The performance of cable screen depends on their construction. There are different types of cable screens available depending on the applications, specialized screen constructions, such as optimized or multiple braids
You can also run an unscreened cable in a shielded conduit, in a separate braided screen, or wrap it with screening or permeable material. For systems or installation engineers, these options are most useful
Lapped wire screens:
Lapped wire screens are helically wound wires on the cable. They are very flexible but have poor screening efficiency and are noticeably high – frequency inductive and are therefore limited to audio
Single braid screens:
Single braid screens consist of wire woven into a braid to provide the cable with a metal frame covering 80–95 percent coverage and reasonable high – frequency performance. The braid significantly adds weight and rigidity to the cable.
Laminated band or film with drain wire provides a complete cover, but with relatively high resistance and therefore only moderate screening efficiency.
It retains light weight, flexibility, low diameter and low cost. It is difficult to properly terminate this type of screen; screen currents tend to flow mainly into the drain wire, making it unsuitable for magnetic screening, although excellent capacitive screening.
Composite tape and braid:
Composite tape and braid combine the advantages of laminated tape and single braid to optimize coverage and performance at high frequency.
Multiple braid screens enhance single braid performance by separating internal and external current flows and allowing the screens to be dedicated to different (low and high frequency) purposes.
Surface Transfer Impedance:
In terms of surface transfer impedance (STI), the screening performance of shielded cables is best expressed.
This is a measure of the voltage induced by an interference current flowing down the outer shield of the cable on the inner conductor(s) of the cable, which varies with frequency and is usually expressed in milliohms per meter.
A perfect screen would not allow the inner conductors to induce any voltage and would have a zero STI, but practical screens will couple some energy through the screen impedance to the inside. It is equal to d.c at low frequencies. The screen’s resistance.