. 4 mA is the bottom of the range and 20 mA is the top.
What is a loop check?
A loop check tests each instrument loop end to end during commissioning, from the field device to the operator's screen and back for outputs. A signal is injected or simulated at the field end and the value, range, alarm and tag are checked at the control system. Each loop is signed off against its loop diagram, and any wiring or drawing difference is marked up as a redline.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
- ISA-5.4-1991 Instrument loop diagrams (International Society of Automation (ISA))
- ANSI/ISA-5.1-2024 Instrumentation and control symbols and identification (International Society of Automation (ISA))
- NE 43 Standardization of the signal level for the failure information of digital transmitters (NAMUR)
Related products, case studies and tools
- Enco Documentation Tool (product)
Related reading
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