Shielding conductor wires

instrumentation

#1

##Why is shielding needed?

When a changing magnetic field cuts across a conductor, it will induce a current flow. The resistance in the circuits will convert this to a voltage. This can occur in conductor wire connected to sensor and instruments.

These unwanted voltages result in erroneous readings from sensors and signal to outputs. Shielding will reduce the effects of the interference. When shielding and grounding are done properly, the effects of electrical noise will be negligible. Shielding is normally used for; all logical signals in noisy environments, high-speed counters or high-speed circuitry, and all analog signals.

Techniques to reduce noise:

Shielding involves encasing conductors and electrical equipment with metal. As a result electrical equipment is normally housed in metal cases.

Wires are normally put in cables with a metal sheath surrounding both wires. The metal sheath may be a thin film, or a woven metal mesh. Shielded wires are connected at one end to “drain” the unwanted signals into the cases of the instruments.

It is shown in above figure a thermocouple connected with a thermocouple. The cross section of the wire contains two insulated conductors. Both of the wires are covered with a metal foil, and final covering of insulation finishes the cable. The wires are connected to the thermocouple as expected, but the shield is only connected on the amplifier end to the case. The case is then connected to the shielding ground, shown here as three diagonal lines.

Another method to reduce noise is twisted pair wire:

The two wires are twisted at regular intervals, effectively forming small loops. In this case the small loops reverse every twist, so any induced currents are cancel out for every two twists. Using a twisted pair wire to shield reduce the effect of noise to large extent.

When designing shielding, the following design points will reduce the effects of electromagnetic interference, Avoid “noisy” equipment when possible, Choose a metal cabinet that will shield the control electronics, Use shielded cables and twisted pair wires, Separate high current, and AC/DC wires from each other when possible, Use current oriented methods such as sourcing and sinking for logical I/O, Use high frequency filters to eliminate high frequency noise.