This part of BS 1646 specifies instrument symbols for use on interconnection diagrams used for the design, installation, and maintenance of process measurement and control systems.

These detailed symbols are not normally intended for drawings that use the functional symbols given in BS 1646‑1 and BS 1646‑2. However, the symbols specified in this part of BS 1646 show, by detailing the components, the external connections between units of equipment.

Information on the internal connections in units is not normally included, but references to the appropriate circuit or wiring diagrams may be provided.

When an instrument is composed of more than one functional part, the different symbols may be combined, for example, recorder controller.

The dimensions of the symbols are unspecified, provided the ratio of the side lengths is maintained according to this part of BS 1646¹⁾.

If not otherwise stated, contact symbols should be shown open.

The diagrams may employ single line or multi-line representation and may be combined with, or replaced by tables, providing clarity is maintained.

For further assistance see IEC 113‑5, Preparation of interconnection diagrams and tables.