Control Cable vs Instrumentation Cable

What is the difference between control cable and instrumentation cable?

The difference starts with the circuit. Control cable carries commands and status between a control panel and field equipment. Typical uses are relay outputs, interlock circuits, actuator circuits and auxiliary power for control equipment.

Instrumentation cable is selected around the signal. Sensors and transmitters produce low-level analogue or digital signals that are easily disturbed by electrical noise. For that reason the cable is normally built with twisted pairs or triads, and the screens are chosen to protect each signal from its neighbours and from outside interference. The main question is what the circuit must carry and how much interference it can tolerate, not only how much current it draws.

The table below summarises the main differences.

Construction and screening options vary by manufacturer and by the signal-system design. Take the exact electrical values from the manufacturer's datasheet.

How does a loop schedule decide which cable you need?

A loop schedule lists each field device and the circuit that connects it to the control system. Reading the schedule shows which circuits are analogue pairs, which are digital communication lines, which are relay outputs and which carry auxiliary power.

The common mistake is to buy one generic multicore cable for the whole job. Circuits with different signal types need different shielding and electrical properties. If they share one cable without separation, the sensitive pairs may lack the screening they need, and the cable may not terminate cleanly at the marshalling points.

• Group circuits by signal family, for example discrete, analogue, digital communication and auxiliary power.
• Record the signal type and the circuit voltage for each group.
• Decide the pair, triad or multicore arrangement for each group.
• Note any intrinsic-safety or grounding requirement that applies to a group.

Which screen arrangement suits your signals?

Screen options include overall foil, individual pair screens, braid, or combinations of these. Individual screens reduce coupling between pairs, which matters where neighbouring signals could interfere with each other. An overall screen addresses interference coming from outside the cable. Many instrumentation designs use both, but the choice depends on the signal and the environment.

A screen only works if it is continuous through the cable, the gland and the termination. A break at the gland or a poor earth connection can leave the signal unprotected even when the cable itself is correctly specified.

Which electrical parameters matter besides conductor size?

For analogue, pulse or communication circuits, capacitance, impedance and pair balance may matter. These properties are not described by conductor cross-section, so a cable chosen only by size can perform poorly on a sensitive circuit. Long signal runs make capacitance more significant, because the total capacitance grows with cable length.

Ask the supplier for the capacitance, impedance and pair balance values on the datasheet, and check them against what the receiving device requires. These values depend on the construction, so they should be taken from the manufacturer's datasheet rather than estimated from the conductor size.

Conductor class also matters where the cable must bend in service. Under IEC 60228, classes 1 and 2 are intended for fixed installation, while classes 5 and 6 are used for flexible cables and cords.

How should control and instrumentation cables be installed?

Separate sensitive signal circuits from power cables and from VFD cables. Where a crossing cannot be avoided, follow the separation rules in the design and installation specification rather than improvising on site.

Maintain shield continuity through the gland and the termination, and use approved gland and grounding methods. Where intrinsic-safety segregation applies, keep the circuits apart as the design requires. See cable gland selection for gland and earthing detail.

VFD cable is built differently. Symmetrical construction, with the phase conductors and earth conductors arranged symmetrically, and an overall shield bonded at both ends are used to reduce common-mode current and EMC problems. That is a property of the VFD cable itself. It does not remove the need to keep instrumentation pairs away from the drive wiring. For the motor side of a drive installation, see VFD motor cable guide.

Signal-system and hazardous-area requirements should be confirmed by the control-system designer before the cable is ordered.

Frequently asked questions