PlantFlow 3D › Guides › Instrument loop diagrams and cable screens
Instrument loop diagrams and cable screens
What an instrument loop diagram shows, how a 2-wire loop is wired from field to PLC, and why cable screens are earthed at one end only.
What a loop diagram is for
A loop diagram shows every connection of one instrument loop on one sheet: the field device, junction box terminals, multicore cable pairs, marshalling terminals, power supply, fuse and PLC channel, with cable numbers. Technicians use it to install, check and fault-find a loop without tracing the whole panel.
A 2-wire transmitter loop
For a loop-powered transmitter, the fused 24 V DC supply goes out on one core to the transmitter positive terminal; the return core from the transmitter negative goes back to the analogue input, and the input returns to 0 V. The transmitter sits in series in the loop and controls the current.
A 3-wire device has separate supply, common and signal cores. A 4-wire device has its own power supply and drives the current itself into a passive input.
Screens and earthing
Instrument cables usually have an overall or individual screen to reduce interference. The screen is normally earthed at one end only, usually the control panel end, and cut back and insulated at the field end. Earthing both ends can let current flow in the screen when the two earth points are at different potentials, which brings noise into the signal.
Checks before commissioning
Check continuity and insulation of each core, terminal numbers against the drawing, polarity, loop supply voltage, and that the PLC channel type matches the signal. Then inject or simulate 4, 12 and 20 mA and confirm the reading at the HMI.
Try it yourself
Open Drawings → Loop drawings, choose LT-101 and change its wiring type in the instrument index. The sheet and its checks update.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.