Cable spacing as a means of noise mitigation

Separation distances

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.

IEEE 518 – 1982 standard

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:

• Level 1 to level 2-30 mm
• Level 1 to level 3-160 mm
• Level 1 to level 4-670 mm

One cable contained in a tray and the other in conduit:

• Level 1 to level 2-30 mm
• Level 1 to level 3-110 mm
• Level 1 to level 4-460 mm

Both cables contained in separate conduit:

• Level 1 to level 2-30 mm
• Level 1 to level 3-80 mm
• Level 1 to level 4-310 mm.

The figures are approximate as the original standard is quoted in inches.

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