Shield Effectiveness

Transfer Impedance
Transfer impedance is a fundamental value of a shield’s Performance.
Transfer impedance relates a current on one surface of the shield To the voltage drop generated by this current on the opposite surface of the shield. This value depends solely on the shield construction.
Transfer impedance test data is obtained with a terminated Triaxial test fixture. This test method, which is a modification of An existing IEC test procedure, is the one that was proposed by Kenneth Simons.

Above Figure illustrates the relationship between the radiated and transfer impedance methods. In the radiated case (A), an antenna-coupled EF induces current into the RF gasket material. For transfer impedance measurements (B), a signal generator is used. The Zst is a complex quantity that combines the material’s DC surface resistance (Rs) and its surface inductive reactance (Xsl).

Transfer Impedance is defined as: Zt = ( 1 / I0 ) x ( dV / dx )
Where I0 is a longitudinal disturbing current generated on one surface (either the inner or the outer surface) of the shield and dv /dx is the longitudinal voltage per unit length, generated by I0 , appearing on the opposite surface of the shield.
In the terminated triaxial test system, the test cable center conductor and the shield form an inner terminated triaxial test system, the test cable center conductor and the shield form an inner transmission system, with the shield and the outer concentric tube forming an outer transmission system .
The outer system is driven by a generator and creates a current I0 on the outer surface of the shield. This current causes a voltage difference on the opposite surface shown as V1, and V2 over a length of shield X.
This generates signals in the test cable which can be related to the transfer impedance value of the shield.

Conclusion :
The lower the value of the transfer impedance, the more effective the shielding.
The transfer impedance value theoretically can be used to determine absolute interference levels.

Types of Noise and Their Effects on the cables

For transducer connection to the instrument , the cable transmits a very low e.m.f. signal.
A noise free signal is important to avoid mistakes.
Thus the cable must be screened against static or magnetic fields which can induce unwanted e.m.f.
There are four different sources of noise, as follows:

Static Noise

This is interference caused by coupling of capacity between external electrical field from power line or another voltage source and the cable. Interposing a single shield which forms a capacitor when connected to the earth.
A shield of aluminium/polyester, 100% coverage is recommended.

Magnetic Noise

Generally low frequency electromagnetic field due to power cable, motor, ctc. Can induce e.m.f. into the instrumentation cable. The twisting of conductors provides a good reduction of the magnetic noise. Other reduction are given by steel conduit, armours (high inductance material).Is some particular cases low resisting screen(i.e.copper braids, copper tapes) may be necessary.

Common-Mode Noise

This is typical interference caused when the instrumentation loop is earthed on two sides with different potential. To avoid this noise the shield, instrument or hot junction of thermocouple must be commonly earthed.

Cross Talk Noise

This is caused by unbalanced capacitance from adjacent cabling elements of different construction . To reduce this noise for pair/triad/quad cables, differing lay of twist is used or more effectively each pair/triad/quad is individually shielded (i.e. aluminium/polyester 100% coverage) and commonly earthed.

Reduction Factor of Screen

External interference

Screening against external interference has to take the influence of both electric and magnetic interference into account and make a distinction between LF and HF fields.’The design of the required screen depends on the type and strength of interference

Screening in the LF-range

For screening in the LF-range – i.e. in the range of frequencies of up to 10kHz – the influence of both electric and magnetic interference can be examined separately. Interference from electric fields can be virtually disregarded if conductive screens are used; the lower the dc resistance, the better the screening effect; however, care must be taken to ensure a high degree of coverage as the electric field may otherwise affect the cable core.Screening against LF magnetic interference requires the use of magnetic materials such as steel wires or(even better) tapes. Materials of high permeability should be used for high levels of screening.
The measure of the effect of the screening in the LF range is the reduction factor. This is the ratio of the interference voltage with a screen to the interference voltage without a screen and is calculated according to the following formula: