Process control depends on the integrity of communication between instruments,information travels through the loop and signals which represents the value of the process variable or the output of the controller. These signal must be uninterrupted if the loop is to work properly.when an instrument transmits electrical signals,those signals are sent through conductors to other process instruments.
Types of Industrial wires and Condutors
1.Solid Conductor wires -
Solid conductor wires are solid metal usually copper covered by insulation.
2.Stranded Conductor Wire
Stranded conductor wire or strands of metal twisted together and covered by insulation.These wires are more flexible than solid conductors.
Shielded cable has a metal shield between the insulated conductors and outer jacket.The shields can be braided metal,stranded metal or foil.A foil shield may have drain wire to remit easy connections to ground
Coaxial cable is shielded cable that carries low voltage signals.In addition to a jacket and braided shield coaxial cable has a dielectric and insulation covering the conductor.
The amount of current a conductor can carry is determined by its gauge or size. The wire gauge and voltage must be marked on the insulation.Voltage ratings are determined by the amount and type of insulation.When wires are spliced or connected they should have the lowest possible resistance,good mechanical strength and sufficient insulation.
Many process instruments are grounded to a facilities ground system.Most facilities use structural steel rod embedded in the earth or other existing metal in the building to carry ground current to a power system ground.Some grounds are designed for safety.These may also called equipment ground. Others are designed to protect instruments from noise in the environment.Many instruments have both types of ground.
Safety / Equipment Ground - Designed for safety
Instrument ground - Protect instrument from noise.
Grounding provides a low resistance path for unwanted or transient electrical current. Safety grounds include ground wires or ground straps that provide the path for current flow between the instrument case and earth.The metal conductor of a safety ground should be a large enough gauge to carry the maximum current expected under fault conditions as well as normal conditions.
Instrument grounds are also wires or straps.However they are designed to protect instruments from electrical noise.Electronic instrument signals are low voltage and low current as a result they are easily distorted by electrical noise. For example an instrument cable that passes near a high voltage alternating current motor may be distorted by electrical noise.When the distorted signal is received by the controller the value it represents wont be accurate.So the controllers output wont be accurate either and the final control element may take incorrect action. The entire may be adversely affected simply because a signal cable wasn't grounded properly. It is a good idea to use shielded cable when signal wires pass near high voltage equipment.The shield screens out noise that can affect the signal carried by the conductor.
Shielded cable work properly only if they are grounded so that transient current can be drained off before it affects the signal.However shields must be grounded only at one end or they may create their own source of signal distortion.If a shield is grounded in more than one point distortion could be caused by a difference in ground voltage potentials. This voltage difference could be large enough to create a current flow through the shield producing an unintentional ground loop. Ground is considered to be zero potential but factors such as weather or electrical equipment can alter the earth's voltage potential. Any current flow in this ground loop could distort the transmitted signal.