There are a lot of possibilities for combining unwanted signals in process variable and other information signals from the industries and from the environment itself. Several techniques are used before wiring installation in an industry.
Noise due to inductive and capacitive coupling can be minimized by ensuring that signal wires are positioned as far away as possible from such noise sources. Because, both the mutual inductance and capacitance between signal wires and other cables are inversely proportional to the square of the distance between the wires and the cable.
Noise due to inductive coupling is also substantially reduced if each pair of signal wires is twisted together along its length. This design is known as a twisted pair.
Noise due to multiple earths can be avoided by good earthing practices. In particular, this means keeping earths for signal wires and earths for high-current equipment entirely separate. The recommended practice is to install four completely isolated earth circuits as follows:
Power earth: Provides a path for fault currents due to power faults.
Logic earth: Provides a common line for all logic circuit potentials.
Analogue earth (ground): Provides a common reference for all analogue signals.
Safety earth: Connected to all metal parts of equipment to protect personnel should power lines come into contact with metal enclosures
Shielding consists of enclosing the signal wires in an earthed, metal shield that is itself isolated electrically from the signal wires. The shield should be earthed at only one point, preferably the signal source end. A shield consisting of braided metal eliminates 85% of noise due to capacitive coupling whilst a lapped metal foil shield eliminates noise almost entirely.
The wires inside such a shield are normally formed as a twisted pair so that protection is also provided against induced noise due to nearby electromagnetic fields. Metal conduit is also sometimes used to provide shielding from
capacitive-coupled noise, but the necessary supports for the conduit provide multiple earth points and lead to the problem of earth loops