In situations where there are a large number of cables varying in voltage and current levels, the IEEE 518-1982 standard has developed a useful set of tables indicating separation distances for the various classes of cables.
There are four classification levels of susceptibility for cables.
Susceptibility, in this context, is understood to be an indication of how well the signal circuit can differentiate between the undesirable noise and required signal. It follows that a data communication physical standard such as RS-232E would have a high susceptibility, and a 1000-V, 200-A AC cable has a low susceptibility.
The four susceptibility levels defined by the IEEE 518 – 1982 standard are briefly:
Level 1 (High) – This is defined as analog signals less than 50 V and digital signals less than 15 V. This would include digital logic buses and telephone circuits. Data communication cables fall into this category.
Level 2 (Medium) – This category includes analog signals greater than 50 V and switching circuits.
Level 3 (Low) – This includes switching signals greater than 50 V and analog signals greater than 50 V. Currents less than 20 A are also included in this category.
Level 4 (Power) – This includes voltages in the range 0–1000 V and currents in the range 20–800 A. This applies to both AC and DC circuits.
The IEEE 518 also provides for three different situations when calculating the separation distance required between the various levels of susceptibilities. In considering the specific case where one cable is a high-susceptibility cable and the other cable has a varying susceptibility, the required separation distance would vary as follows:
Both cables contained in a separate tray:
One cable contained in a tray and the other in conduit:
Both cables contained in separate conduit:
The figures are approximate as the original standard is quoted in inches.
Cable tray/conduit (photo credit: Legrand)
A few words need to be said about the construction of the trays and conduits. It is expected that the trays are manufactured from metal and be firmly earthed with complete continuity throughout the length of the tray. The trays should also be fully covered preventing the possibility of any area being without shielding.
Briefly galvanic noise can easily be avoided by refraining from the use of a shared signal reference conductor, in other words, keeping the two signal channels galvanically separate so that no interference takes place.
Electromagnetic induction can be minimized in several ways. One way is to put the source of electromagnetic flux within a metallic enclosure, a magnetic screen. Such a screen restricts the flow of magnetic flux from going beyond its periphery so that it cannot interfere with external conductors. A similar screen around the receptor of EMI can mitigate noise by not allowing flux lines inside its enclosure but to take a path along the plane of its surface.
Physical separation between the noise source and the receptor will also reduce magnetic coupling and therefore the interference.
Twisting of signal conductors is another way to reduce EMI. The polarity of induced voltage will be reversed at each twist along the length of the signal cable and will cancel out the noise voltage. These are called twisted pair cables.
and Braid) – photo credit: multicable.com
Electrostatic interference can be prevented or at least minimized by the use of shields. A shield is usually made of a highly conductive material such as copper, which is placed in the path of coupling. An example is the use of a shield, which is placed around a signal conductor.
When a noise voltage tries to flow across the capacitance separating two conductors, say a power and a signal conductor (actually through the insulation of the conductors), it encounters the conducting screen, which is connected to ground. The result is that the noise is diverted to ground through the shield rather than flowing through the higher impedance path to the other conductor.
If the shield is not of a high conductive material, the flow of the diverted current through the shield can cause a local rise of voltage in the shield, which can cause part
of the noise current to flow through the capacitance between the shield and the second conductor.
Reference: Practical Grounding, Bonding, Shielding and Surge Protection – G. Vijayaraghavan, Mark Brown (Get hardcopy from Amazon)
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