Basics of Instrument Loop Documents

Instrument Loop Documents represents detailed drawing showing a connection from one point to control system.

It could be connection between:

  • Field instrument to control system (or vice versa)
  • Signal from Control Panel to control system (or vice versa)
  • Signal from MCC to control system (or vice versa)
  • Signal form one control system to another system

Loop diagram shows instrument (in a symbol) and its terminal numbers which are to be connected, instrument cable number, junction box number, terminal number assigned for the specified instrument, multi-pair cable and pair number , marshaling cabinet number, terminal number in marshaling cabinet, control system details (rack, slot, I/O channel).

It also clearly indicates location of each equipment by means of border line as a limit. Loop Diagram usually shows a single control loop which means it could only contains just one input (sensor to control system), just one output (control system to final element) or combination of both

Reference drawing

To have the loop diagram completed and provide complete information, the following are list of data required along with its source/reference:

  • Instrument Terminal number. Most instrument could be assumed to use (+) and (-). Terminals. Instrument which needs special arrangement such as smoke detector or instrument which in series loop, requires manufacturer connection detail to make the cable is properly connected.

  • Junction box terminal number, this information could be obtained from JB wiring connection

  • Marshaling terminal number, this information could be obtained from marshaling wiring connection.

  • I/O point detail information. Obtain this information from I/O assignment which is produced by system integrator or control system vendor.

The purpose of instrument loop diagram

It is used in checking of a correct installation and connection when tested during pre-commissioning, commissioning and also for trouble shooting during operation.

Instrument Loop Diagram Example

Instrument Loop Diagrams.pdf (40.6 KB)