Selection criteria of cables for process instrumentation are very complicated due to the wide variety of instruments available in the market as well as process plants, their increased sensitivity to electrical interference, as well as their desire for more precise information on process parameters.
Factors for choosing cable for a particular application are
- Instrument Types
- Length of the cable required
- Intensity of electrical interference in the area where the cable will be installed
- Environmental criteria
Instrument Type
The type of cable circuit to be used is usually dictated by the type of instrument. A thermocouple probe, for example, would necessitate a thermocouple (pairs) extension cable, the type of which would be determined by the probe type. A triad configuration is also required for a Resistance Temperature Device (RTD).
Length of Cable Run
The selection of conductor size is mainly based upon permittable voltage drop in the circuit. The other parameters such as Mutual Capacitance and L/R ratio may also be of equal importance while selecting cable.
Electrical Interference
Electrical Interference shortly called as EMI is a very complex subject and least understandable subject. There will be no attempt to provide complete solutions in this section; however, guidelines will be provided. Any spurious voltage or current arising from external sources that appears in the signal transmitting circuit is referred to as electrical interference. Errors in the measurement/control circuit occur when this voltage becomes too high and the signal-to-noise ratio is exceeded.
Electrical interference can be categorised into following three fields. They are
- AC fields, such as motors and power cables, cause magnetic coupling.
- Coupling electrically (capacitively) with adjacent circuits
- Direct coupling, such as two or more earth points in an earth current loop or a common return lead for multiple circuits.
There are various methods to reduce the interference in transmission circuits as mentioned below
Magnetic coupling
Direct coupling
Electrical (capacitive) coupling
Magnetic coupling
- Use of twisted pair/triad cores
- Route measurement cables away from AC fields or vice-versa; signal and power cables or wires should not be in the same conduit, tray or junction box.
- Eliminate the source of interference.
- Run measurement cables through properly earthed steel conduits or trays.
Direct coupling
- Use a high-quality insulation eg XLPE (Cross-linked polyethylene)
- Minimize the chance of moisture getting into cables.
- Earth screens at one point only
- Use one pair/triad per circuit ie; avoid the use of “sharing” leads in different circuits.
Electrical (capacitive) coupling
Screen each twisted pair/triad.
Environmental Conditions Identification
- Maximum operating conditions
- Presence of chemicals/moisture
- Abrasion and/or cut through resistance
- Fire retardancy
- Installation route i.e.; duct, direct burial, tray etc. (See table below.)
