Instrumentation Wiring Consideration


The instrumentation cables between the computer and the devices are operated independently from the 480 VAC energy cables and 120 VAC power and control cables.

This is because the electrical areas of 480 and 120 VAC can cause voltage on the structure of the equipment and this higher voltage sound can’ swamp’ or obscure the low voltage signal in the building of the equipment.

It is possible to separate energy from instrumentation wiring in several distinct respects, including:

  • Physical distance separation, such as a minimum of 12” between the two types of wiring; and

  • Ferromagnetic (such as iron or steel) enclosure of one of the wiring types, such as putting the instrumentation wiring in steel conduit or steel wire way.

If the two types of wiring have to pass through the same location as in a control panel, it is generally better to have them perpendicularly cross each other to minimize the amount of induction.

Cable Types used for signal wiring:

A protected bent pair shielded twisted pair (STP) cable is the sort of cable typically used for 2-wire 4 to 20 mA transmissions. For 3-wire 4 to 20 mA transmitters, the shielded triad is the usual choice. For 4-wire 4 to 20 mA transmitters, shielded twisted pair is typically used for the 4 to 20 mA signal.

Grounding of Shields:

Usually protected bent couple wires are the wires used for 4 to 20 mA signals. Due to the shield’s Faraday box impact, the shield appears to avoid electrical sound from reaching the attached couple of drivers.

If electrical noise still makes it to the twisted pair, it appears as common-mode electrical noise, which will be rejected inherently by the differential measurement technique at the analog input.

To shed any electrical sound voltage caught on the shield, one end of the shield must be ground.

Whether the shield is held at the start of the receiver (field) or the device (PLC or DCS) does not matter technically, but the shield grounding links are generally already present at the bottom of the device in the shape of terminal blocks.

This is one reason that the shields of shielded cables are usually grounded at the controller end.

On loop plates and other industrial cable sketches, it is prevalent to use a sign at the transmission end of the protected bent wire that tells “Cut & Tape,” meaning that the shield is to be cut back to this end and the remaining stub is to be fitted with electrical tape to guarantee that the shield is not held by contacting the back panel or terminal of the enclosure.