How to select Instrument cable sizing?

How to select Instrument cable sizing?

Selecting the correct instrument cable size is important to ensure reliable and accurate communication between instruments and control systems. The sizing of instrument cables should be based on several factors, including the distance between the instruments and the control system, the type of instrument signal being transmitted, and the environmental conditions in which the cables will be installed.

Here are the steps to select the right instrument cable size:

The variety of instruments presently used in process plants, their increased sensitivity to electrical interference, and the need for more precise information on the process parameters have all complicated the choice of cables for process instrumentation.

Several aspects must be taken into account when choosing a cable for a certain application; some of them are addressed here.

Determine the distance between the instrument and the control system:

The length of the cable run is one of the most important factors in determining the size of the cable. Longer cable runs will require larger cables to prevent signal loss.

Determine the type of signal being transmitted:

Instrument cables are used to transmit a variety of signals, including analog, digital, and high-frequency signals. The type of signal being transmitted will impact the cable size needed.

Consider environmental conditions:

Environmental factors, such as temperature, moisture, and chemical exposure, can affect the performance of the cable. Choose a cable with insulation and shielding appropriate for the environment in which it will be installed.

Environmental Conditions are identified using the following criteria.

• optimum operating conditions

• the presence of moisture or chemicals’’

• Cut through or abrasion of resistance’

• Fire resistance’

• The installation technique, such as tray, conduit, or directly burying.

Check manufacturer specifications:

Consult the manufacturer’s specifications for the instrument and control system to determine the recommended cable size.

Use industry standards:

Industry standards, such as the National Electrical Code (NEC) or the International Electrotechnical Commission (IEC), may provide guidance on the selection of instrument cable sizes.

Electrical interference level in the region where the cable is installed:

Electrical interference, often known as EMI, is one of the most complicated and least understood topics. While there will be no effort to provide comprehensive answers in this part, there will be some pointers. Electrical interference is any erroneous voltage or current that emerges in the signal-transmitting circuit and originates from outside sources. Errors arise in the measurement/control circuit when this voltage exceeds the threshold and the signal to noise ratio is exceeded.

The three categories of electrical interference sources are:

  • Magnetic coupling from AC fields, such as motors and power lines.
  • Capacitive electrical connection with neighboring circuits
  • Direct connection, such as earth current loops involving two or more earth points or a shared return lead for several circuits.

The following is a description of techniques for reducing interference in transmission circuits:

Magnetic coupling

  • Use of twisted pair/triad cores.
  • Signal and power cables or wires shouldn’t be in the same conduit, tray, or junction box; route measurement cables away from AC fields, and vice versa.
  • Eliminate the source of interference.
  • Laid the measuring wires in enclosed trays or appropriately earthed steel conduits.

Electrical (capacitive) coupling

  • Screen each twisted pair/triad.

Direct coupling

  • Use a high quality insulation eg XLPE.
  • Reduce risk of moisture ingress into cables.
  • Earth screens at one point only.
  • Use one pair/triad per circuit ie; avoid the \suse of “sharing” leads in different circuits.